Extends the hit-latency microbench to a 4th tier: a remote global-KV-store
hit over RDMA, the Mooncake-Store mechanism. Two kv_both MooncakeConnector
instances (run_rdma.sh); for each prefix length, instance B serves the
request by pulling instance A's cached prefix over RDMA (do_remote_prefill,
via microbench/fresh_setup/mb2_kv_transfer.py) instead of recomputing -- the
timed pull is the remote-hit latency.
Result (TTFT p50, 11 reps): strict tier ordering
GPU(HBM) < CPU(local DRAM) < remote-RDMA-store << miss, gaps growing with
context. At 64k: GPU 0.11s, CPU 0.27s, RDMA 0.97s, miss 15.2s -> miss/RDMA
15.8x, RDMA/CPU 3.6x, CPU/GPU 2.4x. So a global RDMA store is a real win
over recompute (the blog's 46x) but pays the NIC tax (~5-7 GB/s effective)
and sits a tier below local CPU and two below GPU -- reinforcing
GPU-hit-first. README + figure updated to four tiers.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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>
