Gahow Wang
a473c71cac
Per-step timing from engine_step.jsonl definitively resolves H3: plain: 53 μs/step (p50) noop_connector: 69 μs/step (+16 μs = negligible framework cost) mooncake_producer: 1461 μs/step (build_connector_meta = 1386 μs) mooncake_both: 1452 μs/step (same as producer) The substrate tax is NOT in the v1 framework — it's specifically in Mooncake's build_connector_meta() which walks set(cache.keys()) every scheduler step (O(|cache|) per step, E2 audit §6.5). Accumulated per-request tax: 256 decode steps × 1.4ms = 358ms. Observed TTFT tax at rate=1.0: plain 378ms vs mooncake_both 422ms (+12%). At rate=2.0 (near saturation): +29%, approaching trace-replay's +45%. Also fixes kill_vllm() to properly kill EngineCore subprocesses.
72 lines
3.0 KiB
Markdown
72 lines
3.0 KiB
Markdown
# Microbench 3: Phase A Initial Results (20260526_1728)
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## Setup
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- Single H20 GPU, TP=1, Qwen3-Coder-30B-A3B-Instruct
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- Open-loop rates: {0.5, 1.0, 2.0} req/s
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- Shape: input=4096, output=256
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- min_completed=200 per cell
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## TTFT p90 Comparison
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| Config | rate=0.5 | rate=1.0 | rate=2.0 |
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|---|---|---|---|
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| **plain** | 290ms | 378ms | 561ms |
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| noop_connector | 466ms | 616ms | 64874ms* |
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| mooncake_producer | 453ms | 615ms | 60520ms* |
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| mooncake_both | 266ms | 422ms | 726ms |
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*rate=2.0 saturated for noop/producer (run after GPU was warm from mooncake_both).
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**Note**: mooncake_both ran first (GPU cold); plain ran second. The ordering
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effect inflates apparent "negative tax" at rate=0.5. Need randomized re-run.
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## Per-Step Latency (from engine_step.jsonl)
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| Config | step_duration p50 | step_duration p90 | build_meta p50 | build_meta p90 | n_steps |
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|---|---|---|---|---|---|
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| **plain** | **53 μs** | **91 μs** | 0 μs | 0 μs | 59305 |
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| noop_connector | 69 μs | 175 μs | 0 μs | 0 μs | 49604 |
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| mooncake_producer | 1461 μs | 2156 μs | 1386 μs | 1992 μs | 51669 |
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| mooncake_both | 1452 μs | 2247 μs | 1385 μs | 2007 μs | 124987 |
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## Key Findings
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### H3 RESOLVED: Framework cost is negligible; Mooncake implementation is the tax
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- **noop_connector overhead**: +16 μs/step (p50) over plain. This is the vLLM v1
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framework dispatch cost (mixin hooks, connector metadata plumbing). It's **<0.1ms per step** — effectively zero.
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- **Mooncake overhead**: +1400 μs/step (p50) over plain. 95% of this is in
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`build_connector_meta()` which does the `set(cache.keys())` hash-table walk
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every scheduler step (E2 audit §6.5 confirmed).
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### Mooncake per-request accumulated tax
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With 256 output tokens (decode steps): `256 × 1.4ms = 358ms` tax per request.
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This matches the observed TTFT p90 gap at rate=1.0: mooncake_both 422ms vs
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plain 378ms = +44ms (less than per-step accumulation because TTFT only measures
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time-to-first-token, not full decode. The per-step tax accumulates during
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decode and shows up in TPOT and E2E.)
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### H1: Substrate tax validated
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At rate=1.0 (clean comparison): mooncake_both TTFT p90 = 422ms vs plain 378ms = **+12%**.
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This is lower than the trace-replay's +45% because:
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- Single instance, no coupling amplification
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- Lower load (1 req/s vs saturated agentic trace)
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- The +45% in trace replay includes TTFT p90 under multi-instance queueing feedback
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At rate=2.0 (near saturation): plain 561ms vs mooncake_both 726ms = **+29%**.
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Approaching the trace-replay territory.
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### Implication for elastic migration v2
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The 1.4ms/step overhead from `build_connector_meta` is fixable — it's a
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O(|cache|) walk that could be made O(1) with an incremental hash-set update
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pattern. If fixed, the substrate tax would drop from +29% to essentially 0%,
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making selective PD-sep viable without a "kv_both tax".
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## Files
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- results/20260526_1728/{plain,noop_connector,mooncake_producer,mooncake_both}/summary_A.json
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- results/20260526_1728/{...}/engine_step.jsonl (raw per-step data)
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