Root cause of 10.1pp APC gap: multi-turn sessions' KV evicted between
turns by cold-start prefills (66% of loss). Inter-turn gap is only 2
requests p50, but LRU cache (550 blocks) can't protect 93 blocks/session
across 14-21 concurrent sessions.
Three approaches designed:
A. Session-sticky routing with KV reservation (proxy-only, no vLLM change)
B. Two-tier KV cache: GPU + DRAM offload via Mooncake
C. Prefill-aware eviction (LFU/ARC instead of LRU, vLLM patch)
Next: simulate LRU vs LFU vs "infinite-for-MT" to quantify upper bounds,
then implement Approach A (lowest effort, immediate benchmark).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
