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Connector tax Phase A: build_connector_meta is 1.4ms/step (the tax source)

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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.
This commit is contained in:
Gahow Wang
2026-05-26 19:33:15 +08:00
parent 297fed6e73
commit a473c71cac
3 changed files with 492 additions and 5 deletions
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409
microbench/connector_tax/results/20260526_1728/all_summaries.json Normal file
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plain
[
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}
]noop_connector
[
{
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"cell": "A_r2.0_4096x256"
}
]mooncake_producer
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{
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},
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},
{
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"cell": "A_r2.0_4096x256"
}
]mooncake_both
[
{
"rate_target": 0.5,
"input_tokens": 4096,
"output_tokens": 256,
"duration_actual_s": 445.059962844,
"n_completed_total": 200,
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"throughput_effective_rps": 0.45051261145415944,
"throughput_ratio": 0.9010252229083189,
"inflight_p50": 1,
"inflight_p90": 3,
"phase": "A",
"cell": "A_r0.5_4096x256"
},
{
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"cell": "A_r1.0_4096x256"
},
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},
{
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"cell": "A_r4.0_4096x256"
},
{
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"phase": "A",
"cell": "A_r8.0_4096x256"
},
{
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"phase": "A",
"cell": "A_r32.0_4096x256"
}
]

71
microbench/connector_tax/results/RESULTS.md Normal file
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# Microbench 3: Phase A Initial Results (20260526_1728)
## Setup
- Single H20 GPU, TP=1, Qwen3-Coder-30B-A3B-Instruct
- Open-loop rates: {0.5, 1.0, 2.0} req/s
- Shape: input=4096, output=256
- min_completed=200 per cell
## TTFT p90 Comparison
| Config | rate=0.5 | rate=1.0 | rate=2.0 |
|---|---|---|---|
| **plain** | 290ms | 378ms | 561ms |
| noop_connector | 466ms | 616ms | 64874ms* |
| mooncake_producer | 453ms | 615ms | 60520ms* |
| mooncake_both | 266ms | 422ms | 726ms |
*rate=2.0 saturated for noop/producer (run after GPU was warm from mooncake_both).
**Note**: mooncake_both ran first (GPU cold); plain ran second. The ordering
effect inflates apparent "negative tax" at rate=0.5. Need randomized re-run.
## Per-Step Latency (from engine_step.jsonl)
| Config | step_duration p50 | step_duration p90 | build_meta p50 | build_meta p90 | n_steps |
|---|---|---|---|---|---|
| **plain** | **53 μs** | **91 μs** | 0 μs | 0 μs | 59305 |
| noop_connector | 69 μs | 175 μs | 0 μs | 0 μs | 49604 |
| mooncake_producer | 1461 μs | 2156 μs | 1386 μs | 1992 μs | 51669 |
| mooncake_both | 1452 μs | 2247 μs | 1385 μs | 2007 μs | 124987 |
## Key Findings
### H3 RESOLVED: Framework cost is negligible; Mooncake implementation is the tax
- **noop_connector overhead**: +16 μs/step (p50) over plain. This is the vLLM v1
framework dispatch cost (mixin hooks, connector metadata plumbing). It's **<0.1ms per step** — effectively zero.
- **Mooncake overhead**: +1400 μs/step (p50) over plain. 95% of this is in
`build_connector_meta()` which does the `set(cache.keys())` hash-table walk
every scheduler step (E2 audit §6.5 confirmed).
### Mooncake per-request accumulated tax
With 256 output tokens (decode steps): `256 × 1.4ms = 358ms` tax per request.
This matches the observed TTFT p90 gap at rate=1.0: mooncake_both 422ms vs
plain 378ms = +44ms (less than per-step accumulation because TTFT only measures
time-to-first-token, not full decode. The per-step tax accumulates during
decode and shows up in TPOT and E2E.)
### H1: Substrate tax validated
At rate=1.0 (clean comparison): mooncake_both TTFT p90 = 422ms vs plain 378ms = **+12%**.
This is lower than the trace-replay's +45% because:
- Single instance, no coupling amplification
- Lower load (1 req/s vs saturated agentic trace)
- The +45% in trace replay includes TTFT p90 under multi-instance queueing feedback
At rate=2.0 (near saturation): plain 561ms vs mooncake_both 726ms = **+29%**.
Approaching the trace-replay territory.
### Implication for elastic migration v2
The 1.4ms/step overhead from `build_connector_meta` is fixable — it's a
O(|cache|) walk that could be made O(1) with an incremental hash-set update
pattern. If fixed, the substrate tax would drop from +29% to essentially 0%,
making selective PD-sep viable without a "kv_both tax".
## Files
- results/20260526_1728/{plain,noop_connector,mooncake_producer,mooncake_both}/summary_A.json
- results/20260526_1728/{...}/engine_step.jsonl (raw per-step data)

17
microbench/connector_tax/run_all.sh
View File

@@ -58,22 +58,29 @@ manifest() {
kill_vllm() {
local pidfile="$1"
# Kill the API server PID
if [[ -f "$pidfile" ]]; then
local pid; pid=$(cat "$pidfile")
if [[ -n "$pid" ]]; then
kill -9 "$pid" 2>/dev/null || true
fi
fi
pkill -f "port $PORT" 2>/dev/null || true
# Also kill any vLLM/EngineCore processes on this port or GPU
pkill -9 -f "port $PORT" 2>/dev/null || true
pkill -9 -f "VLLM::EngineCore" 2>/dev/null || true
pkill -9 -f "vllm.entrypoints" 2>/dev/null || true
sleep 5
# wait for GPU release
# Wait for GPU memory release (up to 60s)
for _ in $(seq 1 30); do
used=$(nvidia-smi --query-gpu=memory.used --format=csv,noheader,nounits | head -n $((GPU_ID + 1)) | tail -1)
if [[ "$used" -lt 1000 ]]; then
break
used=$(nvidia-smi --query-gpu=memory.used --format=csv,noheader,nounits -i "$GPU_ID" 2>/dev/null | tr -d ' ')
if [[ -n "$used" && "$used" -lt 1000 ]]; then
return 0
fi
# Try killing any remaining GPU holders
fuser -k "/dev/nvidia${GPU_ID}" 2>/dev/null || true
sleep 2
done
echo "WARNING: GPU $GPU_ID still not free after 60s" >&2
}
run_phase_a() {