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vllm/v1/attention/backends/utils.py
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314
vllm/v1/attention/backends/utils.py
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# SPDX-License-Identifier: Apache-2.0
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# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
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import abc
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import functools
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from abc import abstractmethod
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from dataclasses import dataclass
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from typing import TYPE_CHECKING, ClassVar, Generic, TypeVar
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import numpy as np
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import torch
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from vllm.utils import cdiv
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if TYPE_CHECKING:
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from vllm.v1.core.sched.output import SchedulerOutput
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from vllm.v1.worker.gpu_input_batch import InputBatch
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import vllm.envs as envs
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from vllm.distributed.kv_transfer.kv_connector.utils import (
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get_kv_connector_cache_layout)
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from vllm.logger import init_logger
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logger = init_logger(__name__)
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@dataclass
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class CommonAttentionMetadata:
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"""
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Per-batch attention metadata, shared across layers and backends.
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AttentionMetadataBuilder instances use it to construct per-layer metadata.
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"""
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query_start_loc: torch.Tensor
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"""(batch_size + 1,), the start location of each request in query Tensor"""
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seq_lens: torch.Tensor
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"""(batch_size,), the length of each request including both computed tokens
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and newly scheduled tokens"""
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num_reqs: int
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"""Number of requests"""
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num_actual_tokens: int
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"""Total number of tokens in batch"""
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max_query_len: int
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"""Longest query in batch"""
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num_speculative_tokens: int = 0
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"""Number of speculative tokens"""
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slot_mapping: torch.Tensor = None
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"""(batch_size, seq_len), slot mapping"""
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spec_layer_decoding: bool = False
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M = TypeVar("M")
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class AttentionMetadataBuilder(abc.ABC, Generic[M]):
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# Does this backend/builder support CUDA Graphs for attention.
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full_cudagraph_supported: ClassVar[bool] = False
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@abstractmethod
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def build(self, common_prefix_len: int,
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common_attn_metadata: CommonAttentionMetadata) -> M:
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"""
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Central method that builds attention metadata.
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Some builders (MLA) require reorder_batch to be called prior to build.
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"""
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raise NotImplementedError
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def can_run_in_cudagraph(
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self, common_attn_metadata: CommonAttentionMetadata) -> bool:
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"""
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Can this batch (with given metadata) use CUDA Graphs for attention.
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"""
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return False
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def build_for_cudagraph_capture(
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self, common_attn_metadata: CommonAttentionMetadata) -> M:
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"""
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Build attention metadata for CUDA graph capture. Uses build by default.
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Subclasses that override this method should call self.build or
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super().build_for_cudagraph_capture.
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"""
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return self.build(common_prefix_len=0,
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common_attn_metadata=common_attn_metadata)
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def use_cascade_attention(
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self,
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common_prefix_len: int,
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query_lens: np.ndarray,
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num_query_heads: int,
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num_kv_heads: int,
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use_alibi: bool,
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use_sliding_window: bool,
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num_sms: int,
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) -> bool:
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return False
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def reorder_batch(self, input_batch: "InputBatch",
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scheduler_output: "SchedulerOutput") -> bool:
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"""
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This method can reorder the batch if desired by the backend.
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:return: Has the batch been reordered (default False).
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"""
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return False
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def validate_kv_sharing_target(current_layer_name, target_layer_name,
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static_forward_context):
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error_msg = (f"Specified KV sharing target layer for {current_layer_name} "
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f"is not valid: target layer {target_layer_name} ")
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if current_layer_name == target_layer_name:
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raise ValueError(error_msg +
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"cannot be the same as the current layer.")
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if target_layer_name not in static_forward_context:
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from vllm.model_executor.models.utils import extract_layer_index
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# If target layer name is not in the static fwd context, it means either
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# a) the target layer does not come BEFORE the current layer, or
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# b) the target layer is not an Attention layer that exists in the model
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current_layer_idx = extract_layer_index(current_layer_name)
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target_layer_idx = extract_layer_index(target_layer_name)
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if current_layer_idx <= target_layer_idx:
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raise ValueError(error_msg + "must come before the current layer.")
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else:
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raise ValueError(error_msg +
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"is not a valid Attention layer in the model.")
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# Currently KV sharing is only supported between layers of the same type
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target_layer_attn_type = static_forward_context[
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target_layer_name].attn_type
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expected = static_forward_context[current_layer_name].attn_type
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if target_layer_attn_type != expected:
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raise ValueError(
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error_msg +
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f"must be the same type as the current layer ({expected}).")
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@functools.lru_cache
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def get_kv_cache_layout():
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# Override with format specified by the user.
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cache_layout = envs.VLLM_KV_CACHE_LAYOUT
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if cache_layout is None:
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cache_layout = get_kv_connector_cache_layout()
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else:
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logger.info_once("`VLLM_KV_CACHE_LAYOUT` environment variable " \
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"detected. Setting KV cache layout to %s.", cache_layout)
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return cache_layout
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#
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# Take in `query_start_loc_np` and `seq_lens_np` and break the sequences into
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# local attention blocks, where each block is passed to the attention kernel
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# as an independent local ("virtual") batch item.
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#
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# For example, if are performing a chunked prefill a batch of 3 sequences:
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# q_seqlens = [4, 10, 5]
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# kv_seqlens = [6, 17, 9]
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# Then normally for regular attention we would compute with an attention mask
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# for batch idx 0 (q_seqlens = 4, kv_seqlens = 6) like:
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# batch idx: 0 (q_seqlens = 4, kv_seqlens = 6)
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# k_toks > 0 1 2 3 4 5
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# q_toks v _____________
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# 0 | 1 1 1
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# 1 | 1 1 1 1
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# 2 | 1 1 1 1 1
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# 3 | 1 1 1 1 1 1
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#
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# for local attention (with attn_chunk_size = 4) we would compute with an
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# attention mask like:
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# batch idx: 0 (q_seqlens = 4, kv_seqlens = 6, attn_chunk_size = 4)
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# k_toks > 0 1 2 3 4 5
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# q_toks v _____________
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# 0 | 1 1 1
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# 1 | 1 1 1 1
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# 2 | 1
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# 3 | 1 1
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#
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# We can simulate this mask using standard flash-attention by breaking the
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# sequences into local ("virtual") batches, where each local batch item is a
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# local attention block, so in this case batch idx 0 would be broken up into:
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#
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# local-batch idx: 0 (q_seqlens = 2, kv_seqlens = 4) (batch 0)
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# k_toks > 0 1 2 3
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# q_toks v _____________
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# 0 | 1 1 1
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# 1 | 1 1 1 1
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# local-batch idx: 1 (q_seqlens = 2, kv_seqlens = 2) (batch 0)
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# k_toks > 4 5
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# q_toks v _____________
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# 2 | 1
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# 3 | 1 1
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#
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# e.g. if we have:
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# attn_chunk_size = 4
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# query_start_loc_np = [0, 4, 14, 19] (q_seqlens = [4, 10, 5])
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# Then this function would return:
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# __b0__ ______b1______ __b2__ < orig batch indices
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# q_seqlens_local = [ 2, 2, 1, 4, 4, 1, 4, 1]
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# cu_seqlens_q_local = [0, 4, 6, 10, 14, 18, 19, 23, 24]
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# seqlens_k_local = [ 4, 2, 4, 4, 4, 1, 4, 1]
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# block_table_local : shape[local_virtual_batches, pages_per_local_batch]
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def make_local_attention_virtual_batches(
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attn_chunk_size: int,
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query_start_loc_np: np.ndarray,
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seq_lens_np: np.ndarray,
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block_table: torch.Tensor,
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block_size: int = 0,
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) -> tuple[np.ndarray, np.ndarray, np.ndarray, torch.Tensor]:
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q_seqlens = query_start_loc_np[1:] - query_start_loc_np[:-1]
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actual_batch_size = seq_lens_np.shape[0]
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# Handle if we are starting in the middle of a local attention block,
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# we assume q_seqlens > 0 (for all elements), for each batch idx we compute
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# the number of tokens that are not in the first local attention block and
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# then we can simply use a cdiv for the rest.
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# For example if we have:
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# attn_chunk_size = 4
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# q_seqlens = [4, 10, 5]
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# k_seqlens = [6, 17, 9]
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# Then we would get:
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# new_tokens_in_first_block = [2, 1, 4]
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# local_blocks = [2, 4, 2]
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q_tokens_in_first_block = np.minimum(
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attn_chunk_size - ((seq_lens_np - q_seqlens) % attn_chunk_size),
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q_seqlens).astype(np.int32)
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tokens_in_last_block = attn_chunk_size + (seq_lens_np % -attn_chunk_size)
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local_blocks = 1 + cdiv(q_seqlens - q_tokens_in_first_block,
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attn_chunk_size)
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# Once we know the number of local blocks we can compute the request spans
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# for each batch idx, we can figure out the number of "virtual" requests we
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# have to make,
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# For the above example we would get:
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# seqlens_q_local = [2, 2, 1, 4, 4, 1, 4, 1]
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#
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# First Get batched arange. (E.g., [2, 4, 2] -> [0, 1, 0, 1, 2, 3, 0, 1])
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# (TODO: max a utility to share this code with _prepare_inputs)
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# arange step 1. [2, 4, 2] -> [2, 6, 8]
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cu_num_blocks = np.cumsum(local_blocks)
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virtual_batches = cu_num_blocks[-1]
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# arange step 2. [2, 6, 8] -> [0, 0, 2, 2, 2, 2, 6, 6]
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block_offsets = np.repeat(cu_num_blocks - local_blocks, local_blocks)
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# arange step 3. [0, 1, 0, 1, 2, 3, 0, 1]
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arange = np.arange(virtual_batches, dtype=np.int32) - block_offsets
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# also compute reverse arange (i.e. [1, 0, 3, 2, 1, 0, 1, 0])
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rarange = np.repeat(local_blocks, local_blocks) - arange - 1
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# Then we can compute the seqlens_q_local, handling the fact that the
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# first and last blocks could be partial
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seqlens_q_local = \
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np.repeat(q_seqlens - q_tokens_in_first_block, local_blocks)
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# set the first block since this may be a partial block
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seqlens_q_local[arange == 0] = q_tokens_in_first_block
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# set the remaining blocks
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seqlens_q_local[arange > 0] = np.minimum(
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seqlens_q_local - attn_chunk_size * (arange - 1),
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attn_chunk_size)[arange > 0]
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# convert from q_seqlens to cu_seqlens_q
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cu_seqlens_q_local = np.pad(np.cumsum(seqlens_q_local), (1, 0))\
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.astype(np.int32)
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# compute the seqlens_k_local,
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# basically a full local attention block for all but the last block in each
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# batch
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# For our example this will be:
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# seqlens_k_local = [4, 2, 4, 4, 4, 1, 4, 1]
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seqlens_k_local = np.full(cu_num_blocks[-1],
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attn_chunk_size,
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dtype=np.int32)
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seqlens_k_local[cu_num_blocks - 1] = tokens_in_last_block
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k_seqstarts_absolute = np.repeat(seq_lens_np, local_blocks) - \
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(rarange * attn_chunk_size + \
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np.repeat(tokens_in_last_block, local_blocks))
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# For the example the local attention blocks start at:
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# _b0_ _____b1_____ _b2_
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# k_seqstarts_absolute = [0, 4, 4, 8, 12, 16, 4, 8]
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block_starts = k_seqstarts_absolute // block_size
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assert attn_chunk_size % block_size == 0, \
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f"attn_chunk_size {attn_chunk_size} is not " \
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f"divisible by block_size {block_size}"
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pages_per_local_batch = attn_chunk_size // block_size
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# Create a block_table for the local attention blocks
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# For out example if we have a block-table like (assuming block_size=2):
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# block_table = [
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# [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9], < batch 0
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# [10, 11, 12, 13, 14, 15, 16, 17, 18, 19], < batch 1
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# [20, 21, 22, 23, 24, 25, 26, 27, 28, 29], < batch 2
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# ]
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# Then for the local batches we would want a block-table like
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# block_table_local = [
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# [ 0, 1 ], < local-batch 0, (batch 0, starting from k[0])
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# [ 2, 3 ], < local-batch 1, (batch 0, starting from k[4])
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# [ 12, 13 ], < local-batch 2, (batch 1, starting from k[4])
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# [ 14, 15 ], < local-batch 3, (batch 1, starting from k[8])
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# [ 16, 17 ], < local-batch 4, (batch 1, starting from k[12])
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# [ 18, 19 ], < local-batch 5, (batch 1, starting from k[16])
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# [ 22, 23 ], < local-batch 6, (batch 2, starting from k[4])
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# [ 24, 25 ], < local-batch 7, (batch 2, starting from k[8])
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# ]
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block_indices= np.broadcast_to(
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np.arange(pages_per_local_batch, dtype=np.int32),
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(virtual_batches, pages_per_local_batch)) \
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+ np.expand_dims(block_starts, axis=1)
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block_indices = block_indices.flatten().clip(max=block_table.shape[1] - 1)
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batch_indices = np.repeat(np.arange(actual_batch_size, dtype=np.int32),
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local_blocks * pages_per_local_batch)
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block_table_local = block_table[batch_indices, block_indices]\
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.view(virtual_batches, -1)
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return seqlens_q_local, cu_seqlens_q_local, seqlens_k_local, \
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block_table_local
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