First commit

vllm/core/block/block_table.py

@@ -0,0 +1,374 @@
+import math
+from typing import List, Optional
+
+from vllm.core.block.common import BlockList
+from vllm.core.block.interfaces import Block, DeviceAwareBlockAllocator
+from vllm.utils import Device, cdiv, chunk_list
+
+
+class BlockTable:
+    """A class to manage blocks for a specific sequence.
+
+    The BlockTable maps a sequence of tokens to a list of blocks, where each
+    block represents a contiguous memory allocation for a portion of the 
+    sequence. The blocks are managed by a DeviceAwareBlockAllocator, which is
+    responsible for allocating and freeing memory for the blocks.
+
+    Args:
+        block_size (int): The maximum number of tokens that can be stored in a
+            single block.
+        block_allocator (DeviceAwareBlockAllocator): The block allocator used to
+            manage memory for the blocks.
+        _blocks (Optional[List[Block]], optional): An optional list of existing
+            blocks to initialize the BlockTable with. If not provided, an empty
+            BlockTable is created.
+        max_block_sliding_window (Optional[int], optional): The number of
+            blocks to keep around for each sequance. If None, all blocks
+            are kept (eg., when sliding window is not used).
+            It should at least fit the sliding window size of the model.
+
+    Attributes:
+        _block_size (int): The maximum number of tokens that can be stored in a
+            single block.
+        _allocator (DeviceAwareBlockAllocator): The block allocator used to
+            manage memory for the blocks.
+        _blocks (Optional[List[Block]]): The list of blocks managed by this
+            BlockTable.
+        _num_full_slots (int): The number of tokens currently stored in the
+            blocks.
+    """
+
+    def __init__(
+        self,
+        block_size: int,
+        block_allocator: DeviceAwareBlockAllocator,
+        _blocks: Optional[List[Block]] = None,
+        max_block_sliding_window: Optional[int] = None,
+    ):
+        self._block_size = block_size
+        self._allocator = block_allocator
+        if _blocks is None:
+            _blocks = []
+        self._blocks: BlockList = BlockList(_blocks)
+
+        self._max_block_sliding_window = max_block_sliding_window
+        self._num_full_slots = self._get_num_token_ids()
+
+    @staticmethod
+    def get_num_required_blocks(token_ids: List[int],
+                                block_size: int,
+                                num_lookahead_slots: int = 0) -> int:
+        """Calculates the minimum number of blocks required to store a given
+        sequence of token IDs along with any look-ahead slots that may be
+        required (like in multi-step + chunked-prefill).
+
+        This assumes worst-case scenario, where every block requires a new
+        allocation (e.g. ignoring prefix caching).
+
+        Args:
+            token_ids (List[int]): The sequence of token IDs to be stored.
+            block_size (int): The maximum number of tokens that can be stored in
+                a single block.
+            num_lookahead_slots (int): look-ahead slots that the sequence may
+                require.
+
+        Returns:
+            int: The minimum number of blocks required to store the given
+                sequence of token IDs along with any required look-ahead slots.
+        """
+        return cdiv(len(token_ids) + num_lookahead_slots, block_size)
+
+    def allocate(self,
+                 token_ids: List[int],
+                 device: Device = Device.GPU) -> None:
+        """Allocates memory blocks for storing the given sequence of token IDs.
+
+        This method allocates the required number of blocks to store the given
+        sequence of token IDs.
+
+        Args:
+            token_ids (List[int]): The sequence of token IDs to be stored.
+            device (Device, optional): The device on which the blocks should be
+                allocated. Defaults to Device.GPU.
+        """
+        assert not self._is_allocated
+        assert token_ids
+        blocks = self._allocate_blocks_for_token_ids(prev_block=None,
+                                                     token_ids=token_ids,
+                                                     device=device)
+        self.update(blocks)
+        self._num_full_slots = len(token_ids)
+
+    def update(self, blocks: List[Block]) -> None:
+        """Resets the table to the newly provided blocks 
+        (with their corresponding block ids)
+        """
+        self._blocks.update(blocks)
+
+    def append_token_ids(self,
+                         token_ids: List[int],
+                         num_lookahead_slots: int = 0,
+                         num_computed_slots: Optional[int] = None) -> None:
+        """Appends a sequence of token IDs to the existing blocks in the
+        BlockTable.
+
+        This method appends the given sequence of token IDs to the existing
+        blocks in the BlockTable. If there is not enough space in the existing
+        blocks, new blocks are allocated using the `ensure_num_empty_slots`
+        method to accommodate the additional tokens.
+
+        The token IDs are divided into chunks of size `block_size` (except for
+        the first chunk, which may be smaller), and each chunk is appended to a
+        separate block.
+
+        Args:
+            token_ids (List[int]): The sequence of token IDs to be appended.
+            num_computed_slots (Optional[int]): The number of KV cache slots
+                that are already filled (computed).
+                When sliding window is enabled, this is used to compute how many
+                blocks to drop at the front of the sequence.
+                Without sliding window, None can be passed.
+                Without chunked prefill, it should be the same as
+                _num_full_slots.
+        """
+        assert self._is_allocated, "no blocks have been allocated"
+        assert len(self._blocks) > 0
+
+        # Drop blocks that are no longer needed due to sliding window
+        if self._max_block_sliding_window is not None:
+            null_block = self._allocator.allocate_or_get_null_block()
+            assert num_computed_slots is not None
+            end_block_idx = (num_computed_slots //
+                             self._block_size) - self._max_block_sliding_window
+            for idx in range(0, end_block_idx):
+                b = self._blocks[idx]
+                if b is not null_block:
+                    self._allocator.free(b)
+                    self._blocks[idx] = null_block
+
+        # Ensure there are enough empty slots for the new tokens plus
+        # lookahead slots
+        self.ensure_num_empty_slots(num_empty_slots=len(token_ids) +
+                                    num_lookahead_slots)
+
+        # Update the blocks with the new tokens
+        first_block_idx = self._num_full_slots // self._block_size
+        token_blocks = self._chunk_token_blocks_for_append(token_ids)
+
+        for i, token_block in enumerate(token_blocks):
+            self._blocks.append_token_ids(first_block_idx + i, token_block)
+
+        self._num_full_slots += len(token_ids)
+
+    def ensure_num_empty_slots(self, num_empty_slots: int) -> None:
+        """Ensures that the BlockTable has at least the specified number of
+        empty slots available.
+
+        This method checks if the BlockTable has enough empty slots (i.e.,
+        available space) to accommodate the requested number of tokens. If not,
+        it allocates additional blocks on the GPU to ensure that the required
+        number of empty slots is available.
+
+        Args:
+            num_empty_slots (int): The minimum number of empty slots required.
+        """
+        # Currently the block table only supports
+        # appending tokens to GPU blocks.
+        device = Device.GPU
+        assert self._is_allocated
+
+        if self._num_empty_slots >= num_empty_slots:
+            return
+
+        slots_to_allocate = num_empty_slots - self._num_empty_slots
+        blocks_to_allocate = cdiv(slots_to_allocate, self._block_size)
+
+        for _ in range(blocks_to_allocate):
+            assert len(self._blocks) > 0
+            self._blocks.append(
+                self._allocator.allocate_mutable_block(
+                    prev_block=self._blocks[-1], device=device))
+
+    def fork(self) -> "BlockTable":
+        """Creates a new BlockTable instance with a copy of the blocks from the
+        current instance.
+
+        This method creates a new BlockTable instance with the same block size,
+        block allocator, and a copy of the blocks from the current instance. The
+        new BlockTable has its own independent set of blocks, but shares the
+        same underlying memory allocation with the original BlockTable.
+
+        Returns:
+            BlockTable: A new BlockTable instance with a copy of the blocks from
+                the current instance.
+        """
+        assert self._is_allocated
+        assert len(self._blocks) > 0
+        forked_blocks = self._allocator.fork(self._blocks[-1])
+        return BlockTable(
+            block_size=self._block_size,
+            block_allocator=self._allocator,
+            _blocks=forked_blocks,
+            max_block_sliding_window=self._max_block_sliding_window,
+        )
+
+    def free(self) -> None:
+        """Frees the memory occupied by the blocks in the BlockTable.
+
+        This method iterates over all the blocks in the `_blocks` list and calls
+        the `free` method of the `_allocator` object to release the memory
+        occupied by each block. After freeing all the blocks, the `_blocks` list
+        is set to `None`.
+        """
+        for block in self.blocks:
+            self._allocator.free(block)
+        self._blocks.reset()
+
+    @property
+    def physical_block_ids(self) -> List[int]:
+        """Returns a list of physical block indices for the blocks in the
+        BlockTable.
+
+        This property returns a list of integers, where each integer represents
+        the physical block index of a corresponding block in the `_blocks` list.
+        The physical block index is a unique identifier for the memory location
+        occupied by the block.
+
+        Returns:
+            List[int]: A list of physical block indices for the blocks in the
+                BlockTable.
+        """
+        return self._blocks.ids()
+
+    def get_unseen_token_ids(self, sequence_token_ids: List[int]) -> List[int]:
+        """Get the number of "unseen" tokens in the sequence.
+
+        Unseen tokens are tokens in the sequence corresponding to this block
+        table, but are not yet appended to this block table.
+
+        Args:
+            sequence_token_ids (List[int]): The list of token ids in the
+                sequence.
+
+        Returns:
+            List[int]: The postfix of sequence_token_ids that has not yet been
+                appended to the block table.
+        """
+
+        # Since the block table is append-only, the unseen token ids are the
+        # ones after the appended ones.
+        return sequence_token_ids[self.num_full_slots:]
+
+    def _allocate_blocks_for_token_ids(self, prev_block: Optional[Block],
+                                       token_ids: List[int],
+                                       device: Device) -> List[Block]:
+        blocks: List[Block] = []
+
+        block_token_ids = []
+        tail_token_ids = []
+        for cur_token_ids in chunk_list(token_ids, self._block_size):
+            if len(cur_token_ids) == self._block_size:
+                block_token_ids.append(cur_token_ids)
+            else:
+                tail_token_ids.append(cur_token_ids)
+
+        if block_token_ids:
+            blocks.extend(
+                self._allocator.allocate_immutable_blocks(
+                    prev_block, block_token_ids=block_token_ids,
+                    device=device))
+            prev_block = blocks[-1]
+
+        if tail_token_ids:
+            assert len(tail_token_ids) == 1
+            cur_token_ids = tail_token_ids[0]
+
+            block = self._allocator.allocate_mutable_block(
+                prev_block=prev_block, device=device)
+            block.append_token_ids(cur_token_ids)
+
+            blocks.append(block)
+
+        return blocks
+
+    def _get_all_token_ids(self) -> List[int]:
+        # NOTE: This function is O(seq_len); use sparingly.
+        token_ids: List[int] = []
+
+        if not self._is_allocated:
+            return token_ids
+
+        for block in self.blocks:
+            token_ids.extend(block.token_ids)
+
+        return token_ids
+
+    def _get_num_token_ids(self) -> int:
+        res = 0
+        for block in self.blocks:
+            res += len(block.token_ids)
+
+        return res
+
+    @property
+    def _is_allocated(self) -> bool:
+        return len(self._blocks) > 0
+
+    @property
+    def blocks(self) -> List[Block]:
+        return self._blocks.list()
+
+    @property
+    def _num_empty_slots(self) -> int:
+        assert self._is_allocated
+        return len(self._blocks) * self._block_size - self._num_full_slots
+
+    @property
+    def num_full_slots(self) -> int:
+        """Returns the total number of tokens currently stored in the
+        BlockTable.
+
+        Returns:
+            int: The total number of tokens currently stored in the BlockTable.
+        """
+        return self._num_full_slots
+
+    def get_num_blocks_touched_by_append_slots(
+            self, token_ids: List[int], num_lookahead_slots: int) -> int:
+        """Determine how many blocks will be "touched" by appending the token
+        ids.
+
+        This is required for the scheduler to determine whether a sequence can
+        continue generation, or if it must be preempted.
+        """
+        # Math below is equivalent to:
+        # all_token_ids = token_ids + [-1] * num_lookahead_slots
+        # token_blocks = self._chunk_token_blocks_for_append(all_token_ids)
+        # return len(token_blocks)
+
+        num_token_ids = len(token_ids) + num_lookahead_slots
+        first_chunk_size = self._block_size - (self._num_full_slots %
+                                               self._block_size)
+        num_token_blocks = (1 + math.ceil(
+            (num_token_ids - first_chunk_size) / self._block_size))
+        return num_token_blocks
+
+    def _chunk_token_blocks_for_append(
+            self, token_ids: List[int]) -> List[List[int]]:
+        """Split the token ids into block-sized chunks so they can be easily
+        appended to blocks. The first such "token block" may have less token ids
+        than the block size, since the last allocated block may be partially
+        full.
+
+        If no token ids are provided, then no chunks are returned.
+        """
+
+        if not token_ids:
+            return []
+
+        first_chunk_size = self._block_size - (self._num_full_slots %
+                                               self._block_size)
+        token_blocks = [token_ids[:first_chunk_size]]
+        token_blocks.extend(
+            chunk_list(token_ids[first_chunk_size:], self._block_size))
+        return token_blocks

vllm/core/block/common.py

@@ -0,0 +1,360 @@
+from collections import deque
+from dataclasses import dataclass
+from typing import Deque, Dict, Iterable, List, Optional, Protocol, Tuple
+
+from vllm.core.block.interfaces import Block, BlockAllocator
+
+BlockId = int
+RefCount = int
+
+
+class RefCounterProtocol(Protocol):
+
+    def incr(self, block_id: BlockId) -> RefCount:
+        raise NotImplementedError
+
+    def decr(self, block_id: BlockId) -> RefCount:
+        raise NotImplementedError
+
+    def get(self, block_id: BlockId) -> RefCount:
+        raise NotImplementedError
+
+
+class RefCounter(RefCounterProtocol):
+    """A class for managing reference counts for a set of block indices.
+
+    The RefCounter class maintains a dictionary that maps block indices to their
+    corresponding reference counts. It provides methods to increment, decrement,
+    and retrieve the reference count for a given block index.
+
+    Args:
+        all_block_indices (Iterable[BlockId]): An iterable of block indices
+            to initialize the reference counter with.
+    """
+
+    def __init__(self, all_block_indices: Iterable[BlockId]):
+        deduped = set(all_block_indices)
+        self._refcounts: Dict[BlockId,
+                              RefCount] = {index: 0
+                                           for index in deduped}
+
+    def incr(self, block_id: BlockId) -> RefCount:
+        assert block_id in self._refcounts
+        pre_incr_refcount = self._refcounts[block_id]
+
+        assert pre_incr_refcount >= 0
+
+        post_incr_refcount = pre_incr_refcount + 1
+        self._refcounts[block_id] = post_incr_refcount
+        return post_incr_refcount
+
+    def decr(self, block_id: BlockId) -> RefCount:
+        assert block_id in self._refcounts
+        refcount = self._refcounts[block_id]
+
+        assert refcount > 0
+        refcount -= 1
+
+        self._refcounts[block_id] = refcount
+
+        return refcount
+
+    def get(self, block_id: BlockId) -> RefCount:
+        assert block_id in self._refcounts
+        return self._refcounts[block_id]
+
+    def as_readonly(self) -> "ReadOnlyRefCounter":
+        return ReadOnlyRefCounter(self)
+
+
+class ReadOnlyRefCounter(RefCounterProtocol):
+    """A read-only view of the RefCounter class.
+
+    The ReadOnlyRefCounter class provides a read-only interface to access the
+    reference counts maintained by a RefCounter instance. It does not allow
+    modifications to the reference counts.
+
+    Args:
+        refcounter (RefCounter): The RefCounter instance to create a read-only
+            view for.
+    """
+
+    def __init__(self, refcounter: RefCounter):
+        self._refcounter = refcounter
+
+    def incr(self, block_id: BlockId) -> RefCount:
+        raise ValueError("Incr not allowed")
+
+    def decr(self, block_id: BlockId) -> RefCount:
+        raise ValueError("Decr not allowed")
+
+    def get(self, block_id: BlockId) -> RefCount:
+        return self._refcounter.get(block_id)
+
+
+class CopyOnWriteTracker:
+    """A class for tracking and managing copy-on-write operations for blocks.
+
+    The CopyOnWriteTracker class maintains a mapping of source block indices to
+        their corresponding copy-on-write destination block indices. It works in
+        conjunction with a RefCounter.
+
+    Args:
+        refcounter (RefCounter): The reference counter used to track block
+            reference counts.
+    """
+
+    def __init__(self, refcounter: RefCounterProtocol):
+        self._copy_on_writes: List[Tuple[BlockId, BlockId]] = []
+        self._refcounter = refcounter
+
+    def is_appendable(self, block: Block) -> bool:
+        """Checks if the block is shared or not. If shared, then it cannot
+        be appended and needs to be duplicated via copy-on-write
+        """
+        block_id = block.block_id
+        if block_id is None:
+            return True
+
+        refcount = self._refcounter.get(block_id)
+        return refcount <= 1
+
+    def record_cow(self, src_block_id: Optional[BlockId],
+                   trg_block_id: Optional[BlockId]) -> None:
+        """Records a copy-on-write operation from source to target block id
+        Args:
+            src_block_id (BlockId): The source block id from which to copy 
+                the data
+            trg_block_id (BlockId): The target block id to which the data
+                is copied
+        """
+        assert src_block_id is not None
+        assert trg_block_id is not None
+        self._copy_on_writes.append((src_block_id, trg_block_id))
+
+    def clear_cows(self) -> List[Tuple[BlockId, BlockId]]:
+        """Clears the copy-on-write tracking information and returns the current
+        state.
+
+        This method returns a list mapping source block indices to
+         destination block indices for the current copy-on-write operations.
+        It then clears the internal tracking information.
+
+        Returns:
+            List[Tuple[BlockId, BlockId]]: A list mapping source
+                block indices to destination block indices for the
+                current copy-on-write operations.
+        """
+        cows = self._copy_on_writes
+        self._copy_on_writes = []
+        return cows
+
+
+class BlockPool:
+    """Used to pre-allocate block objects, in order to avoid excessive python
+    object allocations/deallocations.
+    The pool starts from "pool_size" objects and will increase to more objects
+    if necessary
+
+    Note that multiple block objects may point to the same physical block id,
+    which is why this pool is needed, so that it will be easier to support
+    prefix caching and more complicated sharing of physical blocks.
+    """
+
+    def __init__(self, block_size: int, create_block: Block.Factory,
+                 allocator: BlockAllocator, pool_size: int):
+        self._block_size = block_size
+        self._create_block = create_block
+        self._allocator = allocator
+        self._pool_size = pool_size
+        assert self._pool_size >= 0
+
+        self._free_ids: Deque[int] = deque(range(self._pool_size))
+        self._pool = []
+        for i in range(self._pool_size):
+            self._pool.append(
+                self._create_block(prev_block=None,
+                                   token_ids=[],
+                                   block_size=self._block_size,
+                                   allocator=self._allocator,
+                                   block_id=None))
+
+    def increase_pool(self):
+        """Doubles the internal pool size
+        """
+        cur_pool_size = self._pool_size
+        new_pool_size = cur_pool_size * 2
+        self._pool_size = new_pool_size
+
+        self._free_ids += deque(range(cur_pool_size, new_pool_size))
+
+        for i in range(cur_pool_size, new_pool_size):
+            self._pool.append(
+                self._create_block(prev_block=None,
+                                   token_ids=[],
+                                   block_size=self._block_size,
+                                   allocator=self._allocator,
+                                   block_id=None))
+
+    def init_block(self, prev_block: Optional[Block], token_ids: List[int],
+                   block_size: int, physical_block_id: Optional[int]) -> Block:
+        if len(self._free_ids) == 0:
+            self.increase_pool()
+            assert len(self._free_ids) > 0
+
+        pool_id = self._free_ids.popleft()
+
+        block = self._pool[pool_id]
+        block.__init__(  # type: ignore[misc]
+            prev_block=prev_block,
+            token_ids=token_ids,
+            block_size=block_size,
+            allocator=block._allocator,  # type: ignore[attr-defined] 
+            block_id=physical_block_id)
+        block.pool_id = pool_id  # type: ignore[attr-defined]
+        return block
+
+    def free_block(self, block: Block) -> None:
+        self._free_ids.appendleft(block.pool_id)  # type: ignore[attr-defined]
+
+
+class BlockList:
+    """This class is an optimization to allow fast-access to physical 
+    block ids. It maintains a block id list that is updated with the 
+    block list and this avoids the need to reconstruct the block id 
+    list on every iteration of the block manager
+    """
+
+    def __init__(self, blocks: List[Block]):
+        self._blocks: List[Block] = []
+        self._block_ids: List[int] = []
+
+        self.update(blocks)
+
+    def _add_block_id(self, block_id: Optional[BlockId]) -> None:
+        assert block_id is not None
+        self._block_ids.append(block_id)
+
+    def _update_block_id(self, block_index: int,
+                         new_block_id: Optional[BlockId]) -> None:
+        assert new_block_id is not None
+        self._block_ids[block_index] = new_block_id
+
+    def update(self, blocks: List[Block]):
+        self._blocks = blocks
+
+        # Cache block ids for fast query
+        self._block_ids = []
+        for block in self._blocks:
+            self._add_block_id(block.block_id)
+
+    def append_token_ids(self, block_index: int, token_ids: List[int]) -> None:
+        block = self._blocks[block_index]
+        prev_block_id = block.block_id
+
+        block.append_token_ids(token_ids)
+
+        # CoW or promotion may update the internal block_id
+        if prev_block_id != block.block_id:
+            self._update_block_id(block_index, block.block_id)
+
+    def append(self, new_block: Block):
+        self._blocks.append(new_block)
+        self._add_block_id(new_block.block_id)
+
+    def __len__(self) -> int:
+        return len(self._blocks)
+
+    def __getitem__(self, block_index: int) -> Block:
+        return self._blocks[block_index]
+
+    def __setitem__(self, block_index: int, new_block: Block) -> None:
+        self._blocks[block_index] = new_block
+        self._update_block_id(block_index, new_block.block_id)
+
+    def reset(self):
+        self._blocks = []
+        self._block_ids = []
+
+    def list(self) -> List[Block]:
+        return self._blocks
+
+    def ids(self) -> List[int]:
+        return self._block_ids
+
+
+@dataclass
+class CacheMetricData:
+    """A utility dataclass to maintain cache metric.
+    To avoid overflow, we maintain the hit rate in block granularity, so that
+    we can maintain a single hit rate for n_completed_block x block_size,
+    and calculate the real time hit rate by the following:
+    BS = The number of queries per block.
+    nB = The number of completed blocks.
+    HR = hit rate of (nB x BS) queries.
+    Q = current number of queries (< BS).
+    H = current number of hits (< BS).
+    hit rate = ((HR x nB) + (H / Q) x (Q / BS)) / (nB + Q / BS)
+    """
+    num_completed_blocks: int = 0
+    completed_block_cache_hit_rate: float = 0.0
+    num_incompleted_block_queries: int = 0
+    num_incompleted_block_hit: int = 0
+    block_size: int = 1000
+
+    def query(self, hit: bool):
+        self.num_incompleted_block_queries += 1
+        self.num_incompleted_block_hit += 1 if hit else 0
+
+        # When a block is completed, update the cache hit rate
+        # and reset the incomplete numbers.
+        if self.num_incompleted_block_queries == self.block_size:
+            hit_rate = (self.num_incompleted_block_hit /
+                        self.num_incompleted_block_queries)
+            self.completed_block_cache_hit_rate = (
+                self.completed_block_cache_hit_rate * self.num_completed_blocks
+                + hit_rate) / (self.num_completed_blocks + 1)
+            self.num_incompleted_block_queries = 0
+            self.num_incompleted_block_hit = 0
+            self.num_completed_blocks += 1
+
+    def get_hit_rate(self):
+        incomplete_ratio = self.num_incompleted_block_queries / self.block_size
+        total_blocks = self.num_completed_blocks + incomplete_ratio
+        if total_blocks == 0:
+            return 0.0
+
+        completed_block_hit, incompleted_block_hit = 0.0, 0.0
+        if self.num_completed_blocks > 0:
+            completed_block_hit = (self.completed_block_cache_hit_rate *
+                                   self.num_completed_blocks)
+        if self.num_incompleted_block_queries > 0:
+            incompleted_hit_rate = (self.num_incompleted_block_hit /
+                                    self.num_incompleted_block_queries)
+            incompleted_block_hit = (incompleted_hit_rate * incomplete_ratio)
+        return (completed_block_hit + incompleted_block_hit) / total_blocks
+
+
+def get_all_blocks_recursively(last_block: Block) -> List[Block]:
+    """Retrieves all the blocks in a sequence starting from the last block.
+
+    This function recursively traverses the sequence of blocks in reverse order,
+    starting from the given last block, and returns a list of all the blocks in
+    the sequence.
+
+    Args:
+        last_block (Block): The last block in the sequence.
+
+    Returns:
+        List[Block]: A list of all the blocks in the sequence, in the order they
+            appear.
+    """
+
+    def recurse(block: Block, lst: List[Block]) -> None:
+        if block.prev_block is not None:
+            recurse(block.prev_block, lst)
+        lst.append(block)
+
+    all_blocks: List[Block] = []
+    recurse(last_block, all_blocks)
+    return all_blocks

vllm/core/block/cpu_gpu_block_allocator.py

@@ -0,0 +1,404 @@
+from typing import Dict, FrozenSet, List, Optional, Tuple
+
+from vllm.core.block.interfaces import (Block, BlockAllocator, BlockId,
+                                        DeviceAwareBlockAllocator)
+from vllm.core.block.naive_block import NaiveBlock, NaiveBlockAllocator
+from vllm.core.block.prefix_caching_block import PrefixCachingBlockAllocator
+from vllm.utils import Device
+
+
+class CpuGpuBlockAllocator(DeviceAwareBlockAllocator):
+    """A block allocator that can allocate blocks on both CPU and GPU memory.
+
+    This class implements the `DeviceAwareBlockAllocator` interface and provides
+    functionality for allocating and managing blocks of memory on both CPU and
+    GPU devices.
+
+    The `CpuGpuBlockAllocator` maintains separate memory pools for CPU and GPU
+    blocks, and allows for allocation, deallocation, forking, and swapping of
+    blocks across these memory pools.
+    """
+
+    @staticmethod
+    def create(
+        allocator_type: str,
+        num_gpu_blocks: int,
+        num_cpu_blocks: int,
+        block_size: int,
+    ) -> DeviceAwareBlockAllocator:
+        """Creates a CpuGpuBlockAllocator instance with the specified
+        configuration.
+
+        This static method creates and returns a CpuGpuBlockAllocator instance
+        based on the provided parameters. It initializes the CPU and GPU block
+        allocators with the specified number of blocks, block size, and
+        allocator type.
+
+        Args:
+            allocator_type (str): The type of block allocator to use for CPU
+                and GPU blocks. Currently supported values are "naive" and
+                "prefix_caching".
+            num_gpu_blocks (int): The number of blocks to allocate for GPU
+                memory.
+            num_cpu_blocks (int): The number of blocks to allocate for CPU
+                memory.
+            block_size (int): The size of each block in number of tokens.
+
+        Returns:
+            DeviceAwareBlockAllocator: A CpuGpuBlockAllocator instance with the
+                specified configuration.
+
+        Notes:
+            - The block IDs are assigned contiguously, with GPU block IDs coming
+                before CPU block IDs.
+        """
+        block_ids = list(range(num_gpu_blocks + num_cpu_blocks))
+        gpu_block_ids = block_ids[:num_gpu_blocks]
+        cpu_block_ids = block_ids[num_gpu_blocks:]
+
+        if allocator_type == "naive":
+            gpu_allocator: BlockAllocator = NaiveBlockAllocator(
+                create_block=NaiveBlock,  # type: ignore
+                num_blocks=num_gpu_blocks,
+                block_size=block_size,
+                block_ids=gpu_block_ids,
+            )
+
+            cpu_allocator: BlockAllocator = NaiveBlockAllocator(
+                create_block=NaiveBlock,  # type: ignore
+                num_blocks=num_cpu_blocks,
+                block_size=block_size,
+                block_ids=cpu_block_ids,
+            )
+        elif allocator_type == "prefix_caching":
+            gpu_allocator = PrefixCachingBlockAllocator(
+                num_blocks=num_gpu_blocks,
+                block_size=block_size,
+                block_ids=gpu_block_ids,
+            )
+
+            cpu_allocator = PrefixCachingBlockAllocator(
+                num_blocks=num_cpu_blocks,
+                block_size=block_size,
+                block_ids=cpu_block_ids,
+            )
+        else:
+            raise ValueError(f"Unknown allocator type {allocator_type=}")
+
+        return CpuGpuBlockAllocator(
+            cpu_block_allocator=cpu_allocator,
+            gpu_block_allocator=gpu_allocator,
+        )
+
+    def __init__(self, cpu_block_allocator: BlockAllocator,
+                 gpu_block_allocator: BlockAllocator):
+        assert not (
+            cpu_block_allocator.all_block_ids
+            & gpu_block_allocator.all_block_ids
+        ), "cpu and gpu block allocators can't have intersection of block ids"
+
+        self._allocators = {
+            Device.CPU: cpu_block_allocator,
+            Device.GPU: gpu_block_allocator,
+        }
+
+        self._swap_mapping: Dict[int, int] = {}
+        self._null_block: Optional[Block] = None
+
+        self._block_ids_to_allocator: Dict[int, BlockAllocator] = {}
+        for _, allocator in self._allocators.items():
+            for block_id in allocator.all_block_ids:
+                self._block_ids_to_allocator[block_id] = allocator
+
+    def allocate_or_get_null_block(self) -> Block:
+        if self._null_block is None:
+            self._null_block = NullBlock(
+                self.allocate_mutable_block(None, Device.GPU))
+        return self._null_block
+
+    def allocate_mutable_block(self, prev_block: Optional[Block],
+                               device: Device) -> Block:
+        """Allocates a new mutable block on the specified device.
+
+        Args:
+            prev_block (Optional[Block]): The previous block to in the sequence.
+                Used for prefix hashing.
+            device (Device): The device on which to allocate the new block.
+
+        Returns:
+            Block: The newly allocated mutable block.
+        """
+        return self._allocators[device].allocate_mutable_block(prev_block)
+
+    def allocate_immutable_blocks(self, prev_block: Optional[Block],
+                                  block_token_ids: List[List[int]],
+                                  device: Device) -> List[Block]:
+        """Allocates a new group of immutable blocks with the provided block 
+        token IDs on the specified device.
+
+        Args:
+            prev_block (Optional[Block]): The previous block in the sequence.
+                Used for prefix hashing.
+            block_token_ids (List[int]): The list of block token IDs to be 
+                stored in the new blocks.
+            device (Device): The device on which to allocate the new block.
+
+        Returns:
+            List[Block]: The newly allocated list of immutable blocks 
+                containing the provided block token IDs.
+        """
+        return self._allocators[device].allocate_immutable_blocks(
+            prev_block, block_token_ids)
+
+    def allocate_immutable_block(self, prev_block: Optional[Block],
+                                 token_ids: List[int],
+                                 device: Device) -> Block:
+        """Allocates a new immutable block with the provided token IDs on the
+        specified device.
+
+        Args:
+            prev_block (Optional[Block]): The previous block in the sequence.
+                Used for prefix hashing.
+            token_ids (List[int]): The list of token IDs to be stored in the new
+                block.
+            device (Device): The device on which to allocate the new block.
+
+        Returns:
+            Block: The newly allocated immutable block containing the provided
+                token IDs.
+        """
+        return self._allocators[device].allocate_immutable_block(
+            prev_block, token_ids)
+
+    def free(self, block: Block) -> None:
+        """Frees the memory occupied by the given block.
+
+        Args:
+            block (Block): The block to be freed.
+        """
+        # Null block should never be freed
+        if isinstance(block, NullBlock):
+            return
+        block_id = block.block_id
+        assert block_id is not None
+        allocator = self._block_ids_to_allocator[block_id]
+        allocator.free(block)
+
+    def fork(self, last_block: Block) -> List[Block]:
+        """Creates a new sequence of blocks that shares the same underlying
+            memory as the original sequence.
+
+        Args:
+            last_block (Block): The last block in the original sequence.
+
+        Returns:
+            List[Block]: A new list of blocks that shares the same memory as the
+                original sequence.
+        """
+        # do not attempt to fork the null block
+        assert not isinstance(last_block, NullBlock)
+        block_id = last_block.block_id
+        assert block_id is not None
+        allocator = self._block_ids_to_allocator[block_id]
+        return allocator.fork(last_block)
+
+    def get_num_free_blocks(self, device: Device) -> int:
+        """Returns the number of free blocks available on the specified device.
+
+        Args:
+            device (Device): The device for which to query the number of free
+                blocks. AssertionError is raised if None is passed.
+
+        Returns:
+            int: The number of free blocks available on the specified device.
+        """
+        return self._allocators[device].get_num_free_blocks()
+
+    def get_num_total_blocks(self, device: Device) -> int:
+        return self._allocators[device].get_num_total_blocks()
+
+    def get_physical_block_id(self, device: Device, absolute_id: int) -> int:
+        """Returns the zero-offset block id on certain device given the 
+        absolute block id.
+
+        Args:
+            device (Device): The device for which to query relative block id.
+                absolute_id (int): The absolute block id for the block in 
+                whole allocator.
+
+        Returns:
+            int: The zero-offset block id on certain device.
+        """
+        return self._allocators[device].get_physical_block_id(absolute_id)
+
+    def swap(self, blocks: List[Block], src_device: Device,
+             dst_device: Device) -> Dict[int, int]:
+        """Execute the swap for the given blocks from source_device
+        on to dest_device, save the current swap mapping and append 
+        them to the accumulated `self._swap_mapping` for each 
+        scheduling move.
+
+        Args:
+            blocks: List of blocks to be swapped.
+            src_device (Device): Device to swap the 'blocks' from.
+            dst_device (Device): Device to swap the 'blocks' to.
+        
+        Returns:
+            Dict[int, int]: Swap mapping from source_device
+                on to dest_device.
+        """
+        src_block_ids = [block.block_id for block in blocks]
+        self._allocators[src_device].swap_out(blocks)
+        self._allocators[dst_device].swap_in(blocks)
+        dst_block_ids = [block.block_id for block in blocks]
+
+        current_swap_mapping: Dict[int, int] = {}
+        for src_block_id, dst_block_id in zip(src_block_ids, dst_block_ids):
+            if src_block_id is not None and dst_block_id is not None:
+                self._swap_mapping[src_block_id] = dst_block_id
+                current_swap_mapping[src_block_id] = dst_block_id
+        return current_swap_mapping
+
+    def get_num_full_blocks_touched(self, blocks: List[Block],
+                                    device: Device) -> int:
+        """Returns the number of full blocks that will be touched by
+        swapping in/out the given blocks on to the 'device'.
+
+        Args:
+            blocks: List of blocks to be swapped.
+            device (Device): Device to swap the 'blocks' on.
+
+        Returns:
+            int: the number of full blocks that will be touched by
+                swapping in/out the given blocks on to the 'device'.
+                Non full blocks are ignored when deciding the number
+                of blocks to touch.
+        """
+        return self._allocators[device].get_num_full_blocks_touched(blocks)
+
+    def clear_copy_on_writes(self) -> List[Tuple[int, int]]:
+        """Clears the copy-on-write (CoW) state and returns the mapping of
+            source to destination block IDs.
+
+        Returns:
+            List[Tuple[int, int]]: A list mapping source block IDs to 
+                destination block IDs.
+        """
+        # CoW only supported on GPU
+        device = Device.GPU
+        return self._allocators[device].clear_copy_on_writes()
+
+    def mark_blocks_as_accessed(self, block_ids: List[int],
+                                now: float) -> None:
+        """Mark blocks as accessed, only use for prefix caching."""
+        # Prefix caching only supported on GPU.
+        device = Device.GPU
+        return self._allocators[device].mark_blocks_as_accessed(block_ids, now)
+
+    def mark_blocks_as_computed(self, block_ids: List[int]) -> None:
+        """Mark blocks as accessed, only use for prefix caching."""
+        # Prefix caching only supported on GPU.
+        device = Device.GPU
+        return self._allocators[device].mark_blocks_as_computed(block_ids)
+
+    def get_computed_block_ids(self, prev_computed_block_ids: List[int],
+                               block_ids: List[int],
+                               skip_last_block_id: bool) -> List[int]:
+        # Prefix caching only supported on GPU.
+        device = Device.GPU
+        return self._allocators[device].get_computed_block_ids(
+            prev_computed_block_ids, block_ids, skip_last_block_id)
+
+    def get_common_computed_block_ids(
+            self, computed_seq_block_ids: List[List[int]]) -> List[int]:
+        # Prefix caching only supported on GPU.
+        device = Device.GPU
+        return self._allocators[device].get_common_computed_block_ids(
+            computed_seq_block_ids)
+
+    @property
+    def all_block_ids(self) -> FrozenSet[int]:
+        return frozenset(self._block_ids_to_allocator.keys())
+
+    def get_prefix_cache_hit_rate(self, device: Device) -> float:
+        """Prefix cache hit rate. -1 means not supported or disabled."""
+        assert device in self._allocators
+        return self._allocators[device].get_prefix_cache_hit_rate()
+
+    def get_and_reset_swaps(self) -> List[Tuple[int, int]]:
+        """Returns and clears the mapping of source to destination block IDs.
+        Will be called after every swapping operations for now, and after every
+        schedule when BlockManagerV2 become default. Currently not useful.
+
+        Returns:
+            List[Tuple[int, int]]: A mapping of source to destination block IDs.
+        """
+        mapping = self._swap_mapping.copy()
+        self._swap_mapping.clear()
+        return list(mapping.items())
+
+
+class NullBlock(Block):
+    """
+    Null blocks are used as a placeholders for KV cache blocks that have
+    been dropped due to sliding window.
+    This implementation just wraps an ordinary block and prevents it from
+    being modified. It also allows for testing if a block is NullBlock
+    via isinstance().
+    """
+
+    def __init__(self, proxy: Block):
+        super().__init__()
+        self._proxy = proxy
+
+    def append_token_ids(self, token_ids: List[BlockId]):
+        raise ValueError("null block should not be modified")
+
+    @property
+    def block_id(self):
+        return self._proxy.block_id
+
+    @block_id.setter
+    def block_id(self, value: Optional[BlockId]):
+        raise ValueError("null block should not be modified")
+
+    @property
+    def token_ids(self) -> List[BlockId]:
+        return self._proxy.token_ids
+
+    @property
+    def num_tokens_total(self) -> int:
+        raise NotImplementedError(
+            "num_tokens_total is not used for null block")
+
+    @property
+    def num_empty_slots(self) -> BlockId:
+        return self._proxy.num_empty_slots
+
+    @property
+    def is_full(self):
+        return self._proxy.is_full
+
+    @property
+    def prev_block(self):
+        return self._proxy.prev_block
+
+    @property
+    def computed(self):
+        return self._proxy.computed
+
+    @computed.setter
+    def computed(self, value):
+        self._proxy.computed = value
+
+    @property
+    def last_accessed(self) -> float:
+        return self._proxy.last_accessed
+
+    @last_accessed.setter
+    def last_accessed(self, last_accessed_ts: float):
+        self._proxy.last_accessed = last_accessed_ts
+
+    @property
+    def content_hash(self):
+        return self._proxy.content_hash

vllm/core/block/interfaces.py

@@ -0,0 +1,286 @@
+from abc import ABC, abstractmethod
+from typing import Dict, FrozenSet, List, Optional, Protocol, Tuple
+
+from vllm.utils import Device
+
+BlockId = int
+
+
+class Block(ABC):
+
+    @abstractmethod
+    def append_token_ids(self, token_ids: List[int]) -> None:
+        pass
+
+    @property
+    @abstractmethod
+    def block_id(self) -> Optional[int]:
+        pass
+
+    @block_id.setter
+    @abstractmethod
+    def block_id(self, value: Optional[int]) -> None:
+        """NOTE: Do not use this API outside Block."""
+        self._block_id = value
+
+    @property
+    @abstractmethod
+    def token_ids(self) -> List[int]:
+        pass
+
+    @property
+    @abstractmethod
+    def num_tokens_total(self) -> int:
+        """The number of tokens till the current block (inclusive)
+        """
+        pass
+
+    @property
+    @abstractmethod
+    def num_empty_slots(self) -> int:
+        pass
+
+    @property
+    @abstractmethod
+    def is_full(self) -> bool:
+        pass
+
+    @property
+    @abstractmethod
+    def prev_block(self) -> Optional["Block"]:
+        pass
+
+    @property
+    @abstractmethod
+    def computed(self) -> bool:
+        raise NotImplementedError
+
+    @computed.setter
+    @abstractmethod
+    def computed(self, value) -> bool:
+        """Should be only used by PrefixCacingAllocator"""
+        raise NotImplementedError
+
+    @property
+    @abstractmethod
+    def last_accessed(self) -> float:
+        raise NotImplementedError
+
+    @last_accessed.setter
+    @abstractmethod
+    def last_accessed(self, last_accessed_ts: float):
+        raise NotImplementedError
+
+    class Factory(Protocol):
+
+        @abstractmethod
+        def __call__(
+            self,
+            prev_block: Optional["Block"],
+            token_ids: List[int],
+            block_size: int,
+            allocator: "BlockAllocator",
+            block_id: Optional[int] = None,
+        ) -> "Block":
+            pass
+
+    @property
+    @abstractmethod
+    def content_hash(self) -> Optional[int]:
+        """Return the content-based hash of the current block, or None if it is
+        not yet defined or not supported.
+
+        For the content-based hash to be defined, the current block must be
+        full.
+        """
+        return None
+
+
+class BlockAllocator(ABC):
+
+    @abstractmethod
+    def allocate_mutable_block(self, prev_block: Optional[Block]) -> Block:
+        pass
+
+    @abstractmethod
+    def allocate_immutable_block(self, prev_block: Optional[Block],
+                                 token_ids: List[int]) -> Block:
+        pass
+
+    @abstractmethod
+    def allocate_immutable_blocks(
+            self, prev_block: Optional[Block],
+            block_token_ids: List[List[int]]) -> List[Block]:
+        pass
+
+    @abstractmethod
+    def free(self, block: Block) -> None:
+        pass
+
+    @abstractmethod
+    def fork(self, last_block: Block) -> List[Block]:
+        pass
+
+    @abstractmethod
+    def get_num_total_blocks(self) -> int:
+        pass
+
+    @abstractmethod
+    def get_num_free_blocks(self) -> int:
+        pass
+
+    @abstractmethod
+    def get_physical_block_id(self, absolute_id: int) -> int:
+        pass
+
+    @abstractmethod
+    def swap_out(self, blocks: List[Block]) -> None:
+        pass
+
+    @abstractmethod
+    def swap_in(self, blocks: List[Block]) -> None:
+        pass
+
+    @property
+    @abstractmethod
+    def all_block_ids(self) -> FrozenSet[int]:
+        pass
+
+    @abstractmethod
+    def clear_copy_on_writes(self) -> List[Tuple[int, int]]:
+        pass
+
+    @abstractmethod
+    def mark_blocks_as_accessed(self, block_ids: List[int],
+                                now: float) -> None:
+        pass
+
+    @abstractmethod
+    def mark_blocks_as_computed(self, block_ids: List[int]) -> None:
+        pass
+
+    @abstractmethod
+    def get_computed_block_ids(self, prev_computed_block_ids: List[int],
+                               block_ids: List[int],
+                               skip_last_block_id: bool) -> List[int]:
+        pass
+
+    @abstractmethod
+    def get_common_computed_block_ids(
+            self, computed_seq_block_ids: List[List[int]]) -> List[int]:
+        pass
+
+    @abstractmethod
+    def cow_block_if_not_appendable(self, block: Block) -> BlockId:
+        """NOTE: This should not be used besides Block"""
+        pass
+
+    @abstractmethod
+    def promote_to_immutable_block(self, block: Block) -> BlockId:
+        """NOTE: This should not be used besides Block"""
+        pass
+
+    @abstractmethod
+    def get_num_full_blocks_touched(self, blocks: List[Block]) -> int:
+        pass
+
+    @abstractmethod
+    def get_prefix_cache_hit_rate(self) -> float:
+        """Prefix cache hit rate. -1 means not supported or disabled."""
+        pass
+
+    class NoFreeBlocksError(ValueError):
+        pass
+
+
+class DeviceAwareBlockAllocator(ABC):
+
+    @abstractmethod
+    def allocate_mutable_block(self, prev_block: Optional[Block],
+                               device: Device) -> Block:
+        pass
+
+    @abstractmethod
+    def allocate_immutable_block(self, prev_block: Optional[Block],
+                                 token_ids: List[int],
+                                 device: Device) -> Block:
+        pass
+
+    @abstractmethod
+    def allocate_immutable_blocks(self, prev_block: Optional[Block],
+                                  block_token_ids: List[List[int]],
+                                  device: Device) -> List[Block]:
+        pass
+
+    @abstractmethod
+    def get_num_free_blocks(self, device: Device) -> int:
+        pass
+
+    @abstractmethod
+    def get_num_total_blocks(self, device: Device) -> int:
+        pass
+
+    @abstractmethod
+    def free(self, block: Block) -> None:
+        pass
+
+    @abstractmethod
+    def fork(self, last_block: Block) -> List[Block]:
+        pass
+
+    @property
+    @abstractmethod
+    def all_block_ids(self) -> FrozenSet[int]:
+        pass
+
+    @abstractmethod
+    def clear_copy_on_writes(self) -> List[Tuple[int, int]]:
+        pass
+
+    @abstractmethod
+    def mark_blocks_as_accessed(self, block_ids: List[int],
+                                now: float) -> None:
+        pass
+
+    @abstractmethod
+    def mark_blocks_as_computed(self, block_ids: List[int]) -> None:
+        pass
+
+    @abstractmethod
+    def get_computed_block_ids(self, prev_computed_block_ids: List[int],
+                               block_ids: List[int],
+                               skip_last_block_id: bool) -> List[int]:
+        pass
+
+    @abstractmethod
+    def get_common_computed_block_ids(
+            self, computed_seq_block_ids: List[List[int]]) -> List[int]:
+        pass
+
+    @abstractmethod
+    def get_num_full_blocks_touched(self, blocks: List[Block],
+                                    device: Device) -> int:
+        pass
+
+    @abstractmethod
+    def swap(self, blocks: List[Block], src_device: Device,
+             dst_device: Device) -> Dict[int, int]:
+        pass
+
+    @abstractmethod
+    def get_physical_block_id(self, device: Device, absolute_id: int) -> int:
+        pass
+
+    @abstractmethod
+    def allocate_or_get_null_block(self) -> Block:
+        """
+        Null blocks are used as a placeholders for KV cache blocks that have
+        been dropped due to sliding window.
+        There is at most one null block per allocator.
+        """
+        pass
+
+    @abstractmethod
+    def get_prefix_cache_hit_rate(self, device: Device) -> float:
+        """Prefix cache hit rate. -1 means not supported or disabled."""
+        pass

vllm/core/block/naive_block.py

@@ -0,0 +1,449 @@
+from collections import deque
+from typing import Deque, FrozenSet, Iterable, List, Optional, Tuple
+
+from vllm.core.block.common import (BlockPool, CopyOnWriteTracker, RefCounter,
+                                    get_all_blocks_recursively)
+from vllm.core.block.interfaces import Block, BlockAllocator, BlockId, Device
+
+Refcount = int
+
+
+class NaiveBlockAllocator(BlockAllocator):
+    """A simple block allocator that manages blocks of memory without prefix
+    caching.
+
+    Args:
+        create_block (Block.Factory): A factory function for creating new
+            blocks. This is used when a NaiveBlockAllocator is composed within
+            a prefix caching allocator -- the naive block allocator must
+            construct prefix caching blocks (but shouldn't know anything else
+            about them).
+        num_blocks (int): The total number of blocks to manage.
+        block_size (int): The size of each block in tokens.
+        block_ids (Optional[Iterable[int]], optional): An optional iterable of
+            block IDs. If not provided, block IDs will be assigned sequentially
+            from 0 to num_blocks - 1.
+    """
+
+    def __init__(
+        self,
+        create_block: Block.Factory,
+        num_blocks: int,
+        block_size: int,
+        block_ids: Optional[Iterable[int]] = None,
+        block_pool: Optional[BlockPool] = None,
+    ):
+        if block_ids is None:
+            block_ids = range(num_blocks)
+
+        self._free_block_indices: Deque[BlockId] = deque(block_ids)
+        self._all_block_indices = frozenset(block_ids)
+        assert len(self._all_block_indices) == num_blocks
+
+        self._refcounter = RefCounter(
+            all_block_indices=self._free_block_indices)
+        self._block_size = block_size
+
+        self._cow_tracker = CopyOnWriteTracker(
+            refcounter=self._refcounter.as_readonly())
+
+        if block_pool is None:
+            extra_factor = 4
+            # Pre-allocate "num_blocks * extra_factor" block objects.
+            # The "* extra_factor" is a buffer to allow more block objects
+            # than physical blocks
+            self._block_pool = BlockPool(self._block_size, create_block, self,
+                                         num_blocks * extra_factor)
+        else:
+            # In this case, the block pool is provided by the caller,
+            # which means that there is most likely a need to share
+            # a block pool between allocators
+            self._block_pool = block_pool
+
+    def allocate_immutable_block(self,
+                                 prev_block: Optional[Block],
+                                 token_ids: List[int],
+                                 device: Optional[Device] = None) -> Block:
+        """Allocates a new immutable block with the given token IDs, linked to
+        the previous block.
+
+        Args:
+            prev_block (Optional[Block]): The previous block in the sequence. If
+                None, then the block to be allocated is the first block in the
+                sequence.
+            token_ids (List[int]): The token IDs to be stored in the new block.
+
+        Returns:
+            Block: The newly allocated immutable block.
+        """
+        assert device is None
+        block = self.allocate_mutable_block(prev_block=prev_block)
+        block.append_token_ids(token_ids)
+        return block
+
+    def allocate_immutable_blocks(
+            self,
+            prev_block: Optional[Block],
+            block_token_ids: List[List[int]],
+            device: Optional[Device] = None) -> List[Block]:
+        assert device is None
+        num_blocks = len(block_token_ids)
+
+        block_ids = []
+        for i in range(num_blocks):
+            block_ids.append(self._allocate_block_id())
+
+        blocks = []
+        for i in range(num_blocks):
+            prev_block = self._block_pool.init_block(
+                prev_block=prev_block,
+                token_ids=block_token_ids[i],
+                block_size=self._block_size,
+                physical_block_id=block_ids[i])
+            blocks.append(prev_block)
+
+        return blocks
+
+    def allocate_mutable_block(self,
+                               prev_block: Optional[Block],
+                               device: Optional[Device] = None) -> Block:
+        """Allocates a new mutable block, linked to the previous block.
+
+        Args:
+            prev_block (Optional[Block]): The previous block in the sequence. If
+                None, then the block to be allocated is the first block in the
+                sequence.
+
+        Returns:
+            Block: The newly allocated mutable block.
+        """
+        assert device is None
+        block_id = self._allocate_block_id()
+        block = self._block_pool.init_block(prev_block=prev_block,
+                                            token_ids=[],
+                                            block_size=self._block_size,
+                                            physical_block_id=block_id)
+        return block
+
+    def _allocate_block_id(self) -> BlockId:
+        if not self._free_block_indices:
+            raise BlockAllocator.NoFreeBlocksError()
+
+        block_id = self._free_block_indices.popleft()
+        self._refcounter.incr(block_id)
+        return block_id
+
+    def _free_block_id(self, block: Block) -> None:
+        block_id = block.block_id
+        assert block_id is not None
+
+        refcount = self._refcounter.decr(block_id)
+        if refcount == 0:
+            self._free_block_indices.appendleft(block_id)
+
+        block.block_id = None
+
+    def free(self, block: Block, keep_block_object: bool = False) -> None:
+        # Release the physical block id
+        self._free_block_id(block)
+
+        # Release the block object
+        if not keep_block_object:
+            self._block_pool.free_block(block)
+
+    def fork(self, last_block: Block) -> List[Block]:
+        """Creates a new sequence of blocks that shares the same underlying
+        memory as the original sequence.
+
+        Args:
+            last_block (Block): The last block in the original sequence.
+
+        Returns:
+            List[Block]: The new sequence of blocks that shares the same memory
+                as the original sequence.
+        """
+        source_blocks = get_all_blocks_recursively(last_block)
+
+        forked_blocks: List[Block] = []
+        prev_block = None
+        for block in source_blocks:
+
+            # Increment refcount for each block.
+            assert block.block_id is not None
+            refcount = self._refcounter.incr(block.block_id)
+            assert refcount != 1, "can't fork free'd block"
+
+            forked_block = self._block_pool.init_block(
+                prev_block=prev_block,
+                token_ids=block.token_ids,
+                block_size=self._block_size,
+                physical_block_id=block.block_id)
+
+            forked_blocks.append(forked_block)
+            prev_block = forked_blocks[-1]
+
+        return forked_blocks
+
+    def get_num_free_blocks(self) -> int:
+        return len(self._free_block_indices)
+
+    def get_num_total_blocks(self) -> int:
+        return len(self._all_block_indices)
+
+    def get_physical_block_id(self, absolute_id: int) -> int:
+        """Returns the zero-offset block id on certain block allocator
+        given the absolute block id.
+
+        Args:
+            absolute_id (int): The absolute block id for the block 
+            in whole allocator.
+
+        Returns:
+            int: The zero-offset block id on certain device.
+        """
+        return sorted(self._all_block_indices).index(absolute_id)
+
+    @property
+    def refcounter(self):
+        return self._refcounter
+
+    @property
+    def all_block_ids(self) -> FrozenSet[int]:
+        return self._all_block_indices
+
+    def cow_block_if_not_appendable(self, block: Block) -> BlockId:
+        """Performs a copy-on-write operation on the given block if it is not
+        appendable.
+
+        Args:
+            block (Block): The block to check for copy-on-write.
+
+        Returns:
+            BlockId: The block index of the new block if a copy-on-write 
+                operation was performed, or the original block index if
+                no copy-on-write was necessary.
+        """
+        src_block_id = block.block_id
+        assert src_block_id is not None
+
+        if self._cow_tracker.is_appendable(block):
+            return src_block_id
+
+        self._free_block_id(block)
+        trg_block_id = self._allocate_block_id()
+
+        self._cow_tracker.record_cow(src_block_id, trg_block_id)
+
+        return trg_block_id
+
+    def clear_copy_on_writes(self) -> List[Tuple[BlockId, BlockId]]:
+        """Returns the copy-on-write source->destination mapping and clears it.
+
+        Returns:
+            List[Tuple[BlockId, BlockId]]: A list mapping source
+                block indices to destination block indices.
+        """
+        return self._cow_tracker.clear_cows()
+
+    def mark_blocks_as_accessed(self, block_ids: List[int],
+                                now: float) -> None:
+        """Mark blocks as accessed, used in prefix caching.
+
+        Since the naive allocator does not implement prefix caching, we do
+        nothing.
+        """
+        pass
+
+    def mark_blocks_as_computed(self, block_ids: List[int]) -> None:
+        """Mark blocks as computed, used in prefix caching.
+
+        Since the naive allocator does not implement prefix caching, we do
+        nothing.
+        """
+        pass
+
+    def get_computed_block_ids(self, prev_computed_block_ids: List[int],
+                               block_ids: List[int],
+                               skip_last_block_id: bool) -> List[int]:
+        """No prefix caching here => return empty list
+        """
+        return []
+
+    def get_common_computed_block_ids(
+            self, computed_seq_block_ids: List[List[int]]) -> List[int]:
+        """Determine blocks that can be skipped in prefill.
+
+        Since the naive allocator does not support prefix caching, always return
+        an empty list.
+        """
+        return []
+
+    def promote_to_immutable_block(self, block: Block) -> BlockId:
+        raise NotImplementedError("There is no promotion for naive blocks")
+
+    def get_num_full_blocks_touched(self, blocks: List[Block]) -> int:
+        """Returns the number of full blocks that will be touched by
+        swapping in/out.
+
+        Args:
+            blocks: List of blocks to be swapped.
+        Returns:
+            int: the number of full blocks that will be touched by
+                swapping in/out the given blocks. Non full blocks are ignored
+                when deciding the number of blocks to touch.
+        """
+        # NOTE: for naive block, we use set to eliminate common blocks among
+        # seqs, also we compare the empty slots in the mutable blocks with
+        # lookahead slots to get the number of unique new block that are
+        # needed.
+        old_block_set = set()
+        for block in blocks:
+            if block.is_full:
+                old_block_set.add(block)
+        return len(old_block_set)
+
+    def swap_out(self, blocks: List[Block]) -> None:
+        for block in blocks:
+            self._free_block_id(block)
+
+    def swap_in(self, blocks: List[Block]) -> None:
+        for block in blocks:
+            # Here we allocate either immutable or mutable block and then
+            # extract its block_id. Note that the block object is released
+            # and the block_id is assigned to "block" to allow reusing the
+            # existing "block" object
+            if block.is_full:
+                tmp_block = self.allocate_immutable_block(
+                    prev_block=block.prev_block, token_ids=block.token_ids)
+            else:
+                tmp_block = self.allocate_mutable_block(
+                    prev_block=block.prev_block)
+                tmp_block.append_token_ids(block.token_ids)
+
+            block_id = tmp_block.block_id
+            tmp_block.block_id = None
+            self._block_pool.free_block(tmp_block)
+
+            block.block_id = block_id  # Assign block_id
+
+    def get_prefix_cache_hit_rate(self) -> float:
+        return -1
+
+
+class NaiveBlock(Block):
+    """An implementation of the Block class that does not support prefix
+    caching.
+
+    The NaiveBlock class represents a block of token IDs with a fixed size. It
+    provides methods for appending token IDs to the block and manages copy-on
+    -write operations when necessary.
+
+    Args:
+        prev_block (Block): The previous block in the sequence.
+        token_ids (List[int]): The initial token IDs to be stored in the block.
+        block_size (int): The maximum number of token IDs that can be stored in
+            the block.
+        allocator (BlockAllocator): The block allocator associated with this
+            block.
+        block_id (Optional[int], optional): The physical block index
+            of this block. Defaults to None, which means no allocation has been
+            made.
+        _cow_target (Optional[Block], optional): The copy-on-write target block.
+            If not provided, it defaults to self.
+    """
+
+    def __init__(self,
+                 prev_block: Optional[Block],
+                 token_ids: List[int],
+                 block_size: int,
+                 allocator: BlockAllocator,
+                 block_id: Optional[int] = None,
+                 _cow_target: Optional[Block] = None):
+        self._token_ids: List[int] = []
+        self._block_size = block_size
+        self._prev_block = prev_block
+        self._block_id = block_id
+        self._allocator = allocator
+        self._cow_target = _cow_target if _cow_target is not None else self
+
+        self._append_token_ids_no_cow(token_ids)
+
+    def append_token_ids(self, token_ids: List[int]) -> None:
+        """Appends the given token IDs to the block and performs a 
+        copy-on-write if necessary.
+
+        Args:
+            token_ids (Optional[List[int]]): The token IDs to be appended 
+                to the block.
+        """
+        self._append_token_ids_no_cow(token_ids)
+
+        if self._block_id is not None:
+            self._block_id = (self._allocator.cow_block_if_not_appendable(
+                self._cow_target))
+
+    def _append_token_ids_no_cow(self, token_ids: List[int]) -> None:
+        """Appends the given token IDs to the block
+
+        Args:
+            token_ids (List[int]): The token IDs to be appended to the block.
+        """
+        if len(token_ids) == 0:
+            return
+
+        assert len(token_ids) <= self.num_empty_slots
+
+        self._token_ids.extend(token_ids)
+
+    @property
+    def computed(self) -> bool:
+        raise NotImplementedError
+
+    @computed.setter
+    def computed(self, value) -> None:
+        raise NotImplementedError
+
+    @property
+    def last_accessed(self) -> float:
+        raise NotImplementedError
+
+    @last_accessed.setter
+    def last_accessed(self, last_accessed_ts: float):
+        raise NotImplementedError
+
+    @property
+    def block_id(self) -> Optional[int]:
+        return self._block_id
+
+    @block_id.setter
+    def block_id(self, value: Optional[int]) -> None:
+        self._block_id = value
+
+    @property
+    def is_full(self) -> bool:
+        return self.num_empty_slots == 0
+
+    @property
+    def num_empty_slots(self) -> int:
+        return self._block_size - len(self.token_ids)
+
+    @property
+    def token_ids(self) -> List[int]:
+        return self._token_ids
+
+    @property
+    def num_tokens_total(self) -> int:
+        raise NotImplementedError(
+            "num_tokens_total is not used for naive block")
+
+    @property
+    def block_size(self) -> int:
+        return self._block_size
+
+    @property
+    def prev_block(self) -> Optional["Block"]:
+        return self._prev_block
+
+    @property
+    def content_hash(self) -> Optional[int]:
+        return None

vllm/core/block/prefix_caching_block.py

@@ -0,0 +1,970 @@
+"""Token blocks."""
+from os.path import commonprefix
+from typing import Dict, FrozenSet, Iterable, List, Optional, Set, Tuple
+
+from vllm.core.block.common import (CacheMetricData, CopyOnWriteTracker,
+                                    get_all_blocks_recursively)
+from vllm.core.block.interfaces import Block, BlockAllocator, BlockId, Device
+from vllm.core.block.naive_block import (BlockPool, NaiveBlock,
+                                         NaiveBlockAllocator)
+from vllm.core.evictor_v2 import EvictionPolicy, Evictor, make_evictor
+
+PrefixHash = int
+
+# By default, we init our block access time as _DEFAULT_LAST_ACCESSED_TIME
+# so that if we find one block is still hold _DEFAULT_LAST_ACCESSED_TIME,
+# then we know this block hasn't been accessed yet.
+_DEFAULT_LAST_ACCESSED_TIME = -1
+
+
+class BlockTracker:
+    """Used to track the status of a block inside the prefix caching allocator
+    """
+    __slots__ = ("active", "last_accessed", "computed")
+
+    def reset(self):
+        self.last_accessed: float = _DEFAULT_LAST_ACCESSED_TIME
+        self.computed: bool = False
+
+    def __init__(self):
+        self.active: bool = False
+        self.reset()
+
+    def enable(self):
+        assert not self.active
+        self.active = True
+        self.reset()
+
+    def disable(self):
+        assert self.active
+        self.active = False
+        self.reset()
+
+
+class PrefixCachingBlockAllocator(BlockAllocator):
+    """A block allocator that implements prefix caching.
+
+    The PrefixCachingBlockAllocator maintains a cache of blocks based on their
+    content hash. It reuses blocks with the same content hash to avoid redundant
+    memory allocation. The allocator also supports copy-on-write operations.
+
+    Args:
+        num_blocks (int): The total number of blocks to manage.
+        block_size (int): The size of each block in tokens.
+        block_ids(Optional[Iterable[int]], optional): An optional iterable of
+            block IDs. If not provided, block IDs will be assigned sequentially
+            from 0 to num_blocks - 1.
+    """
+
+    def __init__(
+        self,
+        num_blocks: int,
+        block_size: int,
+        block_ids: Optional[Iterable[int]] = None,
+        eviction_policy: EvictionPolicy = EvictionPolicy.LRU,
+    ):
+        if block_ids is None:
+            block_ids = range(num_blocks)
+
+        self._block_size = block_size
+
+        # A mapping of prefix hash to block index. All blocks which have a
+        # prefix hash will be in this dict, even if they have refcount 0.
+        self._cached_blocks: Dict[PrefixHash, BlockId] = {}
+
+        # A list of immutable block IDs that have been touched by scheduler
+        # and should be marked as computed after an entire batch of sequences
+        # are scheduled.
+        self._touched_blocks: Set[BlockId] = set()
+
+        # Used to track status of each physical block id
+        self._block_tracker: Dict[BlockId, BlockTracker] = {}
+        for block_id in block_ids:
+            self._block_tracker[block_id] = BlockTracker()
+
+        # Pre-allocate "num_blocks * extra_factor" block objects.
+        # The "* extra_factor" is a buffer to allow more block objects
+        # than physical blocks
+        extra_factor = 4
+        self._block_pool = BlockPool(self._block_size, self._create_block,
+                                     self, num_blocks * extra_factor)
+
+        # An allocator for blocks that do not have prefix hashes.
+        self._hashless_allocator = NaiveBlockAllocator(
+            create_block=self._create_block,  # type: ignore
+            num_blocks=num_blocks,
+            block_size=block_size,
+            block_ids=block_ids,
+            block_pool=self._block_pool,  # Share block pool here
+        )
+
+        # Evitor used to maintain how we want to handle those computed blocks
+        # if we find memory pressure is high.
+        self.evictor: Evictor = make_evictor(eviction_policy)
+
+        # We share the refcounter between allocators. This allows us to promote
+        # blocks originally allocated in the hashless allocator to immutable
+        # blocks.
+        self._refcounter = self._hashless_allocator.refcounter
+
+        self._cow_tracker = CopyOnWriteTracker(
+            refcounter=self._refcounter.as_readonly())
+
+        self.metric_data = CacheMetricData()
+
+    # Implements Block.Factory.
+    def _create_block(
+        self,
+        prev_block: Optional[Block],
+        token_ids: List[int],
+        block_size: int,
+        allocator: BlockAllocator,
+        block_id: Optional[int] = None,
+        computed: bool = False,
+    ) -> Block:
+        # Bind block to self.
+        allocator = self
+
+        return PrefixCachingBlock(
+            prev_block=prev_block,
+            token_ids=token_ids,
+            block_size=block_size,
+            block_id=block_id,
+            allocator=allocator,
+            computed=computed,
+        )
+
+    def allocate_immutable_block(self,
+                                 prev_block: Optional[Block],
+                                 token_ids: List[int],
+                                 device: Optional[Device] = None) -> Block:
+        """Allocates an immutable block with the given token IDs, reusing cached
+        blocks if possible.
+
+        Args:
+            prev_block (Optional[Block]): The previous block in the sequence.
+            token_ids (List[int]): The token IDs to be stored in the block.
+
+        Returns:
+            Block: The allocated immutable block.
+        """
+        assert device is None
+        assert_prefix_caching_block_or_none(prev_block)
+
+        # First, try to create a block that points to cached data
+        block = self._block_pool.init_block(prev_block=prev_block,
+                                            token_ids=token_ids,
+                                            block_size=self._block_size,
+                                            physical_block_id=None)
+        assert block.content_hash is not None
+
+        cached_block_id = self._cached_blocks.get(block.content_hash, None)
+        if cached_block_id is not None:
+            self.metric_data.query(hit=True)
+            block.block_id = cached_block_id
+            self._incr_refcount_cached_block(block)
+            return block
+        self.metric_data.query(hit=False)
+        self._block_pool.free_block(block)
+
+        # No cached block => Allocate a new block
+        block = self.allocate_mutable_block(prev_block)
+        block.append_token_ids(token_ids)
+        return block
+
+    def allocate_immutable_blocks(
+            self,
+            prev_block: Optional[Block],
+            block_token_ids: List[List[int]],
+            device: Optional[Device] = None) -> List[Block]:
+        blocks = []
+        for token_ids in block_token_ids:
+            prev_block = self.allocate_immutable_block(prev_block=prev_block,
+                                                       token_ids=token_ids,
+                                                       device=device)
+            blocks.append(prev_block)
+        return blocks
+
+    def allocate_mutable_block(self,
+                               prev_block: Optional[Block],
+                               device: Optional[Device] = None) -> Block:
+        """Allocates a mutable block. If there are no free blocks, this will
+        evict unused cached blocks.
+
+        Args:
+            prev_block (Block): The previous block in the sequence.
+                None is not allowed unlike it is super class.
+
+        Returns:
+            Block: The allocated mutable block.
+        """
+        assert device is None
+        assert_prefix_caching_block_or_none(prev_block)
+
+        block_id = self._allocate_block_id()
+        block = self._block_pool.init_block(prev_block=prev_block,
+                                            token_ids=[],
+                                            block_size=self._block_size,
+                                            physical_block_id=block_id)
+        assert not block.computed
+        assert block.content_hash is None
+        return block
+
+    def _incr_refcount_cached_block(self, block: Block) -> None:
+        # Set this block to be "computed" since it is pointing to a
+        # cached block id (which was already computed)
+        block.computed = True
+
+        block_id = block.block_id
+        assert block_id is not None
+
+        refcount = self._refcounter.incr(block_id)
+        if refcount == 1:
+            # In case a cached block was evicted, restore its tracking
+            if block_id in self.evictor:
+                self.evictor.remove(block_id)
+
+            self._track_block_id(block_id, computed=True)
+
+    def _decr_refcount_cached_block(self, block: Block) -> None:
+        # Ensure this is immutable/cached block
+        assert block.content_hash is not None
+
+        block_id = block.block_id
+        assert block_id is not None
+
+        refcount = self._refcounter.decr(block_id)
+        if refcount > 0:
+            block.block_id = None
+            return
+        else:
+            assert refcount == 0
+
+        # No longer used
+        assert block.content_hash in self._cached_blocks
+
+        # Add the cached block to the evictor
+        # (This keeps the cached block around so it can be reused)
+        self.evictor.add(block_id, block.content_hash, block.num_tokens_total,
+                         self._block_tracker[block_id].last_accessed)
+
+        # Stop tracking the block
+        self._untrack_block_id(block_id)
+
+        block.block_id = None
+
+    def _decr_refcount_hashless_block(self, block: Block) -> None:
+        block_id = block.block_id
+        assert block_id is not None
+
+        # We may have a fork case where block is shared,
+        # in which case, we cannot remove it from tracking
+        refcount = self._refcounter.get(block_id)
+        if refcount == 1:
+            self._untrack_block_id(block_id)
+
+        # Decrement refcount of the block_id, but do not free the block object
+        # itself (will be handled by the caller)
+        self._hashless_allocator.free(block, keep_block_object=True)
+
+    def _allocate_block_id(self) -> BlockId:
+        """First tries to allocate a block id from the hashless allocator,
+        and if there are no blocks, then tries to evict an unused cached block.
+        """
+        hashless_block_id = self._maybe_allocate_hashless_block_id()
+        if hashless_block_id is not None:
+            return hashless_block_id
+
+        evicted_block_id = self._maybe_allocate_evicted_block_id()
+        if evicted_block_id is not None:
+            return evicted_block_id
+
+        # No block available in hashless allocator, nor in unused cache blocks.
+        raise BlockAllocator.NoFreeBlocksError()
+
+    def _maybe_allocate_hashless_block_id(self) -> Optional[BlockId]:
+        try:
+            # Allocate mutable block and extract its block_id
+            block = self._hashless_allocator.allocate_mutable_block(
+                prev_block=None)
+            block_id = block.block_id
+            self._block_pool.free_block(block)
+
+            self._track_block_id(block_id, computed=False)
+            return block_id
+        except BlockAllocator.NoFreeBlocksError:
+            return None
+
+    def _maybe_allocate_evicted_block_id(self) -> Optional[BlockId]:
+        if self.evictor.num_blocks == 0:
+            return None
+
+        # Here we get an evicted block, which is only added
+        # into evictor if its ref counter is 0
+        # and since its content would be changed, we need
+        # to remove it from _cached_blocks's tracking list
+        block_id, content_hash_to_evict = self.evictor.evict()
+
+        # Sanity checks
+        assert content_hash_to_evict in self._cached_blocks
+        _block_id = self._cached_blocks[content_hash_to_evict]
+        assert self._refcounter.get(_block_id) == 0
+        assert _block_id == block_id
+
+        self._cached_blocks.pop(content_hash_to_evict)
+
+        self._refcounter.incr(block_id)
+        self._track_block_id(block_id, computed=False)
+
+        return block_id
+
+    def _free_block_id(self, block: Block) -> None:
+        """Decrements the refcount of the block. The block may be in two 
+        possible states: (1) immutable/cached or (2) mutable/hashless. 
+        In the first case, the refcount is decremented directly and the block
+        may be possibly added to the evictor. In other case, hashless 
+        allocator free(..) with keep_block_object=True is called to only free
+        the block id (since the block object may be reused by the caller)
+        """
+        block_id = block.block_id
+        assert block_id is not None, "Freeing unallocated block is undefined"
+
+        if block.content_hash is not None:
+            # Immutable: This type of block is always cached, and we want to
+            # keep it in the evictor for future reuse
+            self._decr_refcount_cached_block(block)
+        else:
+            # Mutable: This type of block is not cached, so we release it
+            # directly to the hashless allocator
+            self._decr_refcount_hashless_block(block)
+
+        assert block.block_id is None
+
+    def free(self, block: Block, keep_block_object: bool = False) -> None:
+        """Release the block (look at free_block_id(..) docs)
+        """
+        # Release the physical block index
+        self._free_block_id(block)
+
+        # Release the block object to the pool
+        if not keep_block_object:
+            self._block_pool.free_block(block)
+
+    def fork(self, last_block: Block) -> List[Block]:
+        """Creates a new sequence of blocks that shares the same underlying
+        memory as the original sequence.
+
+        Args:
+            last_block (Block): The last block in the original sequence.
+
+        Returns:
+            List[Block]: The new sequence of blocks that shares the same memory
+                as the original sequence.
+        """
+        source_blocks = get_all_blocks_recursively(last_block)
+
+        forked_blocks: List[Block] = []
+        prev_block = None
+        for block in source_blocks:
+            block_id = block.block_id
+            assert block_id is not None
+
+            refcount = self._refcounter.incr(block_id)
+            assert refcount != 1, "can't fork free'd block_id = {}".format(
+                block_id)
+
+            forked_block = self._block_pool.init_block(
+                prev_block=prev_block,
+                token_ids=block.token_ids,
+                block_size=self._block_size,
+                physical_block_id=block_id)
+
+            forked_blocks.append(forked_block)
+            prev_block = forked_blocks[-1]
+
+        return forked_blocks
+
+    def get_num_free_blocks(self, device: Optional[Device] = None) -> int:
+        assert device is None
+        # The number of free blocks is the number of hashless free blocks
+        # plus the number of blocks evictor could free from its list.
+        return self._hashless_allocator.get_num_free_blocks(
+        ) + self.evictor.num_blocks
+
+    def get_num_total_blocks(self) -> int:
+        return self._hashless_allocator.get_num_total_blocks()
+
+    def get_physical_block_id(self, absolute_id: int) -> int:
+        """Returns the zero-offset block id on certain block allocator
+        given the absolute block id.
+
+        Args:
+            absolute_id (int): The absolute block id for the block 
+                in whole allocator.
+
+        Returns:
+            int: The rzero-offset block id on certain device.
+        """
+        return sorted(self.all_block_ids).index(absolute_id)
+
+    @property
+    def all_block_ids(self) -> FrozenSet[int]:
+        return self._hashless_allocator.all_block_ids
+
+    def get_prefix_cache_hit_rate(self) -> float:
+        return self.metric_data.get_hit_rate()
+
+    def is_block_cached(self, block: Block) -> bool:
+        assert block.content_hash is not None
+        return block.content_hash in self._cached_blocks
+
+    def promote_to_immutable_block(self, block: Block) -> BlockId:
+        """Once a mutable block is full, it can be promoted to an immutable
+        block. This means that its content can be referenced by future blocks
+        having the same prefix.
+
+        Note that if we already have a cached block with the same content, we
+        will replace the newly-promoted block's mapping with the existing cached
+        block id.
+
+        Args:
+            block: The mutable block to be promoted.
+
+        Returns:
+            BlockId: Either the original block index, or the block index of
+                the previously cached block matching the same content.
+        """
+        # Ensure block can be promoted
+        assert block.content_hash is not None
+        assert block.block_id is not None
+        assert self._refcounter.get(block.block_id) > 0
+
+        if block.content_hash not in self._cached_blocks:
+            # No cached content hash => Set this block as cached.
+            # Note that this block cannot be marked as computed yet
+            # because other sequences in the same batch cannot reuse
+            # this block.
+            self._cached_blocks[block.content_hash] = block.block_id
+            # Mark this block as touched so that it can be marked as
+            # computed after the entire batch of sequences are scheduled.
+            self._touched_blocks.add(block.block_id)
+            return block.block_id
+
+        # Reuse the cached content hash
+        self._decr_refcount_hashless_block(block)
+        block.block_id = self._cached_blocks[block.content_hash]
+
+        # Increment refcount of the cached block and (possibly) restore
+        # it from the evictor.
+        # Note that in this case, the block is marked as computed
+        self._incr_refcount_cached_block(block)
+
+        return block.block_id
+
+    def cow_block_if_not_appendable(self, block: Block) -> BlockId:
+        """Performs a copy-on-write operation on the given block if it is not
+        appendable.
+
+        Args:
+            block (Block): The block to check for copy-on-write.
+
+        Returns:
+            BlockId: The block index of the new block if a copy-on-write 
+                operation was performed, or the original block index if
+                no copy-on-write was necessary.
+        """
+        src_block_id = block.block_id
+        assert src_block_id is not None
+
+        if self._cow_tracker.is_appendable(block):
+            return src_block_id
+
+        self._free_block_id(block)
+        trg_block_id = self._allocate_block_id()
+
+        self._cow_tracker.record_cow(src_block_id, trg_block_id)
+
+        return trg_block_id
+
+    def clear_copy_on_writes(self) -> List[Tuple[BlockId, BlockId]]:
+        """Returns the copy-on-write source->destination mapping and clears it.
+
+        Returns:
+            List[Tuple[BlockId, BlockId]]: A list mapping source
+                block indices to destination block indices.
+        """
+        return self._cow_tracker.clear_cows()
+
+    def mark_blocks_as_accessed(self, block_ids: List[int],
+                                now: float) -> None:
+        """Mark blocks as accessed, used in prefix caching.
+
+        If the block is added into evictor, we need to update corresponding
+        info in evictor's metadata.
+        """
+
+        for block_id in block_ids:
+            if self._block_tracker[block_id].active:
+                self._block_tracker[block_id].last_accessed = now
+            elif block_id in self.evictor:
+                self.evictor.update(block_id, now)
+            else:
+                raise ValueError(
+                    "Mark block as accessed which is not belonged to GPU")
+
+    def mark_blocks_as_computed(self, block_ids: List[int]) -> None:
+        # Mark all touched blocks as computed.
+        for block_id in self._touched_blocks:
+            self._block_tracker[block_id].computed = True
+        self._touched_blocks.clear()
+
+    def _track_block_id(self, block_id: Optional[BlockId],
+                        computed: bool) -> None:
+        assert block_id is not None
+        self._block_tracker[block_id].enable()
+        self._block_tracker[block_id].computed = computed
+
+    def _untrack_block_id(self, block_id: Optional[BlockId]) -> None:
+        assert block_id is not None
+        self._block_tracker[block_id].disable()
+
+    def block_is_computed(self, block_id: int) -> bool:
+        if self._block_tracker[block_id].active:
+            return self._block_tracker[block_id].computed
+        else:
+            return block_id in self.evictor
+
+    def get_computed_block_ids(self,
+                               prev_computed_block_ids: List[int],
+                               block_ids: List[int],
+                               skip_last_block_id: bool = True) -> List[int]:
+        prev_prefix_size = len(prev_computed_block_ids)
+        cur_size = len(block_ids)
+        if skip_last_block_id:
+            cur_size -= 1
+
+        # Sanity checks
+        assert cur_size >= 0
+        assert prev_prefix_size <= cur_size
+
+        ret = prev_computed_block_ids
+        for i in range(prev_prefix_size, cur_size):
+            block_id = block_ids[i]
+            if self.block_is_computed(block_id):
+                ret.append(block_id)
+        return ret
+
+    def get_common_computed_block_ids(
+            self, computed_seq_block_ids: List[List[int]]) -> List[int]:
+        """Return the block ids that are common for a given sequence group.
+
+        Only those blocks that are immutable and already be marked
+        compyted would be taken consideration.
+        """
+
+        # NOTE We exclude the last block to avoid the case where the entire
+        # prompt is cached. This would cause erroneous behavior in model
+        # runner.
+
+        # It returns a list of int although type annotation says list of string.
+        if len(computed_seq_block_ids) == 1:
+            return computed_seq_block_ids[0]
+
+        return commonprefix([
+            ids for ids in computed_seq_block_ids  # type: ignore
+            if ids
+        ])
+
+    def get_num_full_blocks_touched(self, blocks: List[Block]) -> int:
+        """Returns the number of full blocks that will be touched by
+        swapping in/out.
+
+        Args:
+            blocks: List of blocks to be swapped.
+        Returns:
+            int: the number of full blocks that will be touched by
+                swapping in/out the given blocks. Non full blocks are ignored
+                when deciding the number of blocks to touch.
+        """
+        num_touched_blocks: int = 0
+        for block in blocks:
+            # If the block has a match in the cache and the cached
+            # block is not referenced, then we still count it as a
+            # touched block
+            if block.is_full and (not self.is_block_cached(block) or \
+                (block.content_hash is not None and \
+                self._cached_blocks[block.content_hash] in \
+                        self.evictor)):
+                num_touched_blocks += 1
+        return num_touched_blocks
+
+    def swap_out(self, blocks: List[Block]) -> None:
+        """Execute the swap out actions. Basically just free the 
+        given blocks.
+
+        Args:
+            blocks: List of blocks to be swapped out.
+        """
+        for block in blocks:
+            self._free_block_id(block)
+
+    def swap_in(self, blocks: List[Block]) -> None:
+        """Execute the swap in actions. Change the block id from 
+        old allocator to current allocator for each block to finish 
+        the block table update. 
+
+        Args:
+            blocks: List of blocks to be swapped in.
+        """
+        for block in blocks:
+            # Here we allocate either immutable or mutable block and then
+            # extract its block_id. Note that the block object is released
+            # and the block_id is assigned to "block" to allow reusing the
+            # existing "block" object
+            if block.is_full:
+                tmp_block = self.allocate_immutable_block(
+                    prev_block=block.prev_block, token_ids=block.token_ids)
+            else:
+                tmp_block = self.allocate_mutable_block(
+                    prev_block=block.prev_block)
+                tmp_block.append_token_ids(block.token_ids)
+
+            block_id = tmp_block.block_id
+            self._block_pool.free_block(tmp_block)
+
+            block.block_id = block_id  # Assign block_id
+
+
+class PrefixCachingBlock(Block):
+    """A block implementation that supports prefix caching.
+
+    The PrefixCachingBlock class represents a block of token IDs with prefix
+    caching capabilities. It wraps a NaiveBlock internally and provides
+    additional functionality for content hashing and promoting immutable blocks
+    with the prefix caching allocator.
+
+    Args:
+        prev_block (Optional[PrefixCachingBlock]): The previous block in the
+            sequence.
+        token_ids (List[int]): The initial token IDs to be stored in the block.
+        block_size (int): The maximum number of token IDs that can be stored in
+            the block.
+        allocator (BlockAllocator): The prefix
+            caching block allocator associated with this block.
+        block_id (Optional[int], optional): The physical block index
+            of this block. Defaults to None.
+    """
+
+    def __init__(
+        self,
+        prev_block: Optional[Block],
+        token_ids: List[int],
+        block_size: int,
+        allocator: BlockAllocator,
+        block_id: Optional[int] = None,
+        computed: bool = False,
+    ):
+        assert isinstance(allocator, PrefixCachingBlockAllocator), (
+            "Currently this class is only tested with "
+            "PrefixCachingBlockAllocator. Got instead allocator = {}".format(
+                allocator))
+        assert_prefix_caching_block_or_none(prev_block)
+
+        self._prev_block = prev_block
+        self._cached_content_hash: Optional[int] = None
+        self._cached_num_tokens_total: int = 0
+        self._allocator = allocator
+        self._last_accessed: float = _DEFAULT_LAST_ACCESSED_TIME
+        self._computed = computed
+
+        # On the first time, we create the block object, and next we only
+        # reinitialize it
+        if hasattr(self, "_block"):
+            self._block.__init__(  # type: ignore[has-type]
+                prev_block=prev_block,
+                token_ids=token_ids,
+                block_size=block_size,
+                block_id=block_id,
+                allocator=self._allocator)
+        else:
+            self._block = NaiveBlock(prev_block=prev_block,
+                                     token_ids=token_ids,
+                                     block_size=block_size,
+                                     block_id=block_id,
+                                     allocator=self._allocator)
+
+        self._update_num_tokens_total()
+
+    def _update_num_tokens_total(self):
+        """Incrementally computes the number of tokens that there is
+        till the current block (included)
+        """
+        res = 0
+
+        # Add all previous blocks
+        if self._prev_block is not None:
+            res += self._prev_block.num_tokens_total
+
+        # Add current block
+        res += len(self.token_ids)
+
+        self._cached_num_tokens_total = res
+
+    @property
+    def computed(self) -> bool:
+        return self._computed
+
+    @computed.setter
+    def computed(self, value) -> None:
+        self._computed = value
+
+    @property
+    def last_accessed(self) -> float:
+        return self._last_accessed
+
+    @last_accessed.setter
+    def last_accessed(self, last_accessed_ts: float):
+        self._last_accessed = last_accessed_ts
+
+    def append_token_ids(self, token_ids: List[int]) -> None:
+        """Appends the given token IDs to the block and registers the block as
+        immutable if the block becomes full.
+
+        Args:
+            token_ids (List[int]): The token IDs to be appended to the block.
+        """
+        # Ensure this is mutable block (not promoted)
+        assert self.content_hash is None
+        assert not self.computed
+
+        if len(token_ids) == 0:
+            return
+
+        # Ensure there are input tokens
+        assert token_ids, "Got token_ids = {}".format(token_ids)
+
+        # Naive block handles CoW.
+        self._block.append_token_ids(token_ids)
+        self._update_num_tokens_total()
+
+        # If the content hash is present, then the block can be made immutable.
+        # Register ourselves with the allocator, potentially replacing the
+        # physical block index.
+        if self.content_hash is not None:
+            self.block_id = self._allocator.promote_to_immutable_block(self)
+
+    @property
+    def block_id(self) -> Optional[int]:
+        return self._block.block_id
+
+    @block_id.setter
+    def block_id(self, value) -> None:
+        self._block.block_id = value
+
+    @property
+    def is_full(self) -> bool:
+        return self._block.is_full
+
+    @property
+    def num_empty_slots(self) -> int:
+        return self._block.num_empty_slots
+
+    @property
+    def num_tokens_total(self) -> int:
+        return self._cached_num_tokens_total
+
+    @property
+    def block_size(self) -> int:
+        return self._block.block_size
+
+    @property
+    def token_ids(self) -> List[int]:
+        return self._block.token_ids
+
+    @property
+    def prev_block(self) -> Optional[Block]:
+        return self._prev_block
+
+    @property
+    def content_hash(self) -> Optional[int]:
+        """Return the content-based hash of the current block, or None if it is
+        not yet defined.
+
+        For the content-based hash to be defined, the current block must be
+        full.
+        """
+        # If the hash is already computed, return it.
+        if self._cached_content_hash is not None:
+            return self._cached_content_hash
+
+        # We cannot compute a hash for the current block because it is not full.
+        if not self.is_full:
+            return None
+
+        is_first_block = self._prev_block is None
+        prev_block_hash = (
+            None if is_first_block else
+            self._prev_block.content_hash  # type: ignore
+        )
+
+        # Previous block exists but does not yet have a hash.
+        # Return no hash in this case.
+        if prev_block_hash is None and not is_first_block:
+            return None
+
+        self._cached_content_hash = PrefixCachingBlock.hash_block_tokens(
+            is_first_block,
+            prev_block_hash,
+            cur_block_token_ids=self.token_ids)
+        return self._cached_content_hash
+
+    @staticmethod
+    def hash_block_tokens(is_first_block: bool, prev_block_hash: Optional[int],
+                          cur_block_token_ids: List[int]) -> int:
+        """Computes a hash value corresponding to the contents of a block and
+        the contents of the preceding block(s). The hash value is used for
+        prefix caching.
+
+        NOTE: Content-based hashing does not yet support LoRA.
+
+        Parameters:
+        - is_first_block (bool): A flag indicating if the block is the first in
+            the sequence.
+        - prev_block_hash (Optional[int]): The hash of the previous block. None
+            if this is the first block.
+        - cur_block_token_ids (List[int]): A list of token ids in the current
+            block. The current block is assumed to be full.
+
+        Returns:
+        - int: The computed hash value for the block.
+        """
+        assert (prev_block_hash is None) == is_first_block
+        return hash((is_first_block, prev_block_hash, *cur_block_token_ids))
+
+
+class ComputedBlocksTracker:
+    """Handles caching of per-sequence computed block ids. 
+        When a sequence appears for the first time, it traverses all of the 
+        blocks and detects the prefix of blocks that is computed. On the
+        subsequent times, it only traverses the new blocks that were added 
+        and updates the already recorded prefix of blocks with the newly 
+        computed blocks.
+
+        To avoid redundant traversals, the algorithm also detects when there
+        is a "gap" in the computed prefix. For example, if we have blocks =
+        [1,2,3,4,5], and we have detected [1,2,3] as the computed prefix, then
+        we won't try to add more computed blocks to [1,2,3] in this sequence
+        iteration, and will add more computed blocks only after the sequence is
+        freed and reused again.
+
+        Note that currently, for a given sequence, we also skip the last 
+        block id for caching purposes, to avoid caching of a full sequence
+    """
+
+    def __init__(self, allocator):
+        self._allocator = allocator
+        self._cached_computed_seq_blocks: Dict[int, Tuple[List[int],
+                                                          bool]] = {}
+
+    def add_seq(self, seq_id: int) -> None:
+        """Start tracking seq_id
+        """
+        assert seq_id not in self._cached_computed_seq_blocks
+        self._cached_computed_seq_blocks[seq_id] = ([], False)
+
+    def remove_seq(self, seq_id: int) -> None:
+        """Stop tracking seq_id
+        """
+        assert seq_id in self._cached_computed_seq_blocks
+        del self._cached_computed_seq_blocks[seq_id]
+
+    def get_cached_computed_blocks_and_update(
+            self, seq_id: int, block_ids: List[int]) -> List[int]:
+        """ Look at the class documentation for details
+        """
+        # Ensure seq_id is already tracked
+        assert seq_id in self._cached_computed_seq_blocks
+
+        # Get cached data (may be empty on the first time)
+        prev_computed_block_ids, has_gap = self._cached_computed_seq_blocks[
+            seq_id]
+
+        if has_gap:
+            # When gap is detected, we do not add more computed blocks at this
+            # sequence iteration
+            return prev_computed_block_ids
+
+        # We do not consider the last block id for caching purposes.
+        num_cur_blocks = len(block_ids) - 1
+        assert num_cur_blocks >= 0
+
+        if len(prev_computed_block_ids) >= num_cur_blocks:
+            # Cache HIT
+            assert len(prev_computed_block_ids) == num_cur_blocks
+            return prev_computed_block_ids
+
+        # If here, then we may possibly add more computed blocks. As a result,
+        # traverse the additional blocks after prev_computed_block_ids to
+        # detect more computed blocks and add them.
+
+        # Incremental init for seq_id => Look only at the new blocks
+        computed_block_ids = self._allocator.get_computed_block_ids(  # noqa: E501
+            prev_computed_block_ids,
+            block_ids,
+            skip_last_block_id=
+            True,  # We skip last block id to avoid caching of full seq
+        )
+
+        # Detect if there is a "gap"
+        has_gap = len(computed_block_ids) < num_cur_blocks
+
+        # Record
+        self._cached_computed_seq_blocks[seq_id] = (computed_block_ids,
+                                                    has_gap)
+
+        return computed_block_ids
+
+
+class LastAccessBlocksTracker:
+    """Manages the last access time of the tracked sequences, in order to allow
+    an efficient update of allocator's block last access times
+    """
+
+    def __init__(self, allocator):
+        self._allocator = allocator
+        self._seq_last_access: Dict[int, Optional[float]] = {}
+
+    def add_seq(self, seq_id: int) -> None:
+        """Start tracking seq_id
+        """
+        assert seq_id not in self._seq_last_access
+        self._seq_last_access[seq_id] = None
+
+    def remove_seq(self, seq_id: int) -> None:
+        """Stop tracking seq_id
+        """
+        assert seq_id in self._seq_last_access
+        del self._seq_last_access[seq_id]
+
+    def update_last_access(self, seq_id: int, time: float) -> None:
+        assert seq_id in self._seq_last_access
+        self._seq_last_access[seq_id] = time
+
+    def update_seq_blocks_last_access(self, seq_id: int,
+                                      block_ids: List[int]) -> None:
+        assert seq_id in self._seq_last_access
+
+        ts = self._seq_last_access[seq_id]
+
+        if ts is None:
+            # No last access was recorded, no need to update.
+            return
+
+        self._allocator.mark_blocks_as_accessed(block_ids, ts)
+
+
+def assert_prefix_caching_block_or_none(block: Optional[Block]):
+    if block is None:
+        return
+    assert isinstance(block,
+                      PrefixCachingBlock), "Got block = {}".format(block)

vllm/core/block/utils.py

@@ -0,0 +1,48 @@
+"""Block manager utils."""
+from vllm.sequence import SequenceGroup
+from vllm.utils import (STR_NOT_IMPL_ENC_DEC_PREFIX_CACHE,
+                        STR_NOT_IMPL_ENC_DEC_SWA)
+
+
+def _get_block_mgr_sliding_window_attr(block_mgr):
+    '''
+    BlockManagerV1 and BlockManagerV2 have slightly different
+    members related to sliding window attention (SWA). This
+    function extracts the appropriate member to use for determining
+    whether SWA is enabled.
+
+    Arguments:
+
+    * block_mgr: BlockManagerV1 or BlockManagerV2 instance
+    '''
+
+    if hasattr(block_mgr, 'block_sliding_window'):
+        return block_mgr.block_sliding_window
+    if hasattr(block_mgr, 'max_block_sliding_window'):
+        return block_mgr.max_block_sliding_window
+
+    raise AttributeError("Block manager instance has neither " + \
+                         "block_sliding_window nor " + \
+                         "max_block_sliding_window attributes.")
+
+
+def check_no_caching_or_swa_for_blockmgr_encdec(
+        block_mgr, seq_group: SequenceGroup) -> None:
+    '''
+    Enforce that prefix caching & sliding-window attention (SWA)
+    are currently unsupported *specifically* for encoder/decoder models.
+
+    Raises NotImplementedError if unsupported scenario is detected.
+
+    Arguments:
+
+    * block_mgr: BlockSpaceManager instance
+    * seq_group: SequenceGroup passed to block_mgr
+    '''
+
+    if seq_group.is_encoder_decoder():
+        if _get_block_mgr_sliding_window_attr(block_mgr) is not None:
+            raise NotImplementedError(STR_NOT_IMPL_ENC_DEC_SWA)
+
+        if block_mgr.enable_caching:
+            raise NotImplementedError(STR_NOT_IMPL_ENC_DEC_PREFIX_CACHE)

vllm/core/block_manager_v1.py

@@ -0,0 +1,743 @@
+"""A block manager that manages token blocks."""
+import math
+from abc import ABC, abstractmethod
+from itertools import count, takewhile
+from os.path import commonprefix
+from typing import Dict, List, Optional
+from typing import Sequence as GenericSequence
+from typing import Set, Tuple
+
+from vllm.block import BlockTable, PhysicalTokenBlock
+from vllm.core.block.common import CacheMetricData
+from vllm.core.block.utils import check_no_caching_or_swa_for_blockmgr_encdec
+from vllm.core.evictor_v1 import EvictionPolicy, Evictor, make_evictor
+from vllm.core.interfaces import AllocStatus, BlockSpaceManager
+from vllm.logger import init_logger
+from vllm.sequence import Sequence, SequenceGroup, SequenceStatus
+from vllm.utils import Device
+
+logger = init_logger(__name__)
+
+
+class BlockAllocatorBase(ABC):
+    """Manages free physical token blocks for a device.
+
+    The allocator maintains a list of free blocks and allocates a block when
+    requested. When a block is freed, its reference count is decremented. If
+    the reference count becomes zero, the block is added back to the free list.
+    """
+
+    @abstractmethod
+    def __init__(self,
+                 device: Device,
+                 block_size: int,
+                 num_blocks: int,
+                 eviction_policy: EvictionPolicy = EvictionPolicy.LRU):
+        pass
+
+    @abstractmethod
+    def allocate(self,
+                 block_hash: Optional[int] = None,
+                 num_hashed_tokens: int = 0) -> PhysicalTokenBlock:
+        pass
+
+    @abstractmethod
+    def free(self, block: PhysicalTokenBlock) -> None:
+        pass
+
+    @abstractmethod
+    def get_num_free_blocks(self) -> int:
+        pass
+
+    @abstractmethod
+    def get_num_total_blocks(self) -> int:
+        pass
+
+    @abstractmethod
+    def contains_block(self, block_hash: int) -> bool:
+        pass
+
+    @abstractmethod
+    def update_hash(self, block_hash: int, block: PhysicalTokenBlock):
+        pass
+
+    @abstractmethod
+    def get_prefix_cache_hit_rate(self) -> float:
+        """Prefix cache hit rate. -1 means not supported or disabled."""
+        pass
+
+
+class CachedBlockAllocator(BlockAllocatorBase):
+    """Manages free physical token blocks for a device.
+
+    The allocator maintains a list of free blocks and allocates a block when
+    requested. When a block is freed, its reference count is decremented. If
+    the reference count becomes zero, the block is added back to the free list.
+    """
+
+    def __init__(self,
+                 device: Device,
+                 block_size: int,
+                 num_blocks: int,
+                 eviction_policy: EvictionPolicy = EvictionPolicy.LRU) -> None:
+        self.device = device
+        self.block_size = block_size
+        self.num_blocks = num_blocks
+
+        self.current_num_blocks = 0
+        self.cached_blocks: Dict[int, PhysicalTokenBlock] = {}
+
+        self.evictor: Evictor = make_evictor(eviction_policy)
+
+        self.default_hash_ctr = count()
+
+        self.cache_metric_data = CacheMetricData()
+
+    def allocate_block(self, block_hash: int,
+                       num_hashed_tokens: int) -> PhysicalTokenBlock:
+        if self.current_num_blocks == self.num_blocks:
+            block = self.evictor.evict()
+            block.block_hash = block_hash
+            block.num_hashed_tokens = num_hashed_tokens
+            return block
+        block = PhysicalTokenBlock(device=self.device,
+                                   block_number=self.current_num_blocks,
+                                   block_size=self.block_size,
+                                   block_hash=block_hash,
+                                   num_hashed_tokens=num_hashed_tokens)
+        self.current_num_blocks += 1
+        return block
+
+    def allocate(self,
+                 block_hash: Optional[int] = None,
+                 num_hashed_tokens: int = 0) -> PhysicalTokenBlock:
+        if block_hash is None:
+            block_hash = next(self.default_hash_ctr)
+
+        if block_hash in self.evictor:
+            assert block_hash not in self.cached_blocks
+            block = self.evictor.remove(block_hash)
+            assert block.ref_count == 0
+            self.cached_blocks[block_hash] = block
+
+        if block_hash in self.cached_blocks:
+            self.cache_metric_data.query(hit=True)
+        else:
+            self.cache_metric_data.query(hit=False)
+            self.cached_blocks[block_hash] = self.allocate_block(
+                block_hash, num_hashed_tokens)
+        block = self.cached_blocks[block_hash]
+        assert block.block_hash == block_hash
+        block.ref_count += 1
+        return block
+
+    def free(self, block: PhysicalTokenBlock) -> None:
+        if block.ref_count == 0:
+            raise ValueError(f"Double free! {block} is already freed.")
+        block.ref_count -= 1
+        if block.ref_count == 0:
+            assert block.block_hash not in self.evictor
+            self.evictor.add(block)
+
+            # Remove the block from the cached_blocks
+            del self.cached_blocks[block.block_hash]
+
+    def get_num_free_blocks(self) -> int:
+        return (self.num_blocks - self.current_num_blocks +
+                self.evictor.num_blocks)
+
+    def get_num_total_blocks(self) -> int:
+        return self.num_blocks
+
+    def contains_block(self, block_hash: int) -> bool:
+        return block_hash in self.cached_blocks or block_hash in self.evictor
+
+    def update_hash(self, block_hash: int, block: PhysicalTokenBlock):
+        # Update the hash of block and the cached_blocks dictionary.
+        assert not self.contains_block(block_hash)
+        old_hash = block.block_hash
+        block.block_hash = block_hash
+        del self.cached_blocks[old_hash]
+        self.cached_blocks[block_hash] = block
+
+    def get_prefix_cache_hit_rate(self) -> float:
+        return self.cache_metric_data.get_hit_rate()
+
+
+class UncachedBlockAllocator(BlockAllocatorBase):
+    """Manages free physical token blocks for a device.
+
+    The allocator maintains a list of free blocks and allocates a block when
+    requested. When a block is freed, its reference count is decremented. If
+    the reference count becomes zero, the block is added back to the free list.
+    """
+
+    def __init__(
+        self,
+        device: Device,
+        block_size: int,
+        num_blocks: int,
+    ) -> None:
+        self.device = device
+        self.block_size = block_size
+        self.num_blocks = num_blocks
+
+        # Initialize the free blocks.
+        self.free_blocks: List[PhysicalTokenBlock] = []
+        for i in range(num_blocks):
+            block = PhysicalTokenBlock(device=device,
+                                       block_number=i,
+                                       block_size=block_size,
+                                       block_hash=-1,
+                                       num_hashed_tokens=0)
+            self.free_blocks.append(block)
+
+    def allocate(self,
+                 block_hash: Optional[int] = None,
+                 num_hashed_tokens: int = 0) -> PhysicalTokenBlock:
+        if not self.free_blocks:
+            raise ValueError("Out of memory! No free blocks are available.")
+        block = self.free_blocks.pop()
+        block.ref_count = 1
+        return block
+
+    def free(self, block: PhysicalTokenBlock) -> None:
+        if block.ref_count == 0:
+            raise ValueError(f"Double free! {block} is already freed.")
+        block.ref_count -= 1
+        if block.ref_count == 0:
+            self.free_blocks.append(block)
+
+    def get_num_free_blocks(self) -> int:
+        return len(self.free_blocks)
+
+    def get_num_total_blocks(self) -> int:
+        return self.num_blocks
+
+    def contains_block(self, block_hash: int) -> bool:
+        raise NotImplementedError(
+            "Invalid codepath for uncached block allocator.")
+
+    def update_hash(self, block_hash: int, block: PhysicalTokenBlock):
+        raise NotImplementedError(
+            "Invalid codepath for uncached block allocator.")
+
+    def get_prefix_cache_hit_rate(self) -> float:
+        return -1
+
+
+class BlockSpaceManagerV1(BlockSpaceManager):
+    """Manages the mapping between logical and physical token blocks."""
+
+    def __init__(
+        self,
+        block_size: int,
+        num_gpu_blocks: int,
+        num_cpu_blocks: int,
+        watermark: float = 0.01,
+        sliding_window: Optional[int] = None,
+        enable_caching: bool = False,
+    ) -> None:
+        self.block_size = block_size
+        self.num_total_gpu_blocks = num_gpu_blocks
+        self.num_total_cpu_blocks = num_cpu_blocks
+
+        if enable_caching and sliding_window is not None:
+            raise NotImplementedError(
+                "Sliding window is not allowed with prefix caching enabled!")
+
+        self.block_sliding_window = None
+        if sliding_window is not None:
+            # Round up to nearest block size to regularize sliding window
+            # allocation sizes.
+            self.block_sliding_window = math.ceil(sliding_window / block_size)
+
+        self.watermark = watermark
+        assert watermark >= 0.0
+
+        self.enable_caching = enable_caching
+
+        self.watermark_blocks = int(watermark * num_gpu_blocks)
+
+        if self.enable_caching:
+            logger.info("Automatic prefix caching is enabled.")
+            self.gpu_allocator: BlockAllocatorBase = CachedBlockAllocator(
+                Device.GPU, block_size, num_gpu_blocks)
+            self.cpu_allocator: BlockAllocatorBase = CachedBlockAllocator(
+                Device.CPU, block_size, num_cpu_blocks)
+        else:
+            self.gpu_allocator = UncachedBlockAllocator(
+                Device.GPU, block_size, num_gpu_blocks)
+            self.cpu_allocator = UncachedBlockAllocator(
+                Device.CPU, block_size, num_cpu_blocks)
+        # Mapping: seq_id -> BlockTable.
+        self.block_tables: Dict[int, BlockTable] = {}
+
+        # Mapping: req_id -> BlockTable
+        # Note that each SequenceGroup has a unique
+        # request ID
+        self.cross_block_tables: Dict[str, BlockTable] = {}
+
+    def _get_seq_num_required_blocks(self, seq: Optional[Sequence]) -> int:
+        return 0 if seq is None else seq.n_blocks
+
+    def can_allocate(self,
+                     seq_group: SequenceGroup,
+                     num_lookahead_slots: int = 0) -> AllocStatus:
+        # FIXME(woosuk): Here we assume that all sequences in the group share
+        # the same prompt. This may not be true for preempted sequences.
+
+        assert (num_lookahead_slots == 0
+                ), "lookahead allocation not supported in BlockSpaceManagerV1"
+
+        check_no_caching_or_swa_for_blockmgr_encdec(self, seq_group)
+
+        self_num_required_blocks = self._get_seq_num_required_blocks(
+            seq_group.get_seqs(status=SequenceStatus.WAITING)[0])
+        cross_num_required_blocks = self._get_seq_num_required_blocks(
+            seq_group.get_encoder_seq())
+        num_required_blocks = self_num_required_blocks + \
+                              cross_num_required_blocks
+
+        if self.block_sliding_window is not None:
+
+            num_required_blocks = min(num_required_blocks,
+                                      self.block_sliding_window)
+        num_free_gpu_blocks = self.gpu_allocator.get_num_free_blocks()
+
+        # Use watermark to avoid frequent cache eviction.
+        if (self.num_total_gpu_blocks - num_required_blocks <
+                self.watermark_blocks):
+            return AllocStatus.NEVER
+        if num_free_gpu_blocks - num_required_blocks >= self.watermark_blocks:
+            return AllocStatus.OK
+        else:
+            return AllocStatus.LATER
+
+    def _allocate_sequence(self, \
+                           seq: Optional[Sequence], \
+                           ref_count: int, \
+                           is_encoder_decoder: bool = True) -> BlockTable:
+        # Allocate new physical token blocks that will store the prompt tokens.
+        num_prompt_blocks = self._get_seq_num_required_blocks(seq)
+
+        block_table: BlockTable = BlockTable()
+        assert seq is not None
+        for logical_idx in range(num_prompt_blocks):
+            if (self.block_sliding_window is not None
+                    and logical_idx >= self.block_sliding_window):
+                block = block_table[logical_idx % self.block_sliding_window]
+                # Set the reference counts of the token blocks.
+                block.ref_count = ref_count
+            elif not is_encoder_decoder and self.enable_caching:
+                block = self.gpu_allocator.allocate(
+                    seq.hash_of_block(logical_idx),
+                    seq.num_hashed_tokens_of_block(logical_idx))
+            else:
+                block = self.gpu_allocator.allocate()
+                # Set the reference counts of the token blocks.
+                block.ref_count = ref_count
+            block_table.append(block)
+
+        return block_table
+
+    def allocate(self, seq_group: SequenceGroup) -> None:
+        is_encoder_decoder = seq_group.is_encoder_decoder()
+        check_no_caching_or_swa_for_blockmgr_encdec(self, seq_group)
+
+        # Allocate decoder sequences
+        #
+        # NOTE: Here we assume that all sequences in the group have the same
+        # decoder prompt.
+        wait_seqs = seq_group.get_seqs(status=SequenceStatus.WAITING)
+        seq = wait_seqs[0]
+        block_table: BlockTable = \
+            self._allocate_sequence(seq,
+                                    seq_group.num_seqs(),
+                                    is_encoder_decoder)
+
+        # Assign the self-attention block tables for each sequence.
+        if len(wait_seqs) == 1:
+            self.block_tables[seq.seq_id] = block_table
+        else:
+            for seq in wait_seqs:
+                self.block_tables[seq.seq_id] = block_table.copy()
+
+        # Allocate encoder sequence
+        if is_encoder_decoder:
+            # A SequenceGroup has only a single encoder sequence (at most),
+            # thus allocate with a ref count of 1
+            block_table = self._allocate_sequence(seq_group.get_encoder_seq(),
+                                                  1, is_encoder_decoder)
+            # Assign the cross-attention block table for the SequenceGroup.
+            self.cross_block_tables[seq_group.request_id] = block_table
+
+    def can_append_slots(self,
+                         seq_group: SequenceGroup,
+                         num_lookahead_slots: int = 0) -> bool:
+        assert (num_lookahead_slots == 0
+                ), "lookahead allocation not supported in BlockSpaceManagerV1"
+
+        # Simple heuristic: If there is at least one free block
+        # for each sequence, we can append.
+        num_free_gpu_blocks = self.gpu_allocator.get_num_free_blocks()
+        num_seqs = seq_group.num_seqs(status=SequenceStatus.RUNNING)
+        return num_seqs <= num_free_gpu_blocks
+
+    def _promote_last_block(
+        self,
+        seq: Sequence,
+        last_block: PhysicalTokenBlock,
+    ) -> PhysicalTokenBlock:
+        assert self.enable_caching
+
+        # Compute a new hash for the block so that it can be shared by other
+        # Sequences
+        new_hash = seq.hash_of_block(seq.n_blocks - 1)
+
+        # if new_hash is already in the cached table, then free last_block
+        # and return the cached version
+        if self.gpu_allocator.contains_block(new_hash):
+            self.gpu_allocator.free(last_block)
+            return self.gpu_allocator.allocate(new_hash)
+        else:
+            self.gpu_allocator.update_hash(new_hash, last_block)
+            return last_block
+
+    def _is_last_block_full(
+        self,
+        seq: Sequence,
+    ) -> bool:
+        token_ids_len = seq.data.get_len()
+        return token_ids_len > 0 and token_ids_len % seq.block_size == 0
+
+    def _maybe_promote_last_block(
+        self,
+        seq: Sequence,
+        last_block: PhysicalTokenBlock,
+    ) -> PhysicalTokenBlock:
+        if self._is_last_block_full(seq):
+            return self._promote_last_block(seq, last_block)
+        else:
+            return last_block
+
+    def _allocate_last_physical_block(
+        self,
+        seq: Sequence,
+    ) -> PhysicalTokenBlock:
+        # Called before a new block is appended.
+        # This is in charge of allocating a new physical block (to be appended).
+
+        # None if the last block is not full. Otherwise, we set it to the
+        # content hash.
+        if not self.enable_caching:
+            return self.gpu_allocator.allocate()
+        block_hash: Optional[int] = None
+        n_blocks = seq.n_blocks
+        if (self._is_last_block_full(seq)):
+            block_hash = seq.hash_of_block(n_blocks - 1)
+        num_hashed_tokens = seq.num_hashed_tokens_of_block(n_blocks - 1)
+
+        # num_hashed_tokens is used to compute future hashes
+        # (e.g. in the hashing function, it is used to ask the sequence for
+        # prefix tokens)
+        new_block = self.gpu_allocator.allocate(block_hash, num_hashed_tokens)
+
+        # If the block_hash is None, then the block is not full.
+        # If the block is not full, then we expect it to have a refcount of 1.
+        if block_hash is None:
+            assert new_block.ref_count == 1
+        return new_block
+
+    def append_slots(
+        self,
+        seq: Sequence,
+        num_lookahead_slots: int = 0,
+    ) -> List[Tuple[int, int]]:
+        """Allocate a physical slot for a new token."""
+        n_blocks = seq.n_blocks
+        block_table = self.block_tables[seq.seq_id]
+        # If we need to allocate a new physical block
+        if len(block_table) < n_blocks:
+            # Currently this code only supports adding one physical block
+            assert len(block_table) == n_blocks - 1
+
+            if (self.block_sliding_window
+                    and len(block_table) >= self.block_sliding_window):
+                # reuse a block
+                block_table.append(block_table[len(block_table) %
+                                               self.block_sliding_window])
+            else:
+                # The sequence hash a new logical block.
+                # Allocate a new physical block.
+                new_block = self._allocate_last_physical_block(seq)
+                block_table.append(new_block)
+                return []
+
+        # We want to append the token to the last physical block.
+        last_block = block_table[-1]
+        assert last_block.device == Device.GPU
+        if last_block.ref_count == 1:
+            # Not shared with other sequences. Appendable.
+            if self.enable_caching:
+                # If the last block is now complete, we may reuse an old block
+                # to save memory.
+                maybe_new_block = self._maybe_promote_last_block(
+                    seq, last_block)
+                block_table[-1] = maybe_new_block
+            return []
+        else:
+            # The last block is shared with other sequences.
+            # Copy on Write: Allocate a new block and copy the tokens.
+            new_block = self._allocate_last_physical_block(seq)
+
+            block_table[-1] = new_block
+            self.gpu_allocator.free(last_block)
+            return [(last_block.block_number, new_block.block_number)]
+
+    def fork(self, parent_seq: Sequence, child_seq: Sequence) -> None:
+        # NOTE: fork does not allocate a new physical block.
+        # Thus, it is always safe from OOM.
+        if parent_seq.seq_id not in self.block_tables:
+            # Parent sequence has either been freed or never existed.
+            return
+        src_block_table = self.block_tables[parent_seq.seq_id]
+        self.block_tables[child_seq.seq_id] = src_block_table.copy()
+
+        # When using a sliding window, blocks will be eventually reused.
+        # In this case the block tables will contain repeated blocks.
+        # When forking, we must make sure that each block's `ref_count`
+        # is only incremented by one, so we deduplicate them by wrapping
+        # them in a set.
+        for block in set(src_block_table):
+            block.ref_count += 1
+
+    def _get_physical_blocks(
+            self, seq_group: SequenceGroup) -> List[PhysicalTokenBlock]:
+
+        # NOTE: Here, we assume that the physical blocks are only shared by
+        # the sequences in the same group.
+        request_id = seq_group.request_id
+        blocks: Set[PhysicalTokenBlock] = set()
+        for seq in seq_group.get_seqs():
+            if seq.is_finished():
+                continue
+            blocks.update(self.block_tables[seq.seq_id])
+        # Cross-attention blocks
+        if seq_group.is_encoder_decoder():
+            blocks.update(self.cross_block_tables[request_id])
+        return list(blocks)
+
+    def can_swap_in(self,
+                    seq_group: SequenceGroup,
+                    num_lookahead_slots: int = 0) -> AllocStatus:
+        assert (num_lookahead_slots == 0
+                ), "BlockSpaceManagerV1 does not support lookahead allocation"
+
+        blocks = self._get_physical_blocks(seq_group)
+        num_swapped_seqs = seq_group.num_seqs(status=SequenceStatus.SWAPPED)
+        if seq_group.is_encoder_decoder():
+            num_swapped_seqs += 1
+        num_free_blocks = self.gpu_allocator.get_num_free_blocks()
+        # NOTE: Conservatively, we assume that every sequence will allocate
+        # at least one free block right after the swap-in.
+        # NOTE: This should match the logic in can_append_slot().
+        num_required_blocks = len(blocks) + num_swapped_seqs
+        if self.gpu_allocator.get_num_total_blocks() < num_required_blocks:
+            return AllocStatus.NEVER
+        elif num_free_blocks - num_required_blocks >= self.watermark_blocks:
+            return AllocStatus.OK
+        else:
+            return AllocStatus.LATER
+
+    def _swap_block_table(
+            self, block_table: BlockTable, src_allocator: BlockAllocatorBase,
+            dest_allocator: BlockAllocatorBase,
+            mapping: Dict[PhysicalTokenBlock,
+                          PhysicalTokenBlock]) -> BlockTable:
+        new_block_table: BlockTable = BlockTable()
+
+        for from_block in block_table:
+            if from_block in mapping:
+                to_block = mapping[from_block]
+                to_block.ref_count += 1
+            else:
+                to_block = dest_allocator.allocate(
+                    from_block.block_hash, from_block.num_hashed_tokens)
+                mapping[from_block] = to_block
+            new_block_table.append(to_block)
+            # Free the source block swapped in to destination.
+            src_allocator.free(from_block)
+
+        return new_block_table
+
+    def swap_in(self, seq_group: SequenceGroup) -> List[Tuple[int, int]]:
+
+        request_id = seq_group.request_id
+
+        # CPU block -> GPU block.
+        # dict is efficient in lookup `if cpu_block in mapping`
+        mapping: Dict[PhysicalTokenBlock, PhysicalTokenBlock] = {}
+        for seq in seq_group.get_seqs(status=SequenceStatus.SWAPPED):
+            self.block_tables[seq.seq_id] = \
+                self._swap_block_table(self.block_tables[seq.seq_id],
+                                       self.cpu_allocator, self.gpu_allocator,
+                                       mapping)
+
+        if seq_group.is_encoder_decoder():
+            self.cross_block_tables[request_id] = \
+                self._swap_block_table(self.cross_block_tables[request_id],
+                                       self.cpu_allocator,
+                                       self.gpu_allocator,
+                                       mapping)
+
+        return [(cpu_block.block_number, gpu_block.block_number)
+                for cpu_block, gpu_block in mapping.items()]
+
+    def can_swap_out(self, seq_group: SequenceGroup) -> bool:
+        blocks = self._get_physical_blocks(seq_group)
+        return len(blocks) <= self.cpu_allocator.get_num_free_blocks()
+
+    def swap_out(self, seq_group: SequenceGroup) -> List[Tuple[int, int]]:
+        request_id = seq_group.request_id
+
+        # GPU block -> CPU block.
+        # dict is efficient in lookup `if gpu_block in mapping`
+        mapping: Dict[PhysicalTokenBlock, PhysicalTokenBlock] = {}
+        for seq in seq_group.get_seqs(status=SequenceStatus.RUNNING):
+            self.block_tables[seq.seq_id] = \
+                self._swap_block_table(self.block_tables[seq.seq_id],
+                                       self.gpu_allocator, self.cpu_allocator,
+                                       mapping)
+
+        if seq_group.is_encoder_decoder():
+            self.cross_block_tables[request_id] = \
+                self._swap_block_table(self.cross_block_tables[request_id],
+                                       self.gpu_allocator,
+                                       self.cpu_allocator,
+                                       mapping)
+
+        return [(cpu_block.block_number, gpu_block.block_number)
+                for cpu_block, gpu_block in mapping.items()]
+
+    def _free_block_table(self, block_table: BlockTable) -> None:
+        # when using a sliding window, each seq will only use up
+        # to `self.block_sliding_window` blocks. When freeing
+        # the block table, we must make sure to not free blocks more
+        # than once. If no sliding window is used, there is no block
+        # reuse in the block table, so we must free all blocks.
+        blocks_to_free = (block_table[-self.block_sliding_window:]
+                          if self.block_sliding_window is not None else
+                          block_table)
+        for block in set(blocks_to_free):
+            if block.device == Device.GPU:
+                self.gpu_allocator.free(block)
+            else:
+                self.cpu_allocator.free(block)
+
+    def free(self, seq: Sequence) -> None:
+        if seq.seq_id not in self.block_tables:
+            # Already freed or haven't been scheduled yet.
+            return
+        block_table = self.block_tables[seq.seq_id]
+        self._free_block_table(block_table)
+        del self.block_tables[seq.seq_id]
+
+    def free_cross(self, seq_group: SequenceGroup) -> None:
+        if seq_group.request_id not in self.cross_block_tables:
+            # Already freed or hasn't ben scheduled yet.
+            return
+        block_table = self.cross_block_tables[seq_group.request_id]
+        self._free_block_table(block_table)
+        del self.cross_block_tables[seq_group.request_id]
+
+    def reset(self) -> None:
+        # Free decoder block tables
+        for block_table in self.block_tables.values():
+            self._free_block_table(block_table)
+        self.block_tables.clear()
+        # Free cross-attention block tables
+        for block_table in self.cross_block_tables.values():
+            self._free_block_table(block_table)
+        self.cross_block_tables.clear()
+
+    def get_block_table(self, seq: Sequence) -> List[int]:
+        return self.block_tables[seq.seq_id].ids()
+
+    def get_cross_block_table(self, seq_group: SequenceGroup) -> List[int]:
+        block_table = self.cross_block_tables[seq_group.request_id]
+        return [block.block_number for block in block_table]
+
+    def get_num_free_gpu_blocks(self) -> int:
+        return self.gpu_allocator.get_num_free_blocks()
+
+    def get_num_free_cpu_blocks(self) -> int:
+        return self.cpu_allocator.get_num_free_blocks()
+
+    def access_all_blocks_in_seq(
+        self,
+        seq: Sequence,
+        access_time: float,
+    ) -> None:
+        if self.enable_caching:
+            # Update the last accessed time of all the blocks accessed
+            # in this step.
+            block_table = self.block_tables[seq.seq_id]
+            for block in block_table:
+                block.last_accessed = access_time
+
+    def compute_full_blocks_in_seq(self, seq: Sequence, token_chunk_size: int):
+        if seq.seq_id not in self.block_tables:
+            return
+
+        # When chunked prefill is enabled, the computed full blocks
+        # should be calculated based on the number of computed tokens.
+        max_computed_tokens = (seq.data.get_num_computed_tokens() +
+                               token_chunk_size)
+        computed_full_blocks = max_computed_tokens // self.block_size
+
+        block_table = self.block_tables[seq.seq_id]
+        if computed_full_blocks == 0:
+            return
+        for i in reversed(range(computed_full_blocks)):
+            if block_table[i].computed:
+                break
+            block_table[i].computed = True
+
+    def get_all_computed_blocks(self, seq: Sequence) -> List[int]:
+        if seq.seq_id not in self.block_tables:
+            return []
+        block_table = self.block_tables[seq.seq_id]
+        # NOTE We exclude the last block to avoid the case where the entire
+        # prompt is cached. This would cause erroneous behavior in model
+        # runner.
+        return [
+            b.block_number
+            for b in takewhile(lambda b: b.computed, block_table[:-1])
+        ]
+
+    def get_common_computed_block_ids(
+            self, seqs: List[Sequence]) -> GenericSequence[int]:
+        """Return the block ids that are common for a given sequence group.
+
+        Used in prefill (can skip prefill of some blocks).
+        """
+        # Can return non-empty result only with prefix caching enabled.
+        if not self.enable_caching:
+            return []
+
+        ids_list = [self.get_all_computed_blocks(seq) for seq in seqs]
+        return commonprefix([ids for ids in ids_list if ids != []])
+
+    def mark_blocks_as_computed(self, seq_group: SequenceGroup,
+                                token_chunk_size: int):
+        if self.enable_caching:
+            for seq in seq_group.get_seqs():
+                self.compute_full_blocks_in_seq(seq, token_chunk_size)
+
+    def get_prefix_cache_hit_rate(self, device: Device) -> float:
+        if device == Device.GPU:
+            return self.gpu_allocator.get_prefix_cache_hit_rate()
+        if device == Device.CPU:
+            return self.cpu_allocator.get_prefix_cache_hit_rate()
+        raise ValueError(f"Invalid device: {device}")

vllm/core/block_manager_v2.py

@@ -0,0 +1,505 @@
+"""A block manager that manages token blocks."""
+from typing import Dict, List, Optional
+from typing import Sequence as GenericSequence
+from typing import Tuple
+
+from vllm.core.block.block_table import BlockTable
+from vllm.core.block.cpu_gpu_block_allocator import CpuGpuBlockAllocator
+from vllm.core.block.interfaces import Block
+from vllm.core.block.prefix_caching_block import (ComputedBlocksTracker,
+                                                  LastAccessBlocksTracker)
+from vllm.core.block.utils import check_no_caching_or_swa_for_blockmgr_encdec
+from vllm.core.interfaces import AllocStatus, BlockSpaceManager
+from vllm.sequence import Sequence, SequenceGroup, SequenceStatus
+from vllm.utils import Device
+
+SeqId = int
+EncoderSeqId = str
+
+
+class BlockSpaceManagerV2(BlockSpaceManager):
+    """BlockSpaceManager which manages the allocation of KV cache.
+
+    It owns responsibility for allocation, swapping, allocating memory for
+    autoregressively-generated tokens, and other advanced features such as
+    prefix caching, forking/copy-on-write, and sliding-window memory allocation.
+
+    This class implements the design described in
+    https://github.com/vllm-project/vllm/pull/3492.
+
+    Lookahead slots
+        The block manager has the notion of a "lookahead slot". These are slots
+        in the KV cache that are allocated for a sequence. Unlike the other
+        allocated slots, the content of these slots is undefined -- the worker
+        may use the memory allocations in any way.
+
+        In practice, a worker could use these lookahead slots to run multiple
+        forward passes for a single scheduler invocation. Each successive
+        forward pass would write KV activations to the corresponding lookahead
+        slot. This allows low inter-token latency use-cases, where the overhead
+        of continuous batching scheduling is amortized over >1 generated tokens.
+
+        Speculative decoding uses lookahead slots to store KV activations of
+        proposal tokens.
+
+        See https://github.com/vllm-project/vllm/pull/3250 for more information
+        on lookahead scheduling.
+
+    Args:
+        block_size (int): The size of each memory block.
+        num_gpu_blocks (int): The number of memory blocks allocated on GPU.
+        num_cpu_blocks (int): The number of memory blocks allocated on CPU.
+        watermark (float, optional): The threshold used for memory swapping.
+            Defaults to 0.01.
+        sliding_window (Optional[int], optional): The size of the sliding
+            window. Defaults to None.
+        enable_caching (bool, optional): Flag indicating whether caching is
+            enabled. Defaults to False.
+    """
+
+    def __init__(
+        self,
+        block_size: int,
+        num_gpu_blocks: int,
+        num_cpu_blocks: int,
+        watermark: float = 0.01,
+        sliding_window: Optional[int] = None,
+        enable_caching: bool = False,
+    ) -> None:
+        self.block_size = block_size
+        self.num_total_gpu_blocks = num_gpu_blocks
+        self.num_total_cpu_blocks = num_cpu_blocks
+
+        self.sliding_window = sliding_window
+        # max_block_sliding_window is the max number of blocks that need to be
+        # allocated
+        self.max_block_sliding_window = None
+        if sliding_window is not None:
+            # +1 here because // rounds down
+            num_blocks = sliding_window // block_size + 1
+            # +1 here because the last block may not be full,
+            # and so the sequence stretches one more block at the beginning
+            # For example, if sliding_window is 3 and block_size is 4,
+            # we may need 2 blocks when the second block only holds 1 token.
+            self.max_block_sliding_window = num_blocks + 1
+
+        self.watermark = watermark
+        assert watermark >= 0.0
+
+        self.enable_caching = enable_caching
+
+        self.watermark_blocks = int(watermark * num_gpu_blocks)
+
+        self.block_allocator = CpuGpuBlockAllocator.create(
+            allocator_type="prefix_caching" if enable_caching else "naive",
+            num_gpu_blocks=num_gpu_blocks,
+            num_cpu_blocks=num_cpu_blocks,
+            block_size=block_size,
+        )
+
+        self.block_tables: Dict[SeqId, BlockTable] = {}
+        self.cross_block_tables: Dict[EncoderSeqId, BlockTable] = {}
+
+        self._computed_blocks_tracker = ComputedBlocksTracker(
+            self.block_allocator)
+        self._last_access_blocks_tracker = LastAccessBlocksTracker(
+            self.block_allocator)
+
+    def can_allocate(self,
+                     seq_group: SequenceGroup,
+                     num_lookahead_slots: int = 0) -> AllocStatus:
+        # FIXME(woosuk): Here we assume that all sequences in the group share
+        # the same prompt. This may not be true for preempted sequences.
+
+        check_no_caching_or_swa_for_blockmgr_encdec(self, seq_group)
+
+        seq = seq_group.get_seqs(status=SequenceStatus.WAITING)[0]
+        num_required_blocks = BlockTable.get_num_required_blocks(
+            seq.get_token_ids(),
+            block_size=self.block_size,
+            num_lookahead_slots=num_lookahead_slots,
+        )
+
+        if seq_group.is_encoder_decoder():
+            encoder_seq = seq_group.get_encoder_seq()
+            assert encoder_seq is not None
+            num_required_blocks += BlockTable.get_num_required_blocks(
+                encoder_seq.get_token_ids(),
+                block_size=self.block_size,
+            )
+
+        if self.max_block_sliding_window is not None:
+            num_required_blocks = min(num_required_blocks,
+                                      self.max_block_sliding_window)
+
+        num_free_gpu_blocks = self.block_allocator.get_num_free_blocks(
+            device=Device.GPU)
+
+        # Use watermark to avoid frequent cache eviction.
+        if (self.num_total_gpu_blocks - num_required_blocks <
+                self.watermark_blocks):
+            return AllocStatus.NEVER
+        if num_free_gpu_blocks - num_required_blocks >= self.watermark_blocks:
+            return AllocStatus.OK
+        else:
+            return AllocStatus.LATER
+
+    def _allocate_sequence(self, seq: Sequence) -> BlockTable:
+        block_table = BlockTable(
+            block_size=self.block_size,
+            block_allocator=self.block_allocator,
+            max_block_sliding_window=self.max_block_sliding_window,
+        )
+        if seq.get_token_ids():
+            # Add blocks to the block table only if the sequence is non empty.
+            block_table.allocate(seq.get_token_ids())
+
+        return block_table
+
+    def allocate(self, seq_group: SequenceGroup) -> None:
+
+        # Allocate self-attention block tables for decoder sequences
+        waiting_seqs = seq_group.get_seqs(status=SequenceStatus.WAITING)
+        assert not (set(seq.seq_id for seq in waiting_seqs)
+                    & self.block_tables.keys()), "block table already exists"
+
+        # NOTE: Here we assume that all sequences in the group have the same
+        # prompt.
+        seq = waiting_seqs[0]
+        block_table: BlockTable = self._allocate_sequence(seq)
+        self.block_tables[seq.seq_id] = block_table
+
+        # Track seq
+        self._computed_blocks_tracker.add_seq(seq.seq_id)
+        self._last_access_blocks_tracker.add_seq(seq.seq_id)
+
+        # Assign the block table for each sequence.
+        for seq in waiting_seqs[1:]:
+            self.block_tables[seq.seq_id] = block_table.fork()
+
+            # Track seq
+            self._computed_blocks_tracker.add_seq(seq.seq_id)
+            self._last_access_blocks_tracker.add_seq(seq.seq_id)
+
+        # Allocate cross-attention block table for encoder sequence
+        #
+        # NOTE: Here we assume that all sequences in the group have the same
+        # encoder prompt.
+        request_id = seq_group.request_id
+
+        assert (request_id
+                not in self.cross_block_tables), \
+            "block table already exists"
+
+        check_no_caching_or_swa_for_blockmgr_encdec(self, seq_group)
+
+        if seq_group.is_encoder_decoder():
+            encoder_seq = seq_group.get_encoder_seq()
+            assert encoder_seq is not None
+            block_table = self._allocate_sequence(encoder_seq)
+            self.cross_block_tables[request_id] = block_table
+
+    def can_append_slots(self, seq_group: SequenceGroup,
+                         num_lookahead_slots: int) -> bool:
+        """Determine if there is enough space in the GPU KV cache to continue
+        generation of the specified sequence group.
+
+        We use a worst-case heuristic: assume each touched block will require a
+        new allocation (either via CoW or new block). We can append slots if the
+        number of touched blocks is less than the number of free blocks.
+
+        "Lookahead slots" are slots that are allocated in addition to the slots
+        for known tokens. The contents of the lookahead slots are not defined.
+        This is used by speculative decoding when speculating future tokens.
+        """
+
+        num_touched_blocks = 0
+        for seq in seq_group.get_seqs(status=SequenceStatus.RUNNING):
+            block_table = self.block_tables[seq.seq_id]
+
+            num_touched_blocks += (
+                block_table.get_num_blocks_touched_by_append_slots(
+                    token_ids=block_table.get_unseen_token_ids(
+                        seq.get_token_ids()),
+                    num_lookahead_slots=num_lookahead_slots,
+                ))
+
+        num_free_gpu_blocks = self.block_allocator.get_num_free_blocks(
+            Device.GPU)
+        return num_touched_blocks <= num_free_gpu_blocks
+
+    def append_slots(
+        self,
+        seq: Sequence,
+        num_lookahead_slots: int,
+    ) -> List[Tuple[int, int]]:
+
+        block_table = self.block_tables[seq.seq_id]
+
+        block_table.append_token_ids(
+            token_ids=block_table.get_unseen_token_ids(seq.get_token_ids()),
+            num_lookahead_slots=num_lookahead_slots,
+            num_computed_slots=seq.data.get_num_computed_tokens(),
+        )
+        # Return any new copy-on-writes.
+        new_cows = self.block_allocator.clear_copy_on_writes()
+        return new_cows
+
+    def free(self, seq: Sequence) -> None:
+        seq_id = seq.seq_id
+
+        if seq_id not in self.block_tables:
+            # Already freed or haven't been scheduled yet.
+            return
+
+        # Update seq block ids with the latest access time
+        self._last_access_blocks_tracker.update_seq_blocks_last_access(
+            seq_id, self.block_tables[seq.seq_id].physical_block_ids)
+
+        # Untrack seq
+        self._last_access_blocks_tracker.remove_seq(seq_id)
+        self._computed_blocks_tracker.remove_seq(seq_id)
+
+        # Free table/blocks
+        self.block_tables[seq_id].free()
+        del self.block_tables[seq_id]
+
+    def free_cross(self, seq_group: SequenceGroup) -> None:
+        request_id = seq_group.request_id
+        if request_id not in self.cross_block_tables:
+            # Already freed or hasn't been scheduled yet.
+            return
+        self.cross_block_tables[request_id].free()
+        del self.cross_block_tables[request_id]
+
+    def get_block_table(self, seq: Sequence) -> List[int]:
+        block_ids = self.block_tables[seq.seq_id].physical_block_ids
+        return block_ids  # type: ignore
+
+    def get_cross_block_table(self, seq_group: SequenceGroup) -> List[int]:
+        request_id = seq_group.request_id
+        assert request_id in self.cross_block_tables
+        block_ids = self.cross_block_tables[request_id].physical_block_ids
+        assert all(b is not None for b in block_ids)
+        return block_ids  # type: ignore
+
+    def access_all_blocks_in_seq(self, seq: Sequence, now: float):
+        if self.enable_caching:
+            # Record the latest access time for the sequence. The actual update
+            # of the block ids is deferred to the sequence free(..) call, since
+            # only during freeing of block ids, the blocks are actually added to
+            # the evictor (which is when the most updated time is required)
+            # (This avoids expensive calls to mark_blocks_as_accessed(..))
+            self._last_access_blocks_tracker.update_last_access(
+                seq.seq_id, now)
+
+    def mark_blocks_as_computed(self, seq_group: SequenceGroup,
+                                token_chunk_size: int):
+        # If prefix caching is enabled, mark immutable blocks as computed
+        # right after they have been scheduled (for prefill). This assumes
+        # the scheduler is synchronous so blocks are actually computed when
+        # scheduling the next batch.
+        self.block_allocator.mark_blocks_as_computed([])
+
+    def get_common_computed_block_ids(
+            self, seqs: List[Sequence]) -> GenericSequence[int]:
+        """Determine which blocks for which we skip prefill.
+
+        With prefix caching we can skip prefill for previously-generated blocks.
+        Currently, the attention implementation only supports skipping cached
+        blocks if they are a contiguous prefix of cached blocks.
+
+        This method determines which blocks can be safely skipped for all
+        sequences in the sequence group.
+        """
+        computed_seq_block_ids = []
+        for seq in seqs:
+            computed_seq_block_ids.append(
+                self._computed_blocks_tracker.
+                get_cached_computed_blocks_and_update(
+                    seq.seq_id,
+                    self.block_tables[seq.seq_id].physical_block_ids))
+
+        # NOTE(sang): This assumes seq_block_ids doesn't contain any None.
+        return self.block_allocator.get_common_computed_block_ids(
+            computed_seq_block_ids)  # type: ignore
+
+    def fork(self, parent_seq: Sequence, child_seq: Sequence) -> None:
+        if parent_seq.seq_id not in self.block_tables:
+            # Parent sequence has either been freed or never existed.
+            return
+        src_block_table = self.block_tables[parent_seq.seq_id]
+        self.block_tables[child_seq.seq_id] = src_block_table.fork()
+
+        # Track child seq
+        self._computed_blocks_tracker.add_seq(child_seq.seq_id)
+        self._last_access_blocks_tracker.add_seq(child_seq.seq_id)
+
+    def can_swap_in(self, seq_group: SequenceGroup,
+                    num_lookahead_slots: int) -> AllocStatus:
+        """Returns the AllocStatus for the given sequence_group 
+        with num_lookahead_slots.
+
+        Args:
+            sequence_group (SequenceGroup): The sequence group to swap in.
+            num_lookahead_slots (int): Number of lookahead slots used in 
+                speculative decoding, default to 0.
+
+        Returns:
+            AllocStatus: The AllocStatus for the given sequence group.
+        """
+        return self._can_swap(seq_group, Device.GPU, SequenceStatus.SWAPPED,
+                              num_lookahead_slots)
+
+    def swap_in(self, seq_group: SequenceGroup) -> List[Tuple[int, int]]:
+        """Returns the block id mapping (from CPU to GPU) generated by
+        swapping in the given seq_group with num_lookahead_slots.
+
+        Args:
+            seq_group (SequenceGroup): The sequence group to swap in.
+
+        Returns:
+            List[Tuple[int, int]]: The mapping of swapping block from CPU 
+                to GPU.
+        """
+        physical_block_id_mapping = []
+        for seq in seq_group.get_seqs(status=SequenceStatus.SWAPPED):
+            blocks = self.block_tables[seq.seq_id].blocks
+            if len(blocks) == 0:
+                continue
+
+            seq_swap_mapping = self.block_allocator.swap(blocks=blocks,
+                                                         src_device=Device.CPU,
+                                                         dst_device=Device.GPU)
+
+            # Refresh the block ids of the table (post-swap)
+            self.block_tables[seq.seq_id].update(blocks)
+
+            seq_physical_block_id_mapping = {
+                self.block_allocator.get_physical_block_id(
+                    Device.CPU, cpu_block_id):
+                self.block_allocator.get_physical_block_id(
+                    Device.GPU, gpu_block_id)
+                for cpu_block_id, gpu_block_id in seq_swap_mapping.items()
+            }
+
+            physical_block_id_mapping.extend(
+                list(seq_physical_block_id_mapping.items()))
+
+        return physical_block_id_mapping
+
+    def can_swap_out(self, seq_group: SequenceGroup) -> bool:
+        """Returns whether we can swap out the given sequence_group 
+        with num_lookahead_slots.
+
+        Args:
+            seq_group (SequenceGroup): The sequence group to swap in.
+            num_lookahead_slots (int): Number of lookahead slots used in 
+                speculative decoding, default to 0.
+
+        Returns:
+            bool: Whether it's possible to swap out current sequence group.
+        """
+        alloc_status = self._can_swap(seq_group, Device.CPU,
+                                      SequenceStatus.RUNNING)
+        return alloc_status == AllocStatus.OK
+
+    def swap_out(self, seq_group: SequenceGroup) -> List[Tuple[int, int]]:
+        """Returns the block id mapping (from GPU to CPU) generated by
+        swapping out the given sequence_group with num_lookahead_slots.
+
+        Args:
+            sequence_group (SequenceGroup): The sequence group to swap in.
+
+        Returns:
+            List[Tuple[int, int]]: The mapping of swapping block from 
+                GPU to CPU.
+        """
+        physical_block_id_mapping = []
+        for seq in seq_group.get_seqs(status=SequenceStatus.RUNNING):
+            blocks = self.block_tables[seq.seq_id].blocks
+            if len(blocks) == 0:
+                continue
+
+            seq_swap_mapping = self.block_allocator.swap(blocks=blocks,
+                                                         src_device=Device.GPU,
+                                                         dst_device=Device.CPU)
+
+            # Refresh the block ids of the table (post-swap)
+            self.block_tables[seq.seq_id].update(blocks)
+
+            seq_physical_block_id_mapping = {
+                self.block_allocator.get_physical_block_id(
+                    Device.GPU, gpu_block_id):
+                self.block_allocator.get_physical_block_id(
+                    Device.CPU, cpu_block_id)
+                for gpu_block_id, cpu_block_id in seq_swap_mapping.items()
+            }
+
+            physical_block_id_mapping.extend(
+                list(seq_physical_block_id_mapping.items()))
+
+        return physical_block_id_mapping
+
+    def get_num_free_gpu_blocks(self) -> int:
+        return self.block_allocator.get_num_free_blocks(Device.GPU)
+
+    def get_num_free_cpu_blocks(self) -> int:
+        return self.block_allocator.get_num_free_blocks(Device.CPU)
+
+    def get_prefix_cache_hit_rate(self, device: Device) -> float:
+        return self.block_allocator.get_prefix_cache_hit_rate(device)
+
+    def _can_swap(self,
+                  seq_group: SequenceGroup,
+                  device: Device,
+                  status: SequenceStatus,
+                  num_lookahead_slots: int = 0) -> AllocStatus:
+        """Returns the AllocStatus for swapping in/out the given sequence_group 
+        on to the 'device'.
+
+        Args:
+            sequence_group (SequenceGroup): The sequence group to swap in.
+            device (Device): device to swap the 'seq_group' on.
+            status (SequenceStatus): The status of sequence which is needed
+                for action. RUNNING for swap out and SWAPPED for swap in
+            num_lookahead_slots (int): Number of lookahead slots used in 
+                speculative decoding, default to 0.
+
+        Returns:
+            AllocStatus: The AllocStatus for swapping in/out the given 
+                sequence_group on to the 'device'.
+        """
+        # First determine the number of blocks that will be touched by this
+        # swap. Then verify if there are available blocks in the device
+        # to perform the swap.
+        num_blocks_touched = 0
+        blocks: List[Block] = []
+        for seq in seq_group.get_seqs(status=status):
+            block_table = self.block_tables[seq.seq_id]
+            if block_table.blocks is not None:
+                # Compute the number blocks to touch for the tokens to be
+                # appended. This does NOT include the full blocks that need
+                # to be touched for the swap.
+                num_blocks_touched += \
+                    block_table.get_num_blocks_touched_by_append_slots(
+                        block_table.get_unseen_token_ids(seq.get_token_ids()),
+                        num_lookahead_slots=num_lookahead_slots)
+                blocks.extend(block_table.blocks)
+        # Compute the number of full blocks to touch and add it to the
+        # existing count of blocks to touch.
+        num_blocks_touched += self.block_allocator.get_num_full_blocks_touched(
+            blocks, device=device)
+
+        watermark_blocks = 0
+        if device == Device.GPU:
+            watermark_blocks = self.watermark_blocks
+
+        if self.block_allocator.get_num_total_blocks(
+                device) < num_blocks_touched:
+            return AllocStatus.NEVER
+        elif self.block_allocator.get_num_free_blocks(
+                device) - num_blocks_touched >= watermark_blocks:
+            return AllocStatus.OK
+        else:
+            return AllocStatus.LATER

vllm/core/evictor_v1.py

@@ -0,0 +1,106 @@
+import enum
+from abc import ABC, abstractmethod
+from typing import OrderedDict
+
+from vllm.block import PhysicalTokenBlock
+
+
+class EvictionPolicy(enum.Enum):
+    """Enum for eviction policy used by make_evictor to instantiate the correct
+       Evictor subclass.
+    """
+    LRU = enum.auto()
+
+
+class Evictor(ABC):
+    """The Evictor subclasses should be used by the BlockAllocator class to
+    handle eviction of freed PhysicalTokenBlocks.
+    """
+
+    @abstractmethod
+    def __init__(self):
+        pass
+
+    @abstractmethod
+    def __contains__(self, block_hash: int) -> bool:
+        pass
+
+    @abstractmethod
+    def evict(self) -> PhysicalTokenBlock:
+        """Runs the eviction algorithm and returns the evicted block"""
+        pass
+
+    @abstractmethod
+    def add(self, block: PhysicalTokenBlock):
+        """Adds block to the evictor, making it a candidate for eviction"""
+        pass
+
+    @abstractmethod
+    def remove(self, block_hash: int) -> PhysicalTokenBlock:
+        """Simply removes the block with the hash value block_hash from the
+        evictor. Caller is responsible for making sure that block_hash is
+        contained in the evictor before calling remove. Should be used to
+        "bring back" blocks that have been freed but not evicted yet.
+        """
+        pass
+
+    @property
+    @abstractmethod
+    def num_blocks(self) -> int:
+        pass
+
+
+class LRUEvictor(Evictor):
+    """Evicts in a least-recently-used order using the last_accessed timestamp
+    that's recorded in the PhysicalTokenBlock. If there are multiple blocks with
+    the same last_accessed time, then the one with the largest num_hashed_tokens
+    will be evicted. If two blocks each have the lowest last_accessed time and
+    highest num_hashed_tokens value, then one will be chose arbitrarily
+    """
+
+    def __init__(self):
+        self.free_table: OrderedDict[int, PhysicalTokenBlock] = OrderedDict()
+
+    def __contains__(self, block_hash: int) -> bool:
+        return block_hash in self.free_table
+
+    def evict(self) -> PhysicalTokenBlock:
+        if len(self.free_table) == 0:
+            raise ValueError("No usable cache memory left")
+
+        evicted_block = next(iter(self.free_table.values()))
+        # The blocks with the lowest timestamps should be placed consecutively
+        # at the start of OrderedDict. Loop through all these blocks to
+        # find the one with maximum number of hashed tokens.
+        for _, block in self.free_table.items():
+            if evicted_block.last_accessed < block.last_accessed:
+                break
+            if evicted_block.num_hashed_tokens < block.num_hashed_tokens:
+                evicted_block = block
+
+        self.free_table.pop(evicted_block.block_hash)
+
+        evicted_block.computed = False
+        return evicted_block
+
+    def add(self, block: PhysicalTokenBlock):
+        self.free_table[block.block_hash] = block
+
+    def remove(self, block_hash: int) -> PhysicalTokenBlock:
+        if block_hash not in self.free_table:
+            raise ValueError(
+                "Attempting to remove block that's not in the evictor")
+        block: PhysicalTokenBlock = self.free_table[block_hash]
+        self.free_table.pop(block_hash)
+        return block
+
+    @property
+    def num_blocks(self) -> int:
+        return len(self.free_table)
+
+
+def make_evictor(eviction_policy: EvictionPolicy) -> Evictor:
+    if eviction_policy == EvictionPolicy.LRU:
+        return LRUEvictor()
+    else:
+        raise ValueError(f"Unknown cache eviction policy: {eviction_policy}")

vllm/core/evictor_v2.py

@@ -0,0 +1,131 @@
+import enum
+from abc import ABC, abstractmethod
+from typing import OrderedDict, Tuple
+
+
+class EvictionPolicy(enum.Enum):
+    """Enum for eviction policy used by make_evictor to instantiate the correct
+       Evictor subclass.
+    """
+    LRU = enum.auto()
+
+
+class Evictor(ABC):
+    """The Evictor subclasses should be used by the BlockAllocator class to
+    handle eviction of freed PhysicalTokenBlocks.
+    """
+
+    @abstractmethod
+    def __init__(self):
+        pass
+
+    @abstractmethod
+    def __contains__(self, block_id: int) -> bool:
+        pass
+
+    @abstractmethod
+    def evict(self) -> Tuple[int, int]:
+        """Runs the eviction algorithm and returns the evicted block's
+        content hash along with physical block id along with physical block id
+        """
+        pass
+
+    @abstractmethod
+    def add(self, block_id: int, content_hash: int, num_hashed_tokens: int,
+            last_accessed: float):
+        """Adds block to the evictor, making it a candidate for eviction"""
+        pass
+
+    @abstractmethod
+    def update(self, block_id: int, last_accessed: float):
+        """Update corresponding block's access time in metadata"""
+        pass
+
+    @abstractmethod
+    def remove(self, block_id: int):
+        """Remove a given block id from the cache."""
+        pass
+
+    @property
+    @abstractmethod
+    def num_blocks(self) -> int:
+        pass
+
+
+class BlockMetaData():
+    """Data structure for storing key data describe cached block, so that
+    evitor could use to make its decision which one to choose for eviction
+
+    Here we use physical block id as the dict key, as there maybe several
+    blocks with the same content hash, but their physical id is unique.
+    """
+
+    def __init__(self, content_hash: int, num_hashed_tokens: int,
+                 last_accessed: float):
+        self.content_hash = content_hash
+        self.num_hashed_tokens = num_hashed_tokens
+        self.last_accessed = last_accessed
+
+
+class LRUEvictor(Evictor):
+    """Evicts in a least-recently-used order using the last_accessed timestamp
+    that's recorded in the PhysicalTokenBlock. If there are multiple blocks with
+    the same last_accessed time, then the one with the largest num_hashed_tokens
+    will be evicted. If two blocks each have the lowest last_accessed time and
+    highest num_hashed_tokens value, then one will be chose arbitrarily
+    """
+
+    def __init__(self):
+        self.free_table: OrderedDict[int, BlockMetaData] = OrderedDict()
+
+    def __contains__(self, block_id: int) -> bool:
+        return block_id in self.free_table
+
+    def evict(self) -> Tuple[int, int]:
+        if len(self.free_table) == 0:
+            raise ValueError("No usable cache memory left")
+
+        evicted_block, evicted_block_id = None, None
+        # The blocks with the lowest timestamps should be placed consecutively
+        # at the start of OrderedDict. Loop through all these blocks to
+        # find the one with maximum number of hashed tokens.
+        for _id, block in self.free_table.items():
+            if evicted_block is None:
+                evicted_block, evicted_block_id = block, _id
+                continue
+            if evicted_block.last_accessed < block.last_accessed:
+                break
+            if evicted_block.num_hashed_tokens < block.num_hashed_tokens:
+                evicted_block, evicted_block_id = block, _id
+
+        assert evicted_block is not None
+        assert evicted_block_id is not None
+        self.free_table.pop(evicted_block_id)
+
+        return evicted_block_id, evicted_block.content_hash
+
+    def add(self, block_id: int, content_hash: int, num_hashed_tokens: int,
+            last_accessed: float):
+        self.free_table[block_id] = BlockMetaData(content_hash,
+                                                  num_hashed_tokens,
+                                                  last_accessed)
+
+    def update(self, block_id: int, last_accessed: float):
+        self.free_table[block_id].last_accessed = last_accessed
+
+    def remove(self, block_id: int):
+        if block_id not in self.free_table:
+            raise ValueError(
+                "Attempting to remove block that's not in the evictor")
+        self.free_table.pop(block_id)
+
+    @property
+    def num_blocks(self) -> int:
+        return len(self.free_table)
+
+
+def make_evictor(eviction_policy: EvictionPolicy) -> Evictor:
+    if eviction_policy == EvictionPolicy.LRU:
+        return LRUEvictor()
+    else:
+        raise ValueError(f"Unknown cache eviction policy: {eviction_policy}")

vllm/core/interfaces.py

@@ -0,0 +1,127 @@
+import enum
+from abc import ABC, abstractmethod
+from typing import List
+from typing import Sequence as GenericSequence
+from typing import Tuple
+
+from vllm.sequence import Sequence, SequenceGroup
+from vllm.utils import Device
+
+
+class AllocStatus(enum.Enum):
+    """Result for BlockSpaceManager.can_allocate
+
+    1. Ok: seq_group can be allocated now.
+    2. Later: seq_group cannot be allocated.
+      The capacity of allocator is larger than seq_group required.
+    3. Never: seq_group can never be allocated.
+      The seq_group is too large to allocated in GPU.
+    """
+    OK = enum.auto()
+    LATER = enum.auto()
+    NEVER = enum.auto()
+
+
+class BlockSpaceManager(ABC):
+
+    @staticmethod
+    def get_block_space_manager_class(version: str):
+        version = version.lower()
+
+        if version == "v1":
+            from vllm.core.block_manager_v1 import BlockSpaceManagerV1
+            return BlockSpaceManagerV1
+
+        if version == "v2":
+            from vllm.core.block_manager_v2 import BlockSpaceManagerV2
+            return BlockSpaceManagerV2
+
+        if version == "placeholder":
+            from vllm.core.placeholder_block_space_manager import (
+                PlaceholderBlockSpaceManager)
+            return PlaceholderBlockSpaceManager
+
+        raise ValueError(f"Unknown version {version=}")
+
+    @abstractmethod
+    def can_allocate(self,
+                     seq_group: SequenceGroup,
+                     num_lookahead_slots: int = 0) -> AllocStatus:
+        pass
+
+    @abstractmethod
+    def allocate(self, seq_group: SequenceGroup) -> None:
+        pass
+
+    @abstractmethod
+    def can_append_slots(self, seq_group: SequenceGroup,
+                         num_lookahead_slots: int) -> bool:
+        pass
+
+    @abstractmethod
+    def append_slots(
+        self,
+        seq: Sequence,
+        num_lookahead_slots: int,
+    ) -> List[Tuple[int, int]]:
+        pass
+
+    @abstractmethod
+    def fork(self, parent_seq: Sequence, child_seq: Sequence) -> None:
+        pass
+
+    @abstractmethod
+    def can_swap_in(self, seq_group: SequenceGroup,
+                    num_lookahead_slots: int) -> AllocStatus:
+        pass
+
+    @abstractmethod
+    def swap_in(self, seq_group: SequenceGroup) -> List[Tuple[int, int]]:
+        pass
+
+    @abstractmethod
+    def can_swap_out(self, seq_group: SequenceGroup) -> bool:
+        pass
+
+    @abstractmethod
+    def swap_out(self, seq_group: SequenceGroup) -> List[Tuple[int, int]]:
+        pass
+
+    @abstractmethod
+    def free(self, seq: Sequence) -> None:
+        pass
+
+    @abstractmethod
+    def get_block_table(self, seq: Sequence) -> List[int]:
+        pass
+
+    @abstractmethod
+    def get_num_free_gpu_blocks(self) -> int:
+        pass
+
+    @abstractmethod
+    def get_num_free_cpu_blocks(self) -> int:
+        pass
+
+    @abstractmethod
+    def access_all_blocks_in_seq(
+        self,
+        seq: Sequence,
+        access_time: float,
+    ) -> None:
+        pass
+
+    @abstractmethod
+    def get_common_computed_block_ids(
+            self, seqs: List[Sequence]) -> GenericSequence[int]:
+        pass
+
+    @abstractmethod
+    def mark_blocks_as_computed(self, seq_group: SequenceGroup,
+                                token_chunk_size: int):
+        pass
+
+    @abstractmethod
+    def get_prefix_cache_hit_rate(self, device: Device) -> float:
+        """Prefix cache hit rate. -1 means not supported or disabled."""
+        pass

vllm/core/placeholder_block_space_manager.py

@@ -0,0 +1,91 @@
+from typing import List, Tuple
+
+from vllm.core.interfaces import AllocStatus, BlockSpaceManager
+from vllm.sequence import Sequence, SequenceGroup
+from vllm.utils import Device
+
+
+class PlaceholderBlockSpaceManager(BlockSpaceManager):
+    """A version of BlockSpaceManager for use in environments
+    where block management is not required. 
+    For example: embedding models or attention-free models like Mamba.
+
+    This class provides the same interface as BlockSpaceManager, but its
+    methods perform no actions or return simple values like True in specific
+    actions. It's designed to be used in scenarios where the overhead of
+    block management is unnecessary, such as in an embedding environment.
+    """
+
+    def __init__(
+        self,
+        **kwargs,
+    ) -> None:
+        pass
+
+    def can_allocate(self,
+                     seq_group: SequenceGroup,
+                     num_lookahead_slots: int = 0) -> AllocStatus:
+        # Always return OK for dummy purposes
+        return AllocStatus.OK
+
+    def allocate(self, seq_group: SequenceGroup) -> None:
+        # No actual allocation logic needed
+        pass
+
+    def can_append_slots(self, seq_group: SequenceGroup,
+                         num_lookahead_slots: int) -> bool:
+        return True
+
+    def append_slots(
+        self,
+        seq: Sequence,
+        num_lookahead_slots: int,
+    ) -> List[Tuple[int, int]]:
+        return []
+
+    def fork(self, parent_seq: Sequence, child_seq: Sequence) -> None:
+        pass
+
+    def can_swap_in(self, seq_group: SequenceGroup,
+                    num_lookahead_slots: int) -> AllocStatus:
+        return AllocStatus.OK
+
+    def swap_in(self, seq_group: SequenceGroup) -> List[Tuple[int, int]]:
+        return None  # type: ignore
+
+    def can_swap_out(self, seq_group: SequenceGroup) -> bool:
+        return True
+
+    def swap_out(self, seq_group: SequenceGroup) -> List[Tuple[int, int]]:
+        return None  # type: ignore
+
+    def free(self, seq: Sequence) -> None:
+        # No operation on free
+        return
+
+    def get_block_table(self, seq: Sequence) -> List[int]:
+        return None  # type: ignore
+
+    def get_num_free_gpu_blocks(self) -> int:
+        return 1
+
+    def get_num_free_cpu_blocks(self) -> int:
+        return 1
+
+    def access_all_blocks_in_seq(
+        self,
+        seq: Sequence,
+        access_time: float,
+    ) -> None:
+        pass
+
+    def get_common_computed_block_ids(self,
+                                      seq_group: List[Sequence]) -> List[int]:
+        return []
+
+    def mark_blocks_as_computed(self, seq_group: SequenceGroup,
+                                token_chunk_size: int):
+        pass
+
+    def get_prefix_cache_hit_rate(self, device: Device) -> float:
+        return -1