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.
