Adds `--dispatch-mode {tracets,thinktime}` to the replayer and documents that
agentic serving should be benchmarked with `thinktime` (the faithful load).
- `tracets` (old default): turn-k at the absolute trace timestamp, i.e.
max(prev_finished, trace_ts) -- collapses inter-turn think-time to ~0 when the
system is behind, manufacturing request bursts.
- `thinktime`: turn-1 at trace arrival; turn-k at prev_finished +
time_to_parent_chat (real production gap). scripts/add_time_to_parent.py
annotates a trace with that gap from the raw trace's request_ready/end_ms.
exp(c) ablation (v2/exp_c_dispatch_ablation/): at N=8 (capacity slack) thinktime
beats tracets -- E2E p90 -28% (73.5 vs 102.8s), TTFT p90 -29%, TPS +7%, because
tracets' bursts spike concurrency -> KV pressure -> preemption. At N=6
(saturated) they converge. So tracets makes the system look ~30% worse on tail
latency than realistic agent pacing. Root README.md carries the headline
guidance; raw per-request metrics gitignored (perf_summary.json kept).
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>
