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sjmshsh
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Code Review
This pull request introduces pluggable GPU KV cache eviction policies, including LRU, Two-Queue (2Q), and Adaptive Replacement Cache (ARC). The changes update the CacheConfig, refactor the BlockPool to use a policy interface, and propagate the policy selection through the KV cache coordinator and manager. A review comment identifies a logic issue in the BlockPool's touch method where promotion signals are only sent for blocks with a zero reference count; moving this signal outside the conditional check is recommended to ensure shared blocks are correctly promoted under high concurrency.
| if block.ref_cnt == 0 and not block.is_null: | ||
| self.free_block_queue.remove(block) | ||
| self._policy.remove(block) | ||
| self._policy.touch(block) |
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The promotion signal (touch) is currently only sent if the block's reference count is zero. This means that if a block is hit while it is already in use by another request (i.e., ref_cnt > 0), it will not be marked for promotion to the "hot" queue (in 2Q) or T2 (in ARC).
This significantly degrades the effectiveness of frequency-based eviction policies under high concurrency, as heavily shared blocks (like common system prompts) might never be promoted if they are always in use when hit. The touch call should be moved outside the if block.ref_cnt == 0 block to ensure every cache hit is recorded as a frequency signal.
| if block.ref_cnt == 0 and not block.is_null: | |
| self.free_block_queue.remove(block) | |
| self._policy.remove(block) | |
| self._policy.touch(block) | |
| if block.ref_cnt == 0 and not block.is_null: | |
| self._policy.remove(block) | |
| if not block.is_null: | |
| self._policy.touch(block) |
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This pull request has merge conflicts that must be resolved before it can be |
SachinVarghese
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SachinVarghese
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Implementation looks good for the default LRU policy.
| # Seed the policy with all blocks so that the initial pool is ready. | ||
| # We use a temporary FreeKVCacheBlockQueue to bootstrap the linked | ||
| # list pointers inside KVCacheBlock, then move the blocks into the | ||
| # policy. For LRU the policy wraps its own FreeKVCacheBlockQueue | ||
| # directly; for TwoQueue we need the pointer bootstrapping regardless. | ||
| _bootstrap_queue = FreeKVCacheBlockQueue(self.blocks) | ||
| # Drain the bootstrap queue and feed all blocks into the policy queue. | ||
| all_blocks: list[KVCacheBlock] = _bootstrap_queue.popleft_n(num_gpu_blocks) | ||
| self._policy.insert_n(all_blocks) | ||
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| # Keep a direct reference to the underlying queue for the null-block | ||
| # popleft below (works for both LRU and TwoQueue via evict_n). | ||
| # We use evict_n(1) which is O(1) and policy-agnostic. |
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Seeding the policy with all free blocks can be potentially moved to each policy class and completely abstracted from the block pool.
| from dataclasses import field | ||
| from typing import ClassVar, Literal | ||
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| GpuEvictionPolicy = Literal["lru", "two_queue", "arc"] |
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If Pydantic supports it, consider using a union of immutable data types to signify the choice between these policies. If/when these policies start requiring parametrization, you can then fold these parameters into the types, without needing to extend the parent config type (which is already large) with optional, eviction policy specific parameters. Something like:
class _Lru(BaseModel):
kind: Literal["lru"]
class _TwoQueue(BaseModel):
kind: Literal["two_queue"]
class _Arc(BaseModel):
kind: Literal["arc"]
GpuEvictionPolicy = Annotated[
Union[_Lru, _TwoQueue, _Arc],
Field(discriminator="kind"),
]
It's also better for type-checking, as passing a str as a Literal is not technically correct, as the value of the str is not statically known.
| """Select and remove n eviction candidates, returning them.""" | ||
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| @abstractmethod | ||
| def __len__(self) -> int: |
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I think this overload is more confusing than anything else. Could we have a num_evictable_blocks method instead?
Also, could this type be a protocol?
| """ | ||
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| @abstractmethod | ||
| def evict_n(self, n: int) -> list[KVCacheBlock]: |
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Return a tuple[KVCacheBlock, ...] instead of a list.
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Ah I see the issue with this, you would need to adjust this:
vllm/vllm/v1/core/kv_cache_utils.py
Line 284 in ed41aa2
.append() in a loop if feasible, I haven't checked.
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Would it be possible to prioritize this PR? The two_queue policy looks like it may address #48435: blocks participating in repeated hybrid full-attention/SWA hits would become hot, while one-pass allocation churn remains cold. If so could we also extend the tests to cover that failure mode? |
4 participants
Purpose
Fixes frequency-blind GPU KV cache eviction that causes unnecessary cache misses under mixed workloads.
Problem
BlockPoolcurrently uses a single-queue LRU implemented inFreeKVCacheBlockQueue. Onceref_cntdrops to zero, all blocks are equal eviction candidates regardless of reuse history. This has two critical blind spots:This is particularly painful for:
Additionally, the CPU offload layer (
vllm/v1/kv_offload/cpu/) already has a fullARCCachePolicy. The GPUBlockPoolwas inconsistent with no equivalent protection.Solution
Introduce a pluggable
GPUCachePolicyabstraction and implement three policies behind it:"lru""two_queue""arc"padapts recency/frequency balance via ghost lists B1/B2; deployed in IBM DS8000 and ZFSAll policies expose the same O(1) interface:
insert / insert_n / remove / touch / evict_n / __len__. TheBlockPoolis refactored to delegate all eviction decisions toself._policy, with no changes to callsites outsideblock_pool.py.ARC — Key Adaptation for GPU KV Cache
Classical ARC detects ghost hits at lookup time (cache miss for a key in B1/B2). In vLLM there is no explicit lookup-miss hook, so ghost-hit detection is adapted to
insert()time:This is semantically equivalent to the original ARC algorithm.
Changed Files
vllm/v1/core/eviction_policy.pyGPUCachePolicyABC,LRUGPUCachePolicy,TwoQueueGPUCachePolicy,ARCGPUCachePolicy,make_gpu_eviction_policyfactoryvllm/v1/core/block_pool.pyself._policy;_FreeBlockQueueShimfor backward compat of.free_block_queue.num_free_blocksvllm/config/cache.pygpu_eviction_policy: Literal["lru", "two_queue", "arc"] = "lru"fieldvllm/v1/core/kv_cache_coordinator.pyeviction_policythrough base class + 3 subclasses + factoryvllm/v1/core/kv_cache_manager.pyeviction_policyparam, forward to coordinatorvllm/v1/core/sched/scheduler.pycache_config.gpu_eviction_policytoKVCacheManagertests/v1/core/test_eviction_policy.pyTest Plan
Run the new unit test file:
Test Results
All 40+ unit tests pass: