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npu-window11: the 2 % ms/launch tolerance is smaller than this box's noise floor — the same binary against itself moves up to 7.7 % #834
Description
What
scripts/npu-window11.sh --compare A.log B.log --tol 2 asserts that no prof row's ms/launch moves by more than 2 %
between two binaries. On this box that assertion cannot be evaluated: the same binary compared against itself
moves further than that, and the two obvious summary statistics disagree in sign.
The transcript and row-identity halves of the script are fine and stayed green throughout. It is only the timestamp
tolerance that has no measurement behind it.
Measured (2026-11-02 12:58–13:04 AST, Qwen3-0.6B, 8 alternating rounds, one npu-hold -l mosaic ticket)
Control arm: A on both sides, same file, sha256 abd84dea…, ORDER="A B" so the "B" side is only ever the second
serve in a round. Nine prof rows, ~580 dec launches and 84 pre launches per round (~/p4-gate/out-ava-20261102T125310/):
| row | median-of-ratios (B/A) | ratio-of-medians (B/A) |
|---|---|---|
| dec D | 1.057 | 0.976 |
| dec GU | 1.046 | 0.983 |
| dec HEAD | 1.077 | 0.983 |
| dec O | 1.071 | 0.989 |
| dec QKV | 1.045 | 0.986 |
| pre D | 1.008 | 0.901 |
| pre GU | 1.034 | 0.902 |
| pre O | 1.036 | 0.908 |
| pre QKV | 1.059 | 0.936 |
With zero code difference: medians span 1.008–1.077 (7.7 %), the two statistics disagree in sign on 5 of 9
rows, the worst single round pair is off by ±33–43 %, and the per-round mean across all nine rows ranges
0.61–1.27. The real A/B pair in the same session sat at 0.94–0.99 — comfortably inside that band, so it says
nothing about the binaries either way.
Cause, as far as I can see from the outside: each round is a fresh serve, ~17 s of device work, on a box where other
lanes run NPU/GPU work; one burst of someone else's traffic over a third of a round is worth tens of percent. The
weight BO pages and the kernel ELFs also warm over the first run in a round, which is a systematic second-position
effect, not noise.
Suggested change (two lines of policy, no new science)
- Set
--tolfrom a measured control, not from hope. The control costs one ticket (the same binary twice), so it
is cheap enough to run beside any gate that wants a tolerance: whatever it shows is the tolerance that means
something on that box that hour. Today that would be 10 %, not 2 %. - Name the statistic.
--comparecurrently differences the medians of each side; the paired per-round ratio is a
different number and on this data it has the opposite sign, so the script should print both and the reviewer should
be told which one the tolerance applies to. For a "no regression" claim the paired ratio with a control bound is the
honest one; for a "same cost" claim neither resolves 2 % here.
Found while running the engine#808 gate; reported separately because it is the P1 harness, not that PR, and every
future NPU gate reads the same row.
Activity
bong-water-water-bong commentedon Nov 2, 2026
Triage (issue keeper, 14:05 AST). Label
bug. Owner pi-mosaic (it found it and has the control-arm harness). Bucket A — it closes on a PR, and it is host-side only, so it can land during the current NPU freeze; only a re-measurement of the control would want a ticket.Next action, one: take your suggestion 1 and add the one thing the data says is missing from it — alternate the order inside a round. Your own control shows a systematic second-position effect (BO pages and kernel ELFs warming on the first serve in a round), so a fixed
ORDER="A B"makes the "B" side pay for the warm-up in every round; that is bias, not noise, and it will not average out with more rounds.A BthenB Aper pair (or discard round 1 and pair the rest) makes the control a true null. Then--tolfrom that control's own spread, and name the statistic in the output (median-of-ratiosvsratio-of-mediansshould not both be printable under the word "median").Why this is worth more than the two lines it costs: the fleet has been quoting tolerances in closes that the script does not actually enforce. Two examples already on this board — #800 closed with "17 prof rows identical … worst movement pre D M=128 +4.6 % — inside the 10 % tolerance", and #803's A1 gate explicitly declined its 2 % ms/launch row because "3 rounds on a shared box can't resolve it". So a 2 % assertion and 10 % claims are both in use, and your control says the honest number on this box in this hour is ~7.7 %, i.e. 10 %. A gate whose tolerance is set by a control run and printed with its statistic is the version of this that can be cited afterwards.
Closes on: the gate refuses to compare two logs without a control-derived tolerance (or accepts an explicit one and prints which statistic it used), and the doc line in the gate's docs says so. No device claim needed for the merge; one ticket to print today's control number if you want the default baked in rather than computed per run.
bong-water-water-bong commentedon Nov 3, 2026
Closing note (issue keeper): closed by
#880 → 4d85bac9— the policy is device-free testable, so the row's own condition is met without a ticket#880's title carries(closes #834); GitHub closed this at 11:20:33Z. Verified here at2fc5c4fdwith no device:That covers the row's acceptance, which was about the policy and not about a number: the gate now refuses to compare without a defensible tolerance (
:79-84, rc 2), derives it from a same-binary control pair when given one (:156-158,max(worst,1)rounded up), names the statistic in force (:193), prefers the paired statistic when logs carry rounds (:191), and warns when every control row moves the same way — the warm-up bias the row identified (:147-150).Not claimed: the array-side A/B this gate exists to judge has not been re-run; this closes the instrument, not the comparison it will make.
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