Measures TTFT to serve a reused prefix of length L from each KV tier on a
single H20 (Qwen3-Coder-30B-A3B, vLLM 0.18.1): miss (recompute), CPU-tier
hit (native DRAM offload), GPU-tier hit (HBM prefix cache). Each measured
request is bracketed by /metrics scrapes so the tier is verified
(vllm:prefix_cache_hits vs external_prefix_cache_hits), not assumed.
Result: GPU hit is ~flat (42->111 ms over 1k->64k tokens); CPU hit is
transfer-bound (PCIe H2D ~54 GB/s, 57->272 ms); miss grows superlinearly
(78 ms -> 15.2 s). GPU beats CPU 1.4-2.5x (gap grows with context);
miss/CPU up to 56x, miss/GPU up to 137x. pcie_transfer.py is the
independent CPU-hit floor backstop. Evidence for the GPU-hit-first
principle (paper section 2.2).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Gahow Wang
837df6bc9e