Follow-up to Microbench 3 that finally tests H5 (cache-size
dependence) and instruments worker-side connector callbacks the
original patch missed.
Patch v2 (apply_step_timing_v2.py) adds:
scheduler: `cache_size` field in engine_step.jsonl
worker: `get_finished_us` + `start_load_kv_us` in worker_step.r0.jsonl
uses BLOCK_BEGIN/END sentinels for safe multi-line revert
(the original v1 patch survives this v2's apply/revert cycle)
Driver: continuous open-loop (1.5 req/s, 4096x256 random per req)
that lets APC fill from 0 → ceiling within one vLLM lifetime so a
single run produces the full cache_size sweep. Decode-only steps
are filtered post-hoc to remove prefill-mix variance.
Findings (H20 96GB, ceiling reached ~17.5k blocks; n=15-18k decode
steps per config):
config | slope (μs / 1k blocks) | step_dur p50 @ |cache|=16.6k
---------------|------------------------|-----------------------------
mooncake_both | +85.6 | 1528 μs (build_meta=1442, 94%)
noop_connector | -0.8 (≈0) | 79 μs
plain | +1.0 (≈0) | 84 μs
Worker-side get_finished p50/p90/p99 (μs/step):
mooncake_both: 180 / 257 / 333
noop_connector: 0 / 0 / 2
H5 PASSES. mooncake_both step_duration scales linearly with |cache|
because build_connector_meta walks set(cache.keys()) every step
(`mooncake_connector.py:434-450`). plain and noop are flat.
The previously-uninstrumented get_finished() adds a constant
180 μs/step on top — two `run_coroutine_threadsafe(...).result()`
blocking waits in kv_both mode (`mooncake_connector.py:1107-1137`)
fire every step even when no transfer is pending.
Trace-replay reconciliation (APC ≈ 79% → |cache| ≈ 13k blocks):
build_meta @ 13k ≈ 1060 μs + get_finished ≈ 180 μs = 1.24 ms/step
On ~7 ms decode forward → +15-20% TPOT per step.
This explains most of the trace-replay +25% TPOT p90 gap from
single-instance per-step cost alone, leaving a smaller residual
for multi-instance coupling than originally assumed.
Two clear fixes pointed out in REPORT.md:
1. replace O(|cache|) per-step walk with incremental delta
listener using block_pool's add/remove callbacks
2. short-circuit get_finished() when both producer/consumer
queues are empty in kv_both
Heavy raw artifacts (engine_step.jsonl, vllm_stdout/stderr,
.vllm.pid) are .gitignored — they re-derive from `bash run_all.sh`
and SUMMARY.md / per_config.json fully capture the conclusions.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
The prior write-up presented one specific reading of the data as
the headline without flagging methodology gaps. Three corrections:
1. The "0% low-concurrency tax" comes from a single back-to-back
mooncake_both_v2/plain_v2 rerun. The original Phase A pair
showed TTFT p90 +29%, TPOT p90 +54%, E2E p90 +55% at rate=2
— a 40 percentage-point swing between two consecutive runs
that the original write-up did not call out. The run-to-run
noise floor is too high to claim "0%" at low concurrency.
2. get_finished() was never instrumented. The patch only times
step_duration_us and build_meta_us. "100% of per-step cost is
build_meta" is an upper bound on what was timed, not a true
decomposition.
3. H5 (cache-size dependence) was the central hypothesis but
was never tested in the prior run; random content kept APC
near empty.
The +7-9% high-concurrency (single instance, 512x64, rate=8-16)
and +17% 8-instance-saturated numbers are kept; they were
measured with adequate sample sizes and are reproducible.
The follow-up sweep in cache_sweep/ tests H5 directly and
revises the decomposition.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
8×TP1 + load_only proxy, shape 512×64, rates 32/64/128 req/s total:
Rate=32 (non-saturated, thr=0.95-0.97):
plain TTFT p90=64ms, mooncake_both=65ms → +2% (noise)
Rate=64 (non-saturated, thr=0.96):
plain TTFT p90=114ms, mooncake_both=107ms → -6% (noise)
Rate=128 (saturated, thr=0.70-0.71):
plain TTFT p90=702ms, mooncake_both=822ms → +17%
plain TTFT p50=339ms, mooncake_both=470ms → +39%
Conclusion: The elastic_migration_v2 +45% is a saturation artifact.
Under SLO-compliant load (TTFT<10s, thr_ratio>0.9), mooncake_both's
1.4ms/step build_connector_meta overhead is completely masked by the
scheduler-model async pipeline. The tax only manifests when the system
is already saturated and queueing amplifies per-step differences.
For practical deployment: enabling kv_role=kv_both has effectively zero
cost as long as the serving system stays within SLO capacity bounds.
