384 lines
15 KiB
Python
384 lines
15 KiB
Python
from dataclasses import dataclass
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from typing import List, Optional, Tuple
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import sys
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import torch
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import traceback
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from vllm import _custom_ops as ops
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# from vllm.attention.ops.prefix_prefill import context_attention_fwd
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# NOTE: context_attention_fwd (Triton kernel from prefix_prefill.py) is NOT
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# imported here. On Iluvatar BI-V100 that kernel hangs the GPU card
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# permanently. Chunked-prefill / prefix-caching attention is handled by
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# _forward_prefix_pytorch below (pure PyTorch, no Triton dependency).
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# Should be the same as PARTITION_SIZE in `paged_attention_v2_launcher`.
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_PARTITION_SIZE = 512
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@dataclass
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class PagedAttentionMetadata:
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"""Metadata for PagedAttention."""
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# (batch_size,). The length of sequences (entire tokens seen so far) per
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# sequence.
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seq_lens_tensor: Optional[torch.Tensor]
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# Maximum sequence length in the batch. 0 if it is prefill-only batch.
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max_decode_seq_len: int
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# (batch_size, max_blocks_per_seq).
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# Block addresses per sequence. (Seq id -> list of physical block)
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# E.g., [0, 1, 2] means tokens are stored in 0th, 1st, and 2nd blocks
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# in the kv cache. Each block can contain up to block_size tokens.
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# 2nd dimensions are padded up to max_blocks_per_seq if it is cuda-graph
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# captured.
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block_tables: Optional[torch.Tensor]
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class PagedAttention:
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@staticmethod
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def get_supported_head_sizes() -> List[int]:
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return [64, 80, 96, 112, 120, 128, 192, 256]
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@staticmethod
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def get_kv_cache_shape(
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num_blocks: int,
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block_size: int,
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num_kv_heads: int,
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head_size: int,
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) -> Tuple[int, ...]:
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return (2, num_blocks, block_size * num_kv_heads * head_size)
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@staticmethod
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def split_kv_cache(
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kv_cache: torch.Tensor,
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num_kv_heads: int,
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head_size: int,
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) -> Tuple[torch.Tensor, torch.Tensor]:
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x = 16 // kv_cache.element_size()
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num_blocks = kv_cache.shape[1]
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key_cache = kv_cache[0]
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key_cache = key_cache.view(num_blocks, num_kv_heads, head_size // x,
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-1, x)
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value_cache = kv_cache[1]
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value_cache = value_cache.view(num_blocks, num_kv_heads, head_size, -1)
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return key_cache, value_cache
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@staticmethod
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def write_to_paged_cache(
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key: torch.Tensor,
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value: torch.Tensor,
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key_cache: torch.Tensor,
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value_cache: torch.Tensor,
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slot_mapping: torch.Tensor,
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kv_cache_dtype: str,
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k_scale: float,
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v_scale: float,
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) -> None:
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ops.reshape_and_cache(
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key,
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value,
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key_cache,
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value_cache,
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slot_mapping.flatten(),
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kv_cache_dtype,
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k_scale,
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v_scale,
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)
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@staticmethod
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def forward_decode(
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query: torch.Tensor,
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key_cache: torch.Tensor,
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value_cache: torch.Tensor,
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block_tables: torch.Tensor,
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seq_lens: torch.Tensor,
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max_seq_len: int,
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kv_cache_dtype: str,
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num_kv_heads: int,
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scale: float,
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alibi_slopes: Optional[torch.Tensor],
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k_scale: float,
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v_scale: float,
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tp_rank: int = 0,
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blocksparse_local_blocks: int = 0,
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blocksparse_vert_stride: int = 0,
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blocksparse_block_size: int = 64,
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blocksparse_head_sliding_step: int = 0,
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) -> torch.Tensor:
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if blocksparse_vert_stride is not None and blocksparse_vert_stride > 1:
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# use blocksparse paged attention
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block_size = value_cache.size(-1)
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assert (blocksparse_block_size > 0 and
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blocksparse_block_size % block_size == 0), \
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(f"{blocksparse_block_size=} needs to be a multiple of"
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f"{block_size=} used in block_tables.")
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output = torch.empty_like(query)
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block_size = value_cache.shape[3]
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num_seqs, num_heads, head_size = query.shape
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max_num_partitions = ((max_seq_len + _PARTITION_SIZE - 1) //
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_PARTITION_SIZE)
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# NOTE(woosuk): We use a simple heuristic to decide whether to use
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# PagedAttention V1 or V2. If the number of partitions is 1, we use
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# V1 to avoid the overhead of reduction. Also, if the number of
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# sequences or heads is large, we use V1 since there is enough work
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# to parallelize.
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# TODO(woosuk): Tune this heuristic.
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# For context len > 8192, use V2 kernel to avoid shared memory shortage.
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use_v1 = (max_seq_len <= 8192
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and (max_num_partitions == 1 or num_seqs * num_heads > 512))
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use_v1 = True
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if use_v1:
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# Run PagedAttention V1.
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ops.paged_attention_v1(
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output,
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query,
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key_cache,
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value_cache,
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num_kv_heads,
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scale,
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block_tables,
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seq_lens,
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block_size,
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max_seq_len,
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alibi_slopes,
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)
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else:
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# Run PagedAttention V2.
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assert _PARTITION_SIZE % block_size == 0
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tmp_output = torch.empty(
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size=(num_seqs, num_heads, max_num_partitions, head_size),
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dtype=output.dtype,
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device=output.device,
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)
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exp_sums = torch.empty(
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size=(num_seqs, num_heads, max_num_partitions),
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dtype=torch.float32,
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device=output.device,
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)
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max_logits = torch.empty_like(exp_sums)
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ops.paged_attention_v2(
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output,
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exp_sums,
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max_logits,
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tmp_output,
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query,
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key_cache,
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value_cache,
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num_kv_heads,
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scale,
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block_tables,
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seq_lens,
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block_size,
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max_seq_len,
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alibi_slopes,
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kv_cache_dtype,
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k_scale,
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v_scale,
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tp_rank,
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blocksparse_local_blocks,
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blocksparse_vert_stride,
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blocksparse_block_size,
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blocksparse_head_sliding_step,
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)
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return output
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@staticmethod
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def forward_prefix(
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query: torch.Tensor,
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key: torch.Tensor,
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value: torch.Tensor,
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kv_cache_dtype: str,
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key_cache: torch.Tensor,
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value_cache: torch.Tensor,
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block_tables: torch.Tensor,
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query_start_loc: torch.Tensor,
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seq_lens_tensor: torch.Tensor,
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context_lens: torch.Tensor,
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max_query_len: int,
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alibi_slopes: Optional[torch.Tensor],
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sliding_window: Optional[int],
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k_scale: float,
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v_scale: float,
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) -> torch.Tensor:
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# NOTE: The Triton context_attention_fwd kernel hangs on Iluvatar
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# BI-V100 hardware (same class of issue as cudnnFlashAttnForward).
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# Use a pure-PyTorch fallback that reads the paged KV cache directly.
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return PagedAttention._forward_prefix_pytorch(
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query, key, value,
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key_cache, value_cache,
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block_tables, query_start_loc,
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seq_lens_tensor, context_lens,
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)
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@staticmethod
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def _forward_prefix_pytorch(
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query: torch.Tensor,
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key: torch.Tensor,
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value: torch.Tensor,
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key_cache: torch.Tensor,
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value_cache: torch.Tensor,
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block_tables: torch.Tensor,
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query_start_loc: torch.Tensor,
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seq_lens_tensor: torch.Tensor,
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context_lens: torch.Tensor,
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) -> torch.Tensor:
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"""Pure-PyTorch prefix-attention with query-chunking (no Triton).
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For each sequence, gathers the context KV from the paged KV cache,
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concatenates with the current-chunk K/V, then computes scaled-dot-
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product attention with a causal mask.
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Memory optimisation — query chunking
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------------------------------------
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A full-sequence attention matrix is O(q_len × kv_len) in float32.
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For long sequences (e.g., q_len = kv_len = 20 000) that blows up
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to ~9 GB per layer. Instead we tile the query axis in sub-chunks
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of _ATTN_Q_CHUNK tokens and accumulate the output; peak attn memory
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becomes O(_ATTN_Q_CHUNK × kv_len), e.g. 123 MB per layer for
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chunk=256 and kv_len=20 000.
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This replaces the need for vllm's --enable-chunked-prefill flag
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(which the vendor's vllm 0.6.3 does not properly support for
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has_inner_state=True models on BI-V100).
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Shapes
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------
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query : [total_q_tokens, num_q_heads, head_dim]
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key : [total_q_tokens, num_kv_heads, head_dim]
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value : [total_q_tokens, num_kv_heads, head_dim]
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key_cache : [num_blocks, num_kv_heads, head_dim//x, block_size, x]
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value_cache : [num_blocks, num_kv_heads, head_dim, block_size]
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block_tables : [batch_size, max_blocks_per_seq]
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query_start_loc: [batch_size + 1]
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seq_lens_tensor: [batch_size] total length (context + query)
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context_lens : [batch_size] tokens already in KV cache
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"""
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# Maximum query tokens to process at once per attention step.
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# Tune this to balance memory vs kernel-launch overhead:
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# 256 → ~120 MB peak attn memory (conservative, safe for 20K ctx)
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# 512 → ~240 MB peak attn memory
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# 1024 → ~490 MB peak attn memory
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try:
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_ATTN_Q_CHUNK = 256
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batch_size = seq_lens_tensor.shape[0]
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num_q_heads = query.shape[1]
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num_kv_heads = key_cache.shape[1]
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head_dim = query.shape[2]
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gqa_ratio = num_q_heads // num_kv_heads
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# value_cache: [num_blocks, num_kv_heads, head_dim, block_size]
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block_size = value_cache.shape[3]
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scale = 1.0 / (head_dim ** 0.5)
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output = torch.empty_like(query)
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orig_dtype = query.dtype
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for i in range(batch_size):
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ctx_len = int(context_lens[i].item())
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q_start = int(query_start_loc[i].item())
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q_end = int(query_start_loc[i + 1].item())
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q_len = q_end - q_start
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q_i = query[q_start:q_end] # [q_len, num_q_heads, head_dim]
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k_i = key [q_start:q_end] # [q_len, num_kv_heads, head_dim]
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v_i = value[q_start:q_end]
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# --- Build full K/V (context from cache + current chunk) ----
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if ctx_len > 0:
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num_ctx_blocks = (ctx_len + block_size - 1) // block_size
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blk_ids = block_tables[i, :num_ctx_blocks]
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# key_cache[blk_ids]: [n, kv_h, d//x, blk_sz, x]
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# → permute(0,3,1,2,4) → contiguous → view → [:ctx_len]
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k_ctx = (key_cache[blk_ids]
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.permute(0, 3, 1, 2, 4)
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.contiguous()
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.view(-1, num_kv_heads, head_dim))[:ctx_len]
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# value_cache[blk_ids]: [n, kv_h, d, blk_sz]
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# → permute(0,3,1,2) → contiguous → view → [:ctx_len]
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v_ctx = (value_cache[blk_ids]
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.permute(0, 3, 1, 2)
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.contiguous()
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.view(-1, num_kv_heads, head_dim))[:ctx_len]
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k_full = torch.cat([k_ctx, k_i], dim=0) # [kv_len, kv_h, d]
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v_full = torch.cat([v_ctx, v_i], dim=0)
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else:
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k_full = k_i
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v_full = v_i
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kv_len = k_full.shape[0] # ctx_len + q_len
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# GQA: expand KV heads to match Q heads
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if gqa_ratio > 1:
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k_full = k_full.repeat_interleave(gqa_ratio, dim=1)
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v_full = v_full.repeat_interleave(gqa_ratio, dim=1)
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k_t = k_full.permute(1, 0, 2).float() # [H, kv_len, d]
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v_t = v_full.permute(1, 0, 2).float() # [H, kv_len, d]
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# k_pos used for causal mask: shape [kv_len]
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k_pos = torch.arange(kv_len, device=query.device)
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# --- Query-chunked attention --------------------------------
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# Process _ATTN_Q_CHUNK query tokens at a time.
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# Peak attn tensor: [H, _ATTN_Q_CHUNK, kv_len] float32
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# instead of [H, q_len, kv_len] float32.
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for qc_start in range(0, q_len, _ATTN_Q_CHUNK):
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qc_end = min(qc_start + _ATTN_Q_CHUNK, q_len)
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# [H, qc, d]
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q_t_chunk = (q_i[qc_start:qc_end]
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.permute(1, 0, 2)
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.float())
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# [H, qc, kv_len]
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attn_w = torch.matmul(q_t_chunk * scale,
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k_t.transpose(-1, -2))
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# Causal mask for this sub-chunk:
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# query absolute position = ctx_len + qc_start..qc_end-1
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# can attend to k_pos <= its own absolute position
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qc_q_pos = torch.arange(qc_start, qc_end,
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device=query.device)
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# mask[j, k] = True → future key, block it
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mask = k_pos.unsqueeze(0) > (ctx_len + qc_q_pos.unsqueeze(1))
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attn_w = attn_w.masked_fill(mask.unsqueeze(0), float('-inf'))
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attn_w = torch.softmax(attn_w, dim=-1) # [H, qc, kv_len]
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out_c = torch.matmul(attn_w, v_t) # [H, qc, d]
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output[q_start + qc_start : q_start + qc_end] = (
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out_c.to(orig_dtype).permute(1, 0, 2))
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except Exception as e:
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print(f"[paged_attn ERROR] {type(e).__name__}: {e}", file=sys.stderr, flush=True)
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traceback.print_exc(file=sys.stderr)
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raise
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return output
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@staticmethod
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def swap_blocks(
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src_kv_cache: torch.Tensor,
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dst_kv_cache: torch.Tensor,
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src_to_dst: torch.Tensor,
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) -> None:
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src_key_cache = src_kv_cache[0]
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dst_key_cache = dst_kv_cache[0]
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ops.swap_blocks(src_key_cache, dst_key_cache, src_to_dst)
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src_value_cache = src_kv_cache[1]
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dst_value_cache = dst_kv_cache[1]
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ops.swap_blocks(src_value_cache, dst_value_cache, src_to_dst)
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@staticmethod
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def copy_blocks(
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kv_caches: List[torch.Tensor],
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src_to_dists: torch.Tensor,
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) -> None:
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key_caches = [kv_cache[0] for kv_cache in kv_caches]
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value_caches = [kv_cache[1] for kv_cache in kv_caches]
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ops.copy_blocks(key_caches, value_caches, src_to_dists)
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