[Core] Add per-request prefix-cache write policy - #51981
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Allow requests to reuse existing prefix-cache entries without adding their newly computed blocks to the local cache. Expose the policy across OpenAI, Responses, Anthropic, and beam-search request paths. Fixes vllm-project#51234 Co-authored-by: pi-coding-agent <pi@earendil.works> Signed-off-by: NancyFyong <88076188+NancyFyong@users.noreply.github.com>
Add a deterministic CPU workload that mixes reusable hot prompts with one-off RAG-style suffixes and reports hit rate and recomputed hot-prefix tokens. Co-authored-by: pi-coding-agent <pi@earendil.works> Signed-off-by: NancyFyong <88076188+NancyFyong@users.noreply.github.com>
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Purpose
#51234.
Add a per-request
skip_writing_prefix_cachepolicy. A request can still reuse an existing local prefix-cache entry, but newly computed blocks from that request are not admitted to the local prefix cache.This is useful for mixed workloads such as:
The shared prefix remains reusable, while low-reuse suffixes do not accumulate and evict hotter prefixes.
Method
The policy is carried through OpenAI Chat/Completions/Responses, Anthropic Messages, batch chat, streaming updates, and online/offline beam search. Both the synchronous allocation path and async delayed cache-commit path skip local cache insertion. Runtime KV allocation is unchanged, and external KV connectors are intentionally out of scope.
Benchmark
The deterministic CPU benchmark seeds eight reusable hot prompts, then mixes in 64 one-off RAG-style requests. Each cold request reuses a 4-block common prefix and has a unique 24-block suffix. The cache has 159 usable blocks; an active cold request fits beside the complete hot cache, so this measures admission pollution rather than unavoidable active-request pressure.
No-store saves 11,904 hot-prefix prefill tokens while preserving cold requests' shared-prefix hits. With a large 512-block cache both policies reach 100% hot hits, confirming the benefit appears specifically under cache-capacity contention.
Tests
ruff-check,ruff-format, andmypy-3.12: passedAccountability
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skip_writing_prefix_cache.AI assistance was used. The human submitter has manually reviewed every changed line, understands the implementation and benchmark, and ran/reviewed the reported tests and results.