check: gate a schedule-sensitive jit-stats counter with a declared band - #1281
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WalkthroughThe PR adds per-fixture JIT-stat tolerance bands. It validates band declarations, classifies in-band changes, adjusts stability retries, and reports banded runs as visible non-failing results. The recursive benchmark declares an eight-count guard-failure band. ChangesJIT statistics tolerance bands
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The new band configuration can silently ignore duplicate declarations, making a fixture’s intended allowance differ from what the file declares. This localized validation issue should be fixed before merging; the remaining requested checks should also be run. Sequence Diagram(s)sequenceDiagram
participant Fixture
participant Check
participant JITStatsComparison
participant BackendReporting
Fixture->>Check: provide JIT-stat band directives
Check->>JITStatsComparison: compare counters with configured bands
JITStatsComparison-->>Check: return banded and failing changes
Check->>BackendReporting: report gate status and affected counters
BackendReporting-->>Fixture: display visible non-failing banded result
Possibly related PRs
Suggested reviewers: Poem
🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches📝 Generate docstrings
🧪 Generate unit tests (beta)
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| return "banded", ( | ||
| "jit-stats within band — " + ", ".join(banded) | ||
| + _jit_stats_context(jitstats) | ||
| ) |
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When a counter differs within its declared band, this early return bypasses the later --snapshot-diff output comparison and --threshold time comparison. For the newly annotated fixture, hosts that consistently observe guard_failures slightly above the committed baseline will therefore skip those requested gates on every run, potentially accepting a snapshot or performance regression. Preserve the banded status but continue through the remaining checks before returning it.
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| moved = _jit_stats_change(jitstats, drifted, jitstats_bands) | ||
| # Snapshot text can drift inside a band without making any | ||
| # gated counter unstable. | ||
| if moved[0] or moved[1]: |
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Applying the band between the first run and its repeat makes the stability decision depend on which sample ran first. For example, with baseline 2951 and band 8, an initial 2960 is gated, but a repeat of 2958 differs by only 2 and is ignored here, so the run fails as stable even though the repeat is within the allowed baseline band; reversing the samples passes without any repeats. Compare each repeat with the committed baseline and check whether it reproduces the original classification, rather than treating the baseline-relative band as a tolerance between samples.
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Inline comments:
In `@pyre/check.py`:
- Around line 1509-1541: Update the jit-stats band directive parsing loop to
continue scanning the header after the first matching directive, reject any
subsequent directive with ValueError, and preserve the existing field and width
validation; adjust the return behavior so the final result is produced only
after the header scan completes.
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pyre/bench/synth/generator_tree_recursion.pypyre/check.py
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…tree_recursion's guard_failures `# pyre-check: jitstats-band=<field>=<width>` suppresses an integer-valued jit-stats move whose absolute delta is at or below the width, in both directions. Unknown fields, duplicates, non-integer and non-positive widths are errors rather than silent no-ops, and a badness counter cannot be banded because its healthy value is exactly zero. Read from the first 20 lines like the other `# pyre-check:` headers; below that window it is not read and the fixture gates normally. `_apply_snapshot_gate` passes the fixture's bands to both `_jit_stats_change` calls. The second one selects its `drifted` snapshot by comparing text, so a repeat that moved only inside a band would report "jit-stats unstable — re-running the same binary moved" with nothing after it; the unstable path is now taken only when the banded comparison is non-empty. `generator_tree_recursion` gets `guard_failures=8`. Measured on one host, one binary, one arch: the jitcounter decays by 0.96 every 32 minor collections (majit-trace/src/counter.rs), so the counter follows the minor-collection count during each guard's warm-up. Sweeping the nursery from 512KB to 8MB reads 2951..2958, and `PYRE_JIT=decay=0` reads 2999 at every one of those sizes. `loops_compiled=3` and `bridges_compiled=26` hold throughout and stay gated exactly. Assisted-by: Claude
`_jit_stats_change` returns `(regressions, improvements, banded)`. A field the band suppresses is listed in `banded` as `field old -> new (within band N)` and in neither of the other two, so the move is reclassified rather than hidden. A run whose only movement was absorbed returns the new non-failing `banded` status, which renders like `unstable` — a yellow line, the bench still passes — and is tallied in the end-of-run summary as "jit-stats within band (not gated)". When the row is already failing for another counter, the absorbed move is appended to the same reason as a `within band:` part. `unstable_line` becomes `note_line` at the call site, since it now carries either note, and the label is derived from the status. Assisted-by: Claude
A banded verdict is assigned rather than returned. Every other verdict reached before it is a failure, so leaving the function early costs those nothing; a band means the run is fine, and returning there carried the fixture past the `--snapshot-diff` output comparison and the `--threshold` time comparison — on a host that sits inside its band, on every run. The repeat-drift comparison no longer takes the bands. It compares two samples of the same invocation, so a band applied there measures from whichever sample ran first instead of from the recorded baseline: with baseline 2951 and band 8, a first run of 2960 was gated while a repeat of 2958 was absorbed as a difference of 2, and swapping the samples changed the verdict. Unbanded, two snapshots that differ textually must differ in a `JITSTATS_SNAPSHOT_FIELDS` entry, so the guard added with the bands is unreachable and the original unstable path is restored. `synth_jitstats_bands` scans the whole 20-line window and raises on a second directive instead of returning at the first. It already rejects a duplicate field inside one line, so dropping a duplicate line silently would let one of two per-field directives do nothing. Assisted-by: Claude
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synth/generator_tree_recursionreds CI withguard_failures 2951 -> 2955,identically on ubuntu-x86_64 and windows-x86_64, against a baseline recorded on
darwin-arm64. It is not a regression.
What the counter actually follows
JitCounter::newregistersinvoke_after_minor_collection, which bumps a decaygeneration every 32 minor collections, and
warmstatecallsset_decay(40)— sotickmultiplies every guard counter by0.96 ** generationsbefore using it (majit-trace/src/counter.rs). Withtrace_eagerness = 200, a guard whose warm-up window straddles a decaygeneration needs a different number of failures before its bridge attaches, and
guard_failurescounts exactly the deopts taken before that bridge lands. Thecounter therefore follows the minor-collection count during warm-up, which moves
with allocation volume and layout — including target architecture.
Measured on one machine, one binary, one arch (darwin arm64), with
check.py'sown pins otherwise:
guard_failuresloops_compiled=3 bridges_compiled=26on every row. CI's 2955 sits inside theband one host produces by itself.
The falsifying test:
PYRE_JIT=decay=0reads 2999 at every one of thosenursery sizes. Turning the decay off removes the entire variance. A decay sweep
at a fixed 4MB nursery reads
0 -> 2999,20 -> 3002,40 -> 2951,80 -> 2955— non-monotone, withloops_compiled=3 bridges_compiled=26 FIRED=26on all four.This is upstream-faithful:
rpython/jit/metainterp/counter.pydecays itscounters from the same GC hook. Nothing in the JIT is changed here.
Why the existing devices do not reach it
JITSTATS_STABILITY_RUNSre-runs the same binary in the same invocation andun-gates a fixture that fails to reproduce itself. This fixture reproduces
exactly — five consecutive runs gave byte-identical snapshots — so that device
provably never fires. The variance is across hosts, and
check.pyruns on onehost at a time.
Trip-count reduction (the
str_fstringprecedent) does not apply either:startup allocation alone costs ~49 minor collections, so no shortening reaches
zero decay generations, and cutting 9000 -> 3600 drops
bridges_compiled26 -> 18, deleting the mechanism the fixture exists to cover. A
.<platform>.jitstatsoverlay cannot express arch. Re-recording to 2955 movesthe red to macOS.
What this adds
# pyre-check: jitstats-band=<field>=<width>— a per-fixture, per-counterallowance, symmetric around the recorded baseline, read from the first 20 lines
like the other
# pyre-check:headers.rather than silent no-ops, following
skip-backends.allowance there would let a real defect through.
_jit_stats_changereturns athird list, and a run whose only movement was absorbed gets the new
non-failing
bandedstatus — a yellow line naming the field, both values andthe width, plus an end-of-run summary row. When the row is already failing for
another counter, the absorbed move is appended to the same reason. This keeps
the band as accountable as the
unstablearm it is modelled on.Only
generator_tree_recursionopts in, atguard_failures=8: one unit ofmargin over the measured 2951..2958 span, 0.27% of the counter. The regressions
this gate exists for move by hundreds to thousands (
828 -> 4923,404 -> 812,937 -> 7408), andloops_compiled/bridges_compiled— invariant under everyperturbation above — stay gated exactly.
Verification
Against the real committed baseline and the real fixture header:
guard_failures2951 -> 2958bridges_compiled26 -> 0Fed through the real pipeline (
_jit_stats_snapshoton measured stderr), the4MB run (2951) passes either way and the 768KB run (2958) fails without the band
and passes with it. Parser validation covers unknown / badness / duplicate /
zero / negative / non-integer / empty and the 20-line boundary.
ruffreportsthe same two pre-existing findings as
main.🤖 Generated with Claude Code
https://claude.ai/code/session_01TBNBmnBveAQFbqRtrvQVyQ
Summary by CodeRabbit
New Features
Bug Fixes
Documentation