A single pair cannot say whether its own verdict is stable, and on this device
that is not a theoretical gap. Measured on an S5000, topk eliminated 32
convert_layout ops yet 17 of its 42 shapes changed the SIGN of their delta
between two equally valid runs. The cause is not the timer: graph capture and
direct timing agree to 1.00x on that operator, and the verdict moved a median of
1.1pp between them, inside the 1.7pp noise floor a same-timer control
established. The deltas are simply near zero, and one measurement cannot orient
them.
ab_repeat.sh runs an A/B over N rounds in independent processes. Round 1's A
side records the autotune configs and every later run -- including A's own later
rounds -- replays that one set, so configs are pinned across rounds and sides and
only the switch under test and measurement noise remain. Round order alternates
to cancel one-directional drift. An operator with a failed round gets no report,
since a report missing rounds reads exactly like a complete one.
batch_summary --ab-rounds aggregates <root>/r<k>_<A|B>/run.log and separates the
two dispersions, which answer different questions and were previously conflated:
the spread within one side's samples says how noisy the machine was, while the
spread of the per-round deltas says whether the verdict holds. Shapes whose
delta changes sign across rounds are counted and called out, because their
direction is not a conclusion at any timer.
The path is already validated on NVIDIA: six rounds narrowed router_gemm to
-16.6% ~ -16.2% with 0.4% side noise, which is what turned a single pair into a
statement.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
