839 lines
34 KiB
Python
839 lines
34 KiB
Python
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# SPDX-License-Identifier: Apache-2.0
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"""Attention layer with xFormers and PagedAttention.
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BI-V100 adaptation: ixformer imports + SDPA fallback for head_dim > 128.
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"""
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from dataclasses import dataclass
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from typing import Any, Dict, List, Optional, Tuple, Type
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import torch
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# from xformers import ops as xops
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from ixformer.contrib.xformers import ops as xops
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from xformers.ops.fmha.attn_bias import (AttentionBias,
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BlockDiagonalMask,)
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from ixformer.contrib.xformers.ops.fmha.attn_bias import (BlockDiagonalCausalMask,
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LowerTriangularMaskWithTensorBias)
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from vllm.attention.backends.abstract import (AttentionBackend, AttentionImpl,
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AttentionLayer,
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AttentionMetadata, AttentionType)
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from vllm.attention.backends.utils import (
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CommonAttentionState, CommonMetadataBuilder,
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get_num_prefill_decode_query_kv_tokens, get_seq_len_block_table_args,
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is_all_cross_attn_metadata_set, is_all_encoder_attn_metadata_set)
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from vllm.attention.ops.paged_attn import (PagedAttention,
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PagedAttentionMetadata)
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from vllm.bi100_profile import bi100_timer
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from vllm.logger import init_logger
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logger = init_logger(__name__)
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class XFormersBackend(AttentionBackend):
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@staticmethod
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def get_name() -> str:
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return "XFORMERS"
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@staticmethod
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def get_impl_cls() -> Type["XFormersImpl"]:
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return XFormersImpl
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@staticmethod
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def get_metadata_cls() -> Type["AttentionMetadata"]:
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return XFormersMetadata
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@staticmethod
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def get_builder_cls() -> Type["XFormersMetadataBuilder"]:
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return XFormersMetadataBuilder
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@staticmethod
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def get_state_cls() -> Type["CommonAttentionState"]:
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return CommonAttentionState
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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 PagedAttention.get_kv_cache_shape(num_blocks, block_size,
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num_kv_heads, head_size)
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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: Dict[int, int],
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) -> None:
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PagedAttention.swap_blocks(src_kv_cache, dst_kv_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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PagedAttention.copy_blocks(kv_caches, src_to_dists)
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@dataclass
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class XFormersMetadata(AttentionMetadata, PagedAttentionMetadata):
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"""Metadata for XFormersbackend.
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NOTE: Any python object stored here is not updated when it is
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cuda-graph replayed. If you have values that need to be changed
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dynamically, it should be stored in tensor. The tensor has to be
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updated from `CUDAGraphRunner.forward` API.
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"""
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# |---------- N-1 iteration --------|
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# |---------------- N iteration ---------------------|
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# |- tokenA -|......................|-- newTokens ---|
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# |---------- context_len ----------|
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# |-------------------- seq_len ----------------------|
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# |-- query_len ---|
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# seq_lens stored as a tensor.
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seq_lens_tensor: Optional[torch.Tensor]
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# FIXME: It is for flash attn.
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# Maximum sequence length among prefill batch. 0 if there are decoding
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# requests only.
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max_prefill_seq_len: int
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# Maximum sequence length among decode batch. 0 if there are prefill
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# requests only.
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max_decode_seq_len: int
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# Whether or not if cuda graph is enabled.
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# Cuda-graph is currently enabled for decoding only.
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# TODO(woosuk): Move `use_cuda_graph` out since it's unrelated to attention.
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use_cuda_graph: bool
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# (batch_size,). The sequence length per sequence. Sequence length means
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# the computed tokens + new tokens None if it is a decoding.
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seq_lens: Optional[List[int]] = None
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# FIXME: It is for flash attn.
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# (batch_size + 1,). The cumulative sequence lengths of the sequences in
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# the batch, used to index into sequence. E.g., if the sequence length is
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# [4, 6], it is [0, 4, 10].
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seq_start_loc: Optional[torch.Tensor] = None
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# (batch_size,) A tensor of context lengths (tokens that are computed
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# so far).
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context_lens_tensor: Optional[torch.Tensor] = None
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# Maximum query length in the batch. None for decoding.
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max_query_len: Optional[int] = None
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# Max number of query tokens among request in the batch.
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max_decode_query_len: Optional[int] = None
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# (batch_size + 1,). The cumulative subquery lengths of the sequences in
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# the batch, used to index into subquery. E.g., if the subquery length
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# is [4, 6], it is [0, 4, 10].
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query_start_loc: Optional[torch.Tensor] = None
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# Self-attention prefill/decode metadata cache
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_cached_prefill_metadata: Optional["XFormersMetadata"] = None
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_cached_decode_metadata: Optional["XFormersMetadata"] = None
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# Begin encoder attn & enc/dec cross-attn fields...
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# Encoder sequence lengths representation
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encoder_seq_lens: Optional[List[int]] = None
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encoder_seq_lens_tensor: Optional[torch.Tensor] = None
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# FIXME: It is for flash attn.
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# (batch_size + 1,). The cumulative sequence lengths of the sequences in
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# the batch, used to index into sequence. E.g., if the sequence length is
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# [4, 6], it is [0, 4, 10].
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encoder_seq_start_loc: Optional[torch.Tensor] = None
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# Maximum sequence length among encoder sequences
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max_encoder_seq_len: Optional[int] = None
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# Number of tokens input to encoder
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num_encoder_tokens: Optional[int] = None
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# Cross-attention memory-mapping data structures: slot mapping
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# and block tables
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cross_slot_mapping: Optional[torch.Tensor] = None
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cross_block_tables: Optional[torch.Tensor] = None
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def __post_init__(self):
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# Set during the execution of the first attention op.
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# It is a list because it is needed to set per prompt
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# when alibi slopes is used. It is because of the limitation
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# from xformer API.
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# will not appear in the __repr__ and __init__
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self.attn_bias: Optional[List[AttentionBias]] = None
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self.encoder_attn_bias: Optional[List[AttentionBias]] = None
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self.cross_attn_bias: Optional[List[AttentionBias]] = None
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@property
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def is_all_encoder_attn_metadata_set(self):
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'''
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All attention metadata required for encoder attention is set.
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'''
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return is_all_encoder_attn_metadata_set(self)
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@property
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def is_all_cross_attn_metadata_set(self):
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'''
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All attention metadata required for enc/dec cross-attention is set.
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Superset of encoder attention required metadata.
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'''
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return is_all_cross_attn_metadata_set(self)
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@property
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def prefill_metadata(self) -> Optional["XFormersMetadata"]:
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if self.num_prefills == 0:
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return None
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if self._cached_prefill_metadata is not None:
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return self._cached_prefill_metadata
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assert ((self.seq_lens is not None)
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or (self.encoder_seq_lens is not None))
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assert ((self.seq_lens_tensor is not None)
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or (self.encoder_seq_lens_tensor is not None))
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# Compute some attn_metadata fields which default to None
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query_start_loc = (None if self.query_start_loc is None else
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self.query_start_loc[:self.num_prefills + 1])
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seq_start_loc = (None if self.seq_start_loc is None else
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self.seq_start_loc[:self.num_prefills + 1])
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slot_mapping = (None if self.slot_mapping is None else
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self.slot_mapping[:self.num_prefill_tokens])
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seq_lens = (None if self.seq_lens is None else
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self.seq_lens[:self.num_prefills])
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seq_lens_tensor = (None if self.seq_lens_tensor is None else
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self.seq_lens_tensor[:self.num_prefills])
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context_lens_tensor = (None if self.context_lens_tensor is None else
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self.context_lens_tensor[:self.num_prefills])
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block_tables = (None if self.block_tables is None else
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self.block_tables[:self.num_prefills])
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self._cached_prefill_metadata = XFormersMetadata(
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num_prefills=self.num_prefills,
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num_prefill_tokens=self.num_prefill_tokens,
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num_decode_tokens=0,
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slot_mapping=slot_mapping,
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multi_modal_placeholder_index_maps=self.
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multi_modal_placeholder_index_maps,
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enable_kv_scales_calculation=self.enable_kv_scales_calculation,
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seq_lens=seq_lens,
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seq_lens_tensor=seq_lens_tensor,
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max_query_len=self.max_query_len,
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max_prefill_seq_len=self.max_prefill_seq_len,
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max_decode_seq_len=0,
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query_start_loc=query_start_loc,
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seq_start_loc=seq_start_loc,
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context_lens_tensor=context_lens_tensor,
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block_tables=block_tables,
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use_cuda_graph=False,
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# Begin encoder & cross attn fields below...
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encoder_seq_lens=self.encoder_seq_lens,
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encoder_seq_lens_tensor=self.encoder_seq_lens_tensor,
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max_encoder_seq_len=self.max_encoder_seq_len,
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cross_slot_mapping=self.cross_slot_mapping,
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cross_block_tables=self.cross_block_tables)
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return self._cached_prefill_metadata
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@property
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def decode_metadata(self) -> Optional["XFormersMetadata"]:
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if self.num_decode_tokens == 0:
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return None
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if self._cached_decode_metadata is not None:
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return self._cached_decode_metadata
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assert ((self.seq_lens_tensor is not None)
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or (self.encoder_seq_lens_tensor is not None))
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# Compute some attn_metadata fields which default to None
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slot_mapping = (None if self.slot_mapping is None else
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self.slot_mapping[self.num_prefill_tokens:])
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seq_lens_tensor = (None if self.seq_lens_tensor is None else
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self.seq_lens_tensor[self.num_prefills:])
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block_tables = (None if self.block_tables is None else
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self.block_tables[self.num_prefills:])
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self._cached_decode_metadata = XFormersMetadata(
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num_prefills=0,
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num_prefill_tokens=0,
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num_decode_tokens=self.num_decode_tokens,
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slot_mapping=slot_mapping,
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multi_modal_placeholder_index_maps=None,
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enable_kv_scales_calculation=True,
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seq_lens_tensor=seq_lens_tensor,
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max_prefill_seq_len=0,
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max_decode_seq_len=self.max_decode_seq_len,
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block_tables=block_tables,
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use_cuda_graph=self.use_cuda_graph,
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# Begin encoder & cross attn fields below...
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encoder_seq_lens=self.encoder_seq_lens,
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encoder_seq_lens_tensor=self.encoder_seq_lens_tensor,
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max_encoder_seq_len=self.max_encoder_seq_len,
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cross_slot_mapping=self.cross_slot_mapping,
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cross_block_tables=self.cross_block_tables)
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# Batch may be composed of prefill|decodes, adjust query start indices
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# to refer to the start of decodes when the two are split apart.
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# E.g. in tokens:[3 prefills|6 decodes], query_start_loc=[3,9] => [0,6].
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if self._cached_decode_metadata.query_start_loc is not None:
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qs = self._cached_decode_metadata.query_start_loc
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self._cached_decode_metadata.query_start_loc = qs - qs[0]
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return self._cached_decode_metadata
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def _get_attn_bias(
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attn_metadata: XFormersMetadata,
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attn_type: str,
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) -> Optional[AttentionBias]:
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'''
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Extract appropriate attention bias from attention metadata
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according to attention type.
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'''
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if (attn_type == AttentionType.DECODER
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or attn_type == AttentionType.ENCODER_ONLY):
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return attn_metadata.attn_bias
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elif attn_type == AttentionType.ENCODER:
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return attn_metadata.encoder_attn_bias
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elif attn_type == AttentionType.ENCODER_DECODER:
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return attn_metadata.cross_attn_bias
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else:
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raise AttributeError(f"Invalid attention type {str(attn_type)}")
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def _set_attn_bias(
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attn_metadata: XFormersMetadata,
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attn_bias: List[Optional[AttentionBias]],
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attn_type: str,
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) -> None:
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'''
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Update appropriate attention bias field of attention metadata,
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according to attention type.
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'''
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if (attn_type == AttentionType.DECODER
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or attn_type == AttentionType.ENCODER_ONLY):
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attn_metadata.attn_bias = attn_bias
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elif attn_type == AttentionType.ENCODER:
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attn_metadata.encoder_attn_bias = attn_bias
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elif attn_type == AttentionType.ENCODER_DECODER:
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attn_metadata.cross_attn_bias = attn_bias
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else:
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raise AttributeError(f"Invalid attention type {str(attn_type)}")
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class XFormersMetadataBuilder(CommonMetadataBuilder[XFormersMetadata]):
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_metadata_cls = XFormersMetadata
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class XFormersImpl(AttentionImpl[XFormersMetadata]):
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"""
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If the input tensors contain prompt tokens, the layout is as follows:
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|<--------------- num_prefill_tokens ----------------->|
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|<--prefill_0-->|<--prefill_1-->|...|<--prefill_N-1--->|
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Otherwise, the layout is as follows:
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|<----------------- num_decode_tokens ------------------>|
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|<--decode_0-->|..........|<--decode_M-1-->|<--padding-->|
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Generation tokens can contain padding when cuda-graph is used.
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Currently, prompt tokens don't contain any padding.
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|
|
|
||
|
|
The prompts might have different lengths, while the generation tokens
|
||
|
|
always have length 1.
|
||
|
|
|
||
|
|
If chunked prefill is enabled, prefill tokens and decode tokens can be
|
||
|
|
batched together in a flattened 1D query.
|
||
|
|
|
||
|
|
|<----- num_prefill_tokens ---->|<------- num_decode_tokens --------->|
|
||
|
|
|<-prefill_0->|...|<-prefill_N-1->|<--decode_0-->|...|<--decode_M-1-->|
|
||
|
|
|
||
|
|
Currently, cuda graph is disabled for chunked prefill, meaning there's no
|
||
|
|
padding between prefill and decode tokens.
|
||
|
|
"""
|
||
|
|
|
||
|
|
def __init__(
|
||
|
|
self,
|
||
|
|
num_heads: int,
|
||
|
|
head_size: int,
|
||
|
|
scale: float,
|
||
|
|
num_kv_heads: int,
|
||
|
|
alibi_slopes: Optional[List[float]],
|
||
|
|
sliding_window: Optional[int],
|
||
|
|
kv_cache_dtype: str,
|
||
|
|
blocksparse_params: Optional[Dict[str, Any]] = None,
|
||
|
|
logits_soft_cap: Optional[float] = None,
|
||
|
|
attn_type: str = AttentionType.DECODER,
|
||
|
|
) -> None:
|
||
|
|
if blocksparse_params is not None:
|
||
|
|
raise ValueError(
|
||
|
|
"XFormers does not support block-sparse attention.")
|
||
|
|
if logits_soft_cap is not None:
|
||
|
|
logger.warning_once("XFormers does not support logits soft cap. "
|
||
|
|
"Outputs may be slightly off.")
|
||
|
|
self.num_heads = num_heads
|
||
|
|
self.head_size = head_size
|
||
|
|
self.scale = float(scale)
|
||
|
|
self.num_kv_heads = num_kv_heads
|
||
|
|
if alibi_slopes is not None:
|
||
|
|
alibi_slopes = torch.tensor(alibi_slopes, dtype=torch.float32)
|
||
|
|
self.alibi_slopes = alibi_slopes
|
||
|
|
self.sliding_window = sliding_window
|
||
|
|
self.kv_cache_dtype = kv_cache_dtype
|
||
|
|
|
||
|
|
assert self.num_heads % self.num_kv_heads == 0
|
||
|
|
self.num_queries_per_kv = self.num_heads // self.num_kv_heads
|
||
|
|
|
||
|
|
suppored_head_sizes = PagedAttention.get_supported_head_sizes()
|
||
|
|
if head_size not in suppored_head_sizes:
|
||
|
|
raise ValueError(
|
||
|
|
f"Head size {head_size} is not supported by PagedAttention. "
|
||
|
|
f"Supported head sizes are: {suppored_head_sizes}.")
|
||
|
|
|
||
|
|
self.attn_type = attn_type
|
||
|
|
|
||
|
|
def forward(
|
||
|
|
self,
|
||
|
|
layer: AttentionLayer,
|
||
|
|
query: torch.Tensor,
|
||
|
|
key: Optional[torch.Tensor],
|
||
|
|
value: Optional[torch.Tensor],
|
||
|
|
kv_cache: torch.Tensor,
|
||
|
|
kv_cache_scale: torch.Tensor,
|
||
|
|
attn_metadata: "XFormersMetadata",
|
||
|
|
output: Optional[torch.Tensor] = None,
|
||
|
|
) -> torch.Tensor:
|
||
|
|
"""Forward pass with xFormers and PagedAttention."""
|
||
|
|
# [DEBUG] Check if kv_cache is properly bound
|
||
|
|
_xf_diag = getattr(self, '_xf_diag_cnt', 0)
|
||
|
|
if _xf_diag < 3:
|
||
|
|
self._xf_diag_cnt = _xf_diag + 1
|
||
|
|
logger.info(
|
||
|
|
"[DEBUG_XFORMERS] call=%d kv_cache.shape=%s numel=%d "
|
||
|
|
"num_prefills=%s num_decode=%s",
|
||
|
|
_xf_diag,
|
||
|
|
tuple(kv_cache.shape) if kv_cache.numel() > 0 else "(empty)",
|
||
|
|
kv_cache.numel(),
|
||
|
|
getattr(attn_metadata, 'num_prefills', '?'),
|
||
|
|
getattr(attn_metadata, 'num_decode_tokens', '?'))
|
||
|
|
attn_type = self.attn_type
|
||
|
|
# Check that appropriate attention metadata attributes are
|
||
|
|
# selected for the desired attention type
|
||
|
|
if (attn_type == AttentionType.ENCODER
|
||
|
|
and (not attn_metadata.is_all_encoder_attn_metadata_set)):
|
||
|
|
raise AttributeError("Encoder attention requires setting "
|
||
|
|
"encoder metadata attributes.")
|
||
|
|
|
||
|
|
elif (attn_type == AttentionType.ENCODER_DECODER
|
||
|
|
and (not attn_metadata.is_all_cross_attn_metadata_set)):
|
||
|
|
raise AttributeError("Encoder/decoder cross-attention "
|
||
|
|
"requires setting cross-attention "
|
||
|
|
"metadata attributes.")
|
||
|
|
|
||
|
|
query = query.view(-1, self.num_heads, self.head_size)
|
||
|
|
if key is not None:
|
||
|
|
assert value is not None
|
||
|
|
key = key.view(-1, self.num_kv_heads, self.head_size)
|
||
|
|
value = value.view(-1, self.num_kv_heads, self.head_size)
|
||
|
|
else:
|
||
|
|
assert value is None
|
||
|
|
|
||
|
|
if (attn_type != AttentionType.ENCODER and kv_cache.numel() > 0):
|
||
|
|
key_cache, value_cache = PagedAttention.split_kv_cache(
|
||
|
|
kv_cache, self.num_kv_heads, self.head_size)
|
||
|
|
|
||
|
|
if (key is not None) and (value is not None):
|
||
|
|
|
||
|
|
if attn_type == AttentionType.ENCODER_DECODER:
|
||
|
|
updated_slot_mapping = attn_metadata.cross_slot_mapping
|
||
|
|
else:
|
||
|
|
updated_slot_mapping = attn_metadata.slot_mapping
|
||
|
|
|
||
|
|
with bi100_timer("xformers.kv_write"):
|
||
|
|
PagedAttention.write_to_paged_cache(
|
||
|
|
key, value, key_cache, value_cache,
|
||
|
|
updated_slot_mapping, self.kv_cache_dtype,
|
||
|
|
layer._k_scale, layer._v_scale)
|
||
|
|
|
||
|
|
(num_prefill_query_tokens, num_prefill_kv_tokens,
|
||
|
|
num_decode_query_tokens) = \
|
||
|
|
get_num_prefill_decode_query_kv_tokens(attn_metadata, attn_type)
|
||
|
|
|
||
|
|
output = torch.empty_like(query)
|
||
|
|
# Query for decode. KV is not needed because it is already cached.
|
||
|
|
decode_query = query[num_prefill_query_tokens:]
|
||
|
|
# QKV for prefill.
|
||
|
|
query = query[:num_prefill_query_tokens]
|
||
|
|
if key is not None and value is not None:
|
||
|
|
key = key[:num_prefill_kv_tokens]
|
||
|
|
value = value[:num_prefill_kv_tokens]
|
||
|
|
|
||
|
|
assert query.shape[0] == num_prefill_query_tokens
|
||
|
|
assert decode_query.shape[0] == num_decode_query_tokens
|
||
|
|
|
||
|
|
if prefill_meta := attn_metadata.prefill_metadata:
|
||
|
|
# Prompt run.
|
||
|
|
if kv_cache.numel() == 0 or prefill_meta.block_tables.numel() == 0:
|
||
|
|
# normal attention.
|
||
|
|
with bi100_timer("xformers.dense_prefill"):
|
||
|
|
out = self._run_memory_efficient_xformers_forward(
|
||
|
|
query, key, value, prefill_meta, attn_type=attn_type)
|
||
|
|
assert out.shape == output[:num_prefill_query_tokens].shape
|
||
|
|
output[:num_prefill_query_tokens] = out
|
||
|
|
else:
|
||
|
|
assert attn_type != AttentionType.ENCODER_ONLY, (
|
||
|
|
"Encoder-only models should not have prefix attention.")
|
||
|
|
|
||
|
|
assert prefill_meta.query_start_loc is not None
|
||
|
|
assert prefill_meta.max_query_len is not None
|
||
|
|
|
||
|
|
with bi100_timer("xformers.paged_prefill"):
|
||
|
|
out = PagedAttention.forward_prefix(
|
||
|
|
query,
|
||
|
|
key,
|
||
|
|
value,
|
||
|
|
self.kv_cache_dtype,
|
||
|
|
key_cache,
|
||
|
|
value_cache,
|
||
|
|
prefill_meta.block_tables,
|
||
|
|
prefill_meta.query_start_loc,
|
||
|
|
prefill_meta.seq_lens_tensor,
|
||
|
|
prefill_meta.max_query_len,
|
||
|
|
self.alibi_slopes,
|
||
|
|
self.sliding_window,
|
||
|
|
layer._k_scale,
|
||
|
|
layer._v_scale,
|
||
|
|
)
|
||
|
|
assert output[:num_prefill_query_tokens].shape == out.shape
|
||
|
|
output[:num_prefill_query_tokens] = out
|
||
|
|
|
||
|
|
if decode_meta := attn_metadata.decode_metadata:
|
||
|
|
assert attn_type != AttentionType.ENCODER_ONLY, (
|
||
|
|
"Encoder-only models should not have decode metadata.")
|
||
|
|
|
||
|
|
_dec_diag = getattr(self, '_dec_diag', 0)
|
||
|
|
if _dec_diag < 3:
|
||
|
|
self._dec_diag = _dec_diag + 1
|
||
|
|
logger.info(
|
||
|
|
"[DEBUG_DECODE] call=%d kv_numel=%d "
|
||
|
|
"seq_lens=%s block_tables_shape=%s "
|
||
|
|
"num_prefill_q=%d num_decode_q=%d",
|
||
|
|
_dec_diag, kv_cache.numel(),
|
||
|
|
decode_meta.seq_lens_tensor[:3].tolist()
|
||
|
|
if decode_meta.seq_lens_tensor is not None else None,
|
||
|
|
tuple(decode_meta.block_tables.shape)
|
||
|
|
if decode_meta.block_tables is not None else None,
|
||
|
|
num_prefill_query_tokens, num_decode_query_tokens)
|
||
|
|
|
||
|
|
(
|
||
|
|
seq_lens_arg,
|
||
|
|
max_seq_len_arg,
|
||
|
|
block_tables_arg,
|
||
|
|
) = get_seq_len_block_table_args(decode_meta, False, attn_type)
|
||
|
|
|
||
|
|
output[num_prefill_query_tokens:] = PagedAttention.forward_decode(
|
||
|
|
decode_query,
|
||
|
|
key_cache,
|
||
|
|
value_cache,
|
||
|
|
block_tables_arg,
|
||
|
|
seq_lens_arg,
|
||
|
|
max_seq_len_arg,
|
||
|
|
self.kv_cache_dtype,
|
||
|
|
self.num_kv_heads,
|
||
|
|
self.scale,
|
||
|
|
self.alibi_slopes,
|
||
|
|
layer._k_scale,
|
||
|
|
layer._v_scale,
|
||
|
|
)
|
||
|
|
|
||
|
|
# Reshape the output tensor.
|
||
|
|
return output.view(-1, self.num_heads * self.head_size)
|
||
|
|
|
||
|
|
def _run_sdpa_fallback(
|
||
|
|
self,
|
||
|
|
query: torch.Tensor,
|
||
|
|
key: torch.Tensor,
|
||
|
|
value: torch.Tensor,
|
||
|
|
attn_metadata: "XFormersMetadata",
|
||
|
|
) -> torch.Tensor:
|
||
|
|
"""Pure-math causal attention fallback with Q-tiling for head_size > 128.
|
||
|
|
|
||
|
|
ixformer flash attention doesn't support head_dim > 128.
|
||
|
|
Qwen3.6 uses head_dim=256 -> must use SDPA fallback.
|
||
|
|
|
||
|
|
GQA broadcast matmul avoids materializing expanded KV
|
||
|
|
(6x memory savings for Qwen3.6 ratio=6).
|
||
|
|
Q-tiling: split Q into 256-token chunks to keep peak memory
|
||
|
|
at O(chunk x seq_len) instead of O(seq_len^2).
|
||
|
|
|
||
|
|
Memory: O(_Q_CHUNK x seq_len) instead of O(seq_len^2).
|
||
|
|
"""
|
||
|
|
_Q_CHUNK = 256
|
||
|
|
|
||
|
|
assert attn_metadata.seq_lens is not None
|
||
|
|
orig_dtype = query.dtype
|
||
|
|
num_seqs = len(attn_metadata.seq_lens)
|
||
|
|
|
||
|
|
if (attn_metadata.query_start_loc is not None
|
||
|
|
and len(attn_metadata.query_start_loc) == num_seqs + 1):
|
||
|
|
q_lens = [
|
||
|
|
int(attn_metadata.query_start_loc[i + 1].item()) -
|
||
|
|
int(attn_metadata.query_start_loc[i].item())
|
||
|
|
for i in range(num_seqs)
|
||
|
|
]
|
||
|
|
else:
|
||
|
|
q_lens = list(attn_metadata.seq_lens)
|
||
|
|
|
||
|
|
q_flat = query.squeeze(0)
|
||
|
|
k_flat = key.squeeze(0)
|
||
|
|
v_flat = value.squeeze(0)
|
||
|
|
|
||
|
|
output = torch.empty_like(q_flat)
|
||
|
|
seq_start = 0
|
||
|
|
for q_len in q_lens:
|
||
|
|
seq_end = seq_start + q_len
|
||
|
|
|
||
|
|
k_s = k_flat[seq_start:seq_end].permute(1, 0, 2).float()
|
||
|
|
v_s = v_flat[seq_start:seq_end].permute(1, 0, 2).float()
|
||
|
|
|
||
|
|
gqa_ratio = self.num_heads // k_s.shape[0]
|
||
|
|
if gqa_ratio > 1:
|
||
|
|
k_s = k_s.unsqueeze(1)
|
||
|
|
v_s = v_s.unsqueeze(1)
|
||
|
|
use_gqa_broadcast = True
|
||
|
|
else:
|
||
|
|
use_gqa_broadcast = False
|
||
|
|
|
||
|
|
k_pos = torch.arange(q_len, device=query.device)
|
||
|
|
_max_chunk = min(_Q_CHUNK, q_len)
|
||
|
|
_qc_q_pos_base = torch.arange(_max_chunk, device=query.device)
|
||
|
|
_num_chunks = (q_len + _Q_CHUNK - 1) // _Q_CHUNK
|
||
|
|
|
||
|
|
for qc_idx in range(_num_chunks):
|
||
|
|
qc_start = qc_idx * _Q_CHUNK
|
||
|
|
qc_end = min(qc_start + _Q_CHUNK, q_len)
|
||
|
|
chunk_len = qc_end - qc_start
|
||
|
|
qc_q_pos = _qc_q_pos_base[:chunk_len] + qc_start
|
||
|
|
|
||
|
|
if use_gqa_broadcast:
|
||
|
|
q_c = (q_flat[seq_start + qc_start:seq_start + qc_end]
|
||
|
|
.float()
|
||
|
|
.view(-1, self.num_kv_heads, gqa_ratio,
|
||
|
|
self.head_size)
|
||
|
|
.permute(1, 2, 0, 3))
|
||
|
|
|
||
|
|
attn_w = torch.matmul(
|
||
|
|
q_c, k_s.transpose(-2, -1)) * self.scale
|
||
|
|
|
||
|
|
mask = k_pos.unsqueeze(0) > qc_q_pos.unsqueeze(1)
|
||
|
|
attn_w = attn_w.masked_fill(
|
||
|
|
mask.unsqueeze(0).unsqueeze(0), float("-inf"))
|
||
|
|
|
||
|
|
attn_w = torch.softmax(attn_w, dim=-1)
|
||
|
|
out_c = torch.matmul(attn_w, v_s).to(orig_dtype)
|
||
|
|
out_c = (out_c.permute(2, 0, 1, 3)
|
||
|
|
.contiguous()
|
||
|
|
.view(-1, self.num_heads, self.head_size))
|
||
|
|
output[seq_start + qc_start:seq_start + qc_end] = out_c
|
||
|
|
else:
|
||
|
|
q_c = q_flat[seq_start + qc_start:seq_start + qc_end] \
|
||
|
|
.permute(1, 0, 2).float()
|
||
|
|
|
||
|
|
attn_w = torch.matmul(
|
||
|
|
q_c, k_s.transpose(-2, -1)) * self.scale
|
||
|
|
|
||
|
|
mask = k_pos.unsqueeze(0) > qc_q_pos.unsqueeze(1)
|
||
|
|
attn_w = attn_w.masked_fill(
|
||
|
|
mask.unsqueeze(0), float("-inf"))
|
||
|
|
|
||
|
|
attn_w = torch.softmax(attn_w, dim=-1)
|
||
|
|
out_c = torch.matmul(attn_w, v_s).to(orig_dtype)
|
||
|
|
|
||
|
|
output[seq_start + qc_start:seq_start + qc_end] = (
|
||
|
|
out_c.permute(1, 0, 2))
|
||
|
|
|
||
|
|
seq_start = seq_end
|
||
|
|
|
||
|
|
return output.unsqueeze(0)
|
||
|
|
|
||
|
|
def _run_memory_efficient_xformers_forward(
|
||
|
|
self,
|
||
|
|
query: torch.Tensor,
|
||
|
|
key: torch.Tensor,
|
||
|
|
value: torch.Tensor,
|
||
|
|
attn_metadata: XFormersMetadata,
|
||
|
|
attn_type: str = AttentionType.DECODER,
|
||
|
|
) -> torch.Tensor:
|
||
|
|
"""Attention for 1D query of multiple prompts. Multiple prompt
|
||
|
|
tokens are flattened in to `query` input.
|
||
|
|
|
||
|
|
See https://facebookresearch.github.io/xformers/components/ops.html
|
||
|
|
for API spec.
|
||
|
|
|
||
|
|
Args:
|
||
|
|
output: shape = [num_prefill_tokens, num_heads, head_size]
|
||
|
|
query: shape = [num_prefill_tokens, num_heads, head_size]
|
||
|
|
key: shape = [num_prefill_tokens, num_kv_heads, head_size]
|
||
|
|
value: shape = [num_prefill_tokens, num_kv_heads, head_size]
|
||
|
|
attn_metadata: Metadata for attention.
|
||
|
|
attn_type: Select attention type, between encoder attention,
|
||
|
|
decoder self-attention, or encoder/decoder cross-
|
||
|
|
attention. Defaults to decoder self-attention,
|
||
|
|
which is the vLLM default generally
|
||
|
|
"""
|
||
|
|
|
||
|
|
original_query = query
|
||
|
|
# GQA/MQA is NOT reshaped here for BI-V100 because the
|
||
|
|
# _run_sdpa_fallback handles GQA internally with broadcast matmul.
|
||
|
|
# The standard xformers path (head_size <= 128) also does not need
|
||
|
|
# GQA reshape because ixformer handles it natively.
|
||
|
|
|
||
|
|
# Set attention bias if not provided. This typically happens at
|
||
|
|
# the very attention layer of every iteration.
|
||
|
|
# FIXME(woosuk): This is a hack.
|
||
|
|
attn_bias = _get_attn_bias(attn_metadata, attn_type)
|
||
|
|
if attn_bias is None:
|
||
|
|
if self.alibi_slopes is None:
|
||
|
|
|
||
|
|
# Cross attention block of decoder branch of encoder-decoder
|
||
|
|
# model uses seq_lens for dec / encoder_seq_lens for enc
|
||
|
|
if (attn_type == AttentionType.ENCODER_DECODER):
|
||
|
|
assert attn_metadata.seq_lens is not None
|
||
|
|
assert attn_metadata.encoder_seq_lens is not None
|
||
|
|
|
||
|
|
# Cross-attention mask is non-causal
|
||
|
|
attn_bias = BlockDiagonalMask.from_seqlens(
|
||
|
|
attn_metadata.seq_lens, attn_metadata.encoder_seq_lens)
|
||
|
|
|
||
|
|
# Encoder branch of encoder-decoder model uses
|
||
|
|
# attn_metadata.encoder_seq_lens
|
||
|
|
elif attn_type == AttentionType.ENCODER:
|
||
|
|
|
||
|
|
assert attn_metadata.encoder_seq_lens is not None
|
||
|
|
|
||
|
|
# Encoder self-attention mask is non-causal
|
||
|
|
attn_bias = BlockDiagonalMask.from_seqlens(
|
||
|
|
attn_metadata.encoder_seq_lens)
|
||
|
|
|
||
|
|
# Self-attention block of encoder-only model just
|
||
|
|
# uses the seq_lens directly.
|
||
|
|
elif attn_type == AttentionType.ENCODER_ONLY:
|
||
|
|
assert attn_metadata.seq_lens is not None
|
||
|
|
|
||
|
|
# Encoder self-attention mask is non-causal
|
||
|
|
attn_bias = BlockDiagonalMask.from_seqlens(
|
||
|
|
attn_metadata.seq_lens)
|
||
|
|
|
||
|
|
# Self-attention block of decoder branch just
|
||
|
|
# uses the seq_lens directly
|
||
|
|
elif attn_type == AttentionType.DECODER:
|
||
|
|
assert attn_metadata.seq_lens is not None
|
||
|
|
|
||
|
|
# Decoder self-attention mask is causal
|
||
|
|
attn_bias = BlockDiagonalCausalMask.from_seqlens(
|
||
|
|
attn_metadata.seq_lens)
|
||
|
|
else:
|
||
|
|
raise ValueError("Unknown AttentionType: %s", attn_type)
|
||
|
|
|
||
|
|
if self.sliding_window is not None:
|
||
|
|
attn_bias = attn_bias.make_local_attention(
|
||
|
|
self.sliding_window)
|
||
|
|
attn_bias = [attn_bias]
|
||
|
|
else:
|
||
|
|
assert attn_type == AttentionType.DECODER
|
||
|
|
assert attn_metadata.seq_lens is not None
|
||
|
|
attn_bias = _make_alibi_bias(self.alibi_slopes,
|
||
|
|
self.num_kv_heads, query.dtype,
|
||
|
|
attn_metadata.seq_lens)
|
||
|
|
|
||
|
|
_set_attn_bias(attn_metadata, attn_bias, attn_type)
|
||
|
|
|
||
|
|
# No alibi slopes.
|
||
|
|
# TODO(woosuk): Too many view operations. Let's try to reduce
|
||
|
|
# them in the future for code readability.
|
||
|
|
self.attn_op = xops.fmha.flash.FwOp()
|
||
|
|
if self.alibi_slopes is None:
|
||
|
|
# Add the batch dimension.
|
||
|
|
query = query.unsqueeze(0)
|
||
|
|
key = key.unsqueeze(0)
|
||
|
|
value = value.unsqueeze(0)
|
||
|
|
# BI-V100: ixformer flash attention doesn't support head_dim > 128.
|
||
|
|
# Qwen3.6 uses head_dim=256 -> must use SDPA fallback.
|
||
|
|
if self.head_size > 128:
|
||
|
|
out = self._run_sdpa_fallback(
|
||
|
|
query, key, value, attn_metadata)
|
||
|
|
else:
|
||
|
|
out = xops.memory_efficient_attention_forward(
|
||
|
|
query,
|
||
|
|
key,
|
||
|
|
value,
|
||
|
|
attn_bias=attn_bias[0],
|
||
|
|
p=0.0,
|
||
|
|
scale=self.scale,
|
||
|
|
op=self.attn_op
|
||
|
|
)
|
||
|
|
return out.view_as(original_query)
|
||
|
|
|
||
|
|
# Attention with alibi slopes.
|
||
|
|
# FIXME(woosuk): Because xformers does not support dynamic sequence
|
||
|
|
# lengths with custom attention bias, we process each prompt one by
|
||
|
|
# one. This is inefficient, especially when we have many short prompts.
|
||
|
|
assert attn_metadata.seq_lens is not None
|
||
|
|
output = torch.empty_like(original_query)
|
||
|
|
start = 0
|
||
|
|
for i, seq_len in enumerate(attn_metadata.seq_lens):
|
||
|
|
end = start + seq_len
|
||
|
|
out = xops.memory_efficient_attention_forward(
|
||
|
|
query[None, start:end],
|
||
|
|
key[None, start:end],
|
||
|
|
value[None, start:end],
|
||
|
|
attn_bias=attn_bias[i],
|
||
|
|
p=0.0,
|
||
|
|
scale=self.scale,
|
||
|
|
)
|
||
|
|
# TODO(woosuk): Unnecessary copy. Optimize.
|
||
|
|
output[start:end].copy_(out.view_as(original_query[start:end]))
|
||
|
|
start += seq_len
|
||
|
|
return output
|
||
|
|
|
||
|
|
|
||
|
|
def _make_alibi_bias(
|
||
|
|
alibi_slopes: torch.Tensor,
|
||
|
|
num_kv_heads: int,
|
||
|
|
dtype: torch.dtype,
|
||
|
|
seq_lens: List[int],
|
||
|
|
) -> List[AttentionBias]:
|
||
|
|
attn_biases: List[AttentionBias] = []
|
||
|
|
for seq_len in seq_lens:
|
||
|
|
bias = torch.arange(seq_len, dtype=dtype)
|
||
|
|
# NOTE(zhuohan): HF uses
|
||
|
|
# `bias = bias[None, :].repeat(seq_len, 1)`
|
||
|
|
# here. We find that both biases give the same results, but
|
||
|
|
# the bias below more accurately follows the original ALiBi
|
||
|
|
# paper.
|
||
|
|
# Calculate a matrix where each element represents ith element- jth
|
||
|
|
# element.
|
||
|
|
bias = bias[None, :] - bias[:, None]
|
||
|
|
|
||
|
|
padded_len = (seq_len + 7) // 8 * 8
|
||
|
|
num_heads = alibi_slopes.shape[0]
|
||
|
|
bias = torch.empty(
|
||
|
|
1, # batch size
|
||
|
|
num_heads,
|
||
|
|
seq_len,
|
||
|
|
padded_len,
|
||
|
|
device=alibi_slopes.device,
|
||
|
|
dtype=dtype,
|
||
|
|
)[:, :, :, :seq_len].copy_(bias)
|
||
|
|
bias.mul_(alibi_slopes[:, None, None])
|
||
|
|
attn_biases.append(LowerTriangularMaskWithTensorBias(bias))
|
||
|
|
|
||
|
|
return attn_biases
|