Cached blocks never hit eviction first - #51909
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Hi maintainers, could someone please add the ready-run-all-tests label to this PR? |
njhill
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njhill
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Thanks @shanrow-amd this makes sense in theory!
Could you provide some e2e benchmarks though to demonstrate the performance benefits.
| # only cumulative reference count for cached block hit. | ||
| _ref_cnt: int = 0 |
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I think this should be called something different, like cache_hit_count
| @@ -713,6 +713,7 @@ def touch(self, blocks: Sequence[KVCacheBlock]) -> None: | |||
| if block.ref_cnt == 0 and not block.is_null: | |||
| self.free_block_queue.remove(block) | |||
| block.ref_cnt += 1 | |||
| block._ref_cnt += 1 | |||
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This is not used just for cache hits, it's also used to temporarily pin blocks when offloading via kv connectors. I think we would want to exclude that case from incrementing here.
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Thanks njhill for your reply.
I would follow your suggestions and recommit once.
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See test result above. The performance has no change.
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Most cached blocks with block_hash that never hit. Now they are appended to the free list mixed with fewer hit blocks. We should differentiate the both cases because of the non-hit blocks amount that is huge and far greater than hit blocks. So we should append the non-hit blocks first and then append the hit blocks so that non-hit blocks are allocated first in free list and keep alive for hit blocks. Signed-off-by: Zhou Xianrong <Xianrong.Zhou@amd.com>
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Thanks @shanrow-amd, so it seems like this change isn't worth adding? |
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Hi @njhill I think this might be due to the "vllm bench throughput" test that has very lower hit ratio actually. I think this refinement is valuable to adding because the idea has no coasts and is very simple. |
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@shanrow-amd that would be great if you could find a benchmark workload to demonstrate that it helps! |
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Purpose
Most cached blocks with block_hash that never hit. Now they are appended to the free list mixed with
fewer hit blocks. We should differentiate the both cases because of the non-hit blocks amount that is
huge and far greater than hit blocks. So we should append the non-hit blocks first and then append
the hit blocks so that non-hit blocks are allocated first in free list and keep alive for hit blocks.
Test Plan
default testcases
python -m pytest tests/v1/core/test_kv_cache_utils.py -v
python -m pytest tests/v1/core/test_prefix_caching.py -v
python -m pytest tests/v1/core/test_single_type_kv_cache_manager.py -v
VLLM_CACHE_ROOT=/data/hf/vllm/ vllm bench throughput --model /data/hf/Qwen3.6-35B-A3B --input-len 1024 --output-len 1024 --dtype float16 --num-prompts 2000 --tensor-parallel-size 4 --enable-prefix-caching
Test Result
python -m pytest tests/v1/core/test_kv_cache_utils.py -v
76 passed, 14 warnings
python -m pytest tests/v1/core/test_prefix_caching.py -v
89 passed, 14 warnings
python -m pytest tests/v1/core/test_single_type_kv_cache_manager.py -v
9 passed, 14 warnings
before:
Throughput: 6.61 requests/s, 7610.42 total tokens/s, 845.60 output tokens/s
Throughput: 6.77 requests/s, 7794.73 total tokens/s, 866.08 output tokens/s
Throughput: 6.75 requests/s, 7776.25 total tokens/s, 864.03 output tokens/s
Throughput: 6.85 requests/s, 7886.45 total tokens/s, 876.27 output tokens/s
after:
Throughput: 6.73 requests/s, 7752.48 total tokens/s, 861.39 output tokens/s
Throughput: 6.73 requests/s, 7752.48 total tokens/s, 861.39 output tokens/s
Throughput: 6.86 requests/s, 7897.19 total tokens/s, 877.47 output tokens/s
Throughput: 6.71 requests/s, 7734.48 total tokens/s, 859.39 output tokens/s
Performance has no change.