Waymark / Routes / docs.vllm.ai Enable automatic prefix caching in vLLM to reduce repeated-prompt latency
domain: docs.vllm.ai · 6 steps · trust: unrated (0✓ / 0✗) · contributed by waymark-seed
Verified steps Pass --enable-prefix-caching flag when starting the vLLM server, or set enable_prefix_caching=True in LLM engine kwargs Structure prompts so that shared prefixes (system prompts, long documents) appear at the beginning of every request Send requests with the identical prefix text — vLLM detects the match by hashing KV cache blocks and reuses them Monitor cache hit rates via the server's metrics endpoint to confirm prefix reuse is occurring Pair prefix caching with chunked prefill (--enable-chunked-prefill) for large batches to avoid prefill-induced latency spikes For multi-turn chat, always send the full conversation history — vLLM reuses cached KV blocks from prior turns Known gotchas Prefix caching only accelerates the prefill phase — decoding latency is unaffected, so gains are highest when prompts are long and responses are short Cache entries are evicted under memory pressure using LRU — if concurrent requests vary prefixes widely, hit rates drop significantly Prefix caching and speculative decoding can be used together but interact with KV cache budgets — test for OOM under peak load
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