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[Perf] Early return in KVCacheManager.allocate_slots #29206

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[Perf] Early return in KVCacheManager.allocate_slots #29206

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27 changes: 17 additions & 10 deletions vllm/v1/core/kv_cache_manager.py
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Original file line number Diff line number Diff line change
Expand Up @@ -267,16 +267,6 @@ def allocate_slots(
else:
new_computed_block_list = self.empty_kv_cache_blocks.blocks

# Free the blocks that are skipped during the attention computation
# (e.g., tokens outside the sliding window).
# We can do this even if we cannot schedule this request due to
# insufficient free blocks.
# Should call this function before allocating new blocks to reduce
# the number of evicted blocks.
self.coordinator.remove_skipped_blocks(
request.request_id, request.num_computed_tokens
)

# The number of computed tokens is the number of computed tokens plus
# the new prefix caching hits
num_computed_tokens = request.num_computed_tokens + num_new_computed_tokens
Expand All @@ -292,6 +282,23 @@ def allocate_slots(
num_encoder_tokens=num_encoder_tokens,
)

if (
num_blocks_to_allocate == 0
and new_computed_block_list is self.empty_kv_cache_blocks.blocks
):
# Early return as no new blocks needed to be allocated
return self.empty_kv_cache_blocks
Comment on lines +285 to +290
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@chatgpt-codex-connector chatgpt-codex-connector bot Nov 21, 2025

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P1 Badge Skipped sliding-window cleanup when no new blocks allocated

allocate_slots now returns early when num_blocks_to_allocate==0 and no prefix-cache hits, so coordinator.remove_skipped_blocks() is never called on those steps. For sliding-window/ChunkedLocal attention this call is what frees blocks that have fallen outside the window; skipping it leaves those blocks held until the request hits a block boundary or finishes, inflating block_pool usage and causing unnecessary evictions or scheduling failures for long prompts that keep generating within an existing block.

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# Free the blocks that are skipped during the attention computation
# (e.g., tokens outside the sliding window).
# We can do this even if we cannot schedule this request due to
# insufficient free blocks.
# Should call this function before allocating new blocks to reduce
# the number of evicted blocks.
self.coordinator.remove_skipped_blocks(
request.request_id, request.num_computed_tokens
)

if num_blocks_to_allocate > self.block_pool.get_num_free_blocks():
# Cannot allocate new blocks
return None
Expand Down