fix(gc): price the tiny-parse pressure guard by the productivity backoff (#9831) - #9838
fix(gc): price the tiny-parse pressure guard by the productivity backoff (#9831)#9838proggeramlug wants to merge 3 commits into
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Issue PerryTS#9831 measured the ArenaBytes arm firing 51 times in one 66-delta claude-code reply, each collection freeing a median 131 KB, while the adaptive step sat saturated at 1 GiB. The issue located the discarded backoff in the arm's own re-arm arithmetic; correcting that (the issue's refuted branch) bought -10.8 % CPU for +22 % settled footprint and was rightly rejected. The arm's re-arm is not what re-fires it. Between two consecutive firings the arena grows a few hundred KB, against a trigger armed 16 MB (and below the ceiling, up to 128 MB) above the post-collection total. What pulls the trigger back down is the tiny-parse pressure guard: after every `JSON.parse` that grew the arena by <= 1 MB, `gc_bump_malloc_trigger` (and `gc_schedule_parse_boundary_collection_ if_pressure`, and the boundary collector they arm) tests the absolute `arena_in_use_bytes() >= 48 MB` and, if so, sets the trigger to "now". That threshold is a quantity no collection can lower below the live set, so on a program whose live set never drops under it every small parse -- one per SSE delta -- forced a minor at the next safepoint. The step those minors doubled was consulted by nothing. The guard now also requires the arena to have grown, since the last collection of any kind ended, by a headroom priced from the step: the step rescaled so that its power-on value (128 MB, the ceiling) buys the 16 MB floor, and each doubling the arm's ceiling clamp discards buys the guard one more doubling, bounded by the same ceiling. A productive collection halves the step and the guard keeps the cadence it always had; an unproductive one earns it room. The boundary collector re-prices a pending request so a collection that already satisfied it is not followed by a second one. Measured on the compiled claude-code TUI (cli_2.1.112.js, Linux, same perry binary, runtime-only A/B, 7 interleaved rounds, 3300-char streamed reply, chunk 50): turn CPU base 30.2-41.5 s (mean 35.1) fix 27.8-29.2 s (mean 28.6) post-turn RSS base 754-1057 MB (mean 803) fix 733-855 MB (mean 786) post-idle RSS base 527-1073 MB (mean 736) fix 517-843 MB (mean 722) peak RSS 1964-2062 MB both arms The fix wins CPU in every pair (-8 % to -30 %); footprint is flat within the base's own spread. The base arm is bimodal in both, which is what an absolute in-use threshold does. PERRY_GC_DIAG on one reply: copying minors 104 -> 84 (ArenaBytes 41 -> 13), old-gen fulls 19 -> 7, and the guard forced exactly one collection, after a genuine 16 MB of growth (`[gc-tiny-parse]` is the new witness line). test_memory_json_churn -- the guard's motivating shape -- is byte-identical in output and RSS in all four GC modes; 48/48 test_gap_gc_* and 8/8 test_gap_json_* pass. The arm's own arithmetic is left as it was and now says why. Claude-Session: https://claude.ai/code/session_015kqVkH6rHzfvXskGAj3tRv
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Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review. 📝 WalkthroughWalkthroughThe GC tiny-parse pressure guard now prices arena growth using the adaptive GC step. It tracks post-collection occupancy, rechecks pending collections, emits optional diagnostics, and adds regression tests for pricing and baseline updates. ChangesTiny-parse pressure control
Estimated code review effort: 3 (Moderate) | ~25 minutes Merge Risk: ⚪ Minimal · up to The change updates GC pressure pricing and collection baselines; no unresolved current merge-readiness risk is recorded. Sequence Diagram(s)sequenceDiagram
participant ParseBoundary
participant GCPolicy
participant Collection
ParseBoundary->>GCPolicy: evaluate tiny-parse pressure
GCPolicy-->>ParseBoundary: schedule or skip collection
ParseBoundary->>GCPolicy: recheck pending pressure
GCPolicy->>Collection: force collection when pressure remains due
Collection-->>GCPolicy: complete collection and rebase baseline
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 78.26% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 23 functions across 3 files. (1 skipped: 1 unsupported.)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Coordination note from the cc-perf campaign (session 014UZWia6L37DpA93VLtNK9m): a companion defect fix is in progress in the same area — |
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Inline comments:
In `@changelog.d/9838-tiny-parse-pressure-pricing.md`:
- Line 8: Update the release-note sentence containing the issue reference so it
begins with “Issue `#9831`” instead of “#9831”, preserving the existing
description as one coherent changelog entry and avoiding Markdown heading
syntax.
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… a bare issue reference
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Landing note for whoever merges this: |
…rs change The structural JSON merge recomputed the census-window pin from the tree but did not carry the author's written re-audit. A pin whose hash tracks the tree while its justification lags is exactly the gap the pin exists to catch: the gate stays green and nobody has re-argued the window.
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Fixes #9831.
What the arm was actually re-firing on
#9831 measured the
ArenaBytesarm firing 51 times in one 66-delta claude-code reply, each collection freeing a median 131 KB while the adaptive step sat saturated at 1 GiB, and located the discarded backoff ingc_finish_arena_trigger_collection's ceiling clamp. Correcting that clamp (the issue'srefuted/arena-trigger-pricingbranch) was measured at −10.8 % CPU for +22 % settled footprint and rightly rejected.The clamp is not what re-fires the arm. In the captures, consecutive
ArenaBytesminors are separated by a few hundred KB of arena growth, against a trigger armed 16 MB above the post-collection total (128 MB, the ceiling, while the arena is below it). Nothing in that arithmetic can be due again after 500 KB. What pulls the trigger down is the tiny-parse pressure guard: after everyJSON.parsethat grew the arena by ≤ 1 MB,gc_bump_malloc_trigger(andgc_schedule_parse_boundary_collection_if_pressure, and the boundary collector both arm) tests the absolutearena_in_use_bytes() >= 48 MBand, if it holds, setsGC_NEXT_TRIGGER_BYTESto "now" and asks for a collection. That threshold is a quantity no collection can lower below the live set, so on a program whose live set never drops under it — claude-code holds 59–297 MB through one reply — every small parse, one per SSE delta, forced a minor at the next safepoint. The step those minors doubled was computed, stored, and consulted by nothing. This is the #9589 shape one trigger over: an absolute threshold on a number the collection cannot move.The fix
The guard now also requires the arena to have grown, since the last collection of any kind ended, by a headroom priced from the step (
tiny_parse_pressure_headroom_bytes): the step rescaled so that its power-on value (128 MB, which equals the ceiling) buys the 16 MB headroom floor, and each doubling the arm's ceiling clamp discards buys the guard one more doubling, bounded by the same ceiling. A productive collection halves the step and the guard keeps the cadence it always had; an unproductive one earns it room.gc_collect_pending_suppressed_parsere-prices a pending request, so a collection that already satisfied it is not followed by a second one. The base isarena_in_use_bytes()recorded atnote_collection_finished_arena_occupancy, the funnel every cycle finishes through, in the guard's own units (bump offsets, so swept holes never read as growth).PERRY_GC_DIAG=1gains a[gc-tiny-parse] forced collection site=… in_use=… base=… headroom=… step=…line — the witness that the guard, and not the arm's arithmetic, was the thing firing.The arm's own re-arm is left exactly as it was, with a comment recording the measured reason (the forbidden trade) and where the step is consumed instead.
Measured (perrymaster, Linux, cli_2.1.112.js, same perry binary, runtime-only relink, 7 interleaved rounds, 3300-char streamed reply, chunk 50)
The fix wins CPU in every pair (−8 % to −30 %); footprint is flat within the base's own spread, never above it. The base arm is bimodal in both CPU and RSS, which is what an absolute in-use threshold does when the live set sits near it. 400-char reply: CPU 5.19 → 4.57 s median, RSS unchanged.
Diag run of one 3300-char reply: copying minors 104 → 84 (
ArenaBytes41 → 13,MallocCount63 → 71), old-gen fulls 19 → 7,[gc-step]lines freeing ≤ 1 % 33 → 3, and the guard forced one collection in the whole reply, after a genuine 16 MB of growth.Tests
gc::tests::tiny_parse_pressure(new, 9 tests): the pricing (power-on step buys the floor; productive steps keep it; each discarded doubling doubles it; bounded by the ceiling), the predicate (below the in-use trigger never due; the measured shape — a few KB past a collection that freed nothing — not due; growth of exactly the headroom due, one byte short not), the live cells, and that a finishedjs_gc_collect()moves the base to the post-collectionarena_in_use_bytes()reading. Sabotage-proved: restoring the absolute guard fails the measured-shape test and the boundary halves; pricing at the rawfloor.max(step.min(ceiling))fails the power-on test.test_memory_json_churn.ts(the guard's motivating shape),test_memory_string_churn.ts,test_memory_long_lived_loop.ts: byte-identical output and RSS within noise on both arms in all four GC modes (default /PERRY_GEN_GC=0/=1/ force-evac+verify).test_gap_gc_*, 8/8test_gap_json_*pass on the fix build; 42/42gc::tests::triggers+ new tests pass.cargo fmt,cargo check/clippy -p perry-runtime(no findings in the new code),check_thread_locals.py(the cell lives in an existing block),gc_runtime_root_holders.py(researchednot_a_gc_pointerverdict for the new byte-count cell;PASS1_MARKED's window re-audited and re-pinned — the only in-cycle touch is oneCellstore in the Publish subphase, after the sweep consumed the snapshot).No version bump (maintainer bumps at merge).
https://claude.ai/code/session_015kqVkH6rHzfvXskGAj3tRv
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Bug Fixes
Tests
Documentation