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[JSC] CodeBlock aging: let idle FTL code (and counter-less DFG code) age out once the mutator goes quiet - #557
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…unter is jettisoned once the mutator has gone quiet FTL CodeBlocks (and DFG CodeBlocks compiled without tier-up checks) never aged out because they have no execution counter to prove idleness with. Each one pins its baseline alternative and every baseline CodeBlock it inlined, together with their metadata and JIT data, so after a burst of work a process keeps most of its CodeBlock graph alive for as long as it lives. Age them against the mutator as a whole instead: Heap keeps a monotonic totalBytesAllocated(); at each old-age check such a block compares it with the value at its last renewal. If more than optimizedCodeAgingQuietAllocationMB (1 MB) was allocated since, the lease is renewed; if not, and the lease is older than optimizedCodeAgingQuietSeconds (30 s), the block is let go for old age and takes its alternative and inlinees with it. Full collections in a busy mutator are allocation-driven, so they always renew; an embedder's idle-time collections see the quiet heap and release the code. optimizedCodeAgingQuietAllocationMB=0 keeps the old behaviour.
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Included review availability: 1 review is currently available. Your included PR review attempts over the past 7 days set your current allowance at 2 reviews per hour. WalkthroughChangesCodeBlock aging now tracks mutator allocation quietness for selected FTL and DFG blocks. Heap allocation totals and quietness thresholds support this logic. New stress tests cover allocation-based lease renewal and aging after quiet intervals. Mutator-quietness code aging
Merge Risk: ⚪ Minimal · up to The declaration and storage changes are guarded for Bun builds and preserve the intended CodeBlock layout behavior. No current merge-blocking risk is identified. 🚥 Pre-merge checks | ✅ 3 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (3 passed)
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In `@JSTests/stress/codeblock-aging-ftl-idle-off.js`:
- Line 1: Update the runDefault options in the test to set
optimizedCodeAgingQuietSeconds below the 0.2-second wait, such as 0.1, while
preserving optimizedCodeAgingQuietAllocationMB=0 so the disabled-option behavior
is exercised after the quiet-time threshold.
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…t32 MB field instead of the float counter; quiet window comes from optimizedCodeAgingQuietSeconds (capped at the TTL under eager timing); gate the tests to FTL platforms and shrink their waits
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In `@Source/JavaScriptCore/bytecode/CodeBlock.h`:
- Line 935: Guard the agesByMutatorQuietness() declaration with `#if`
USE(BUN_JSC_ADDITIONS) so it is exposed only in Bun-enabled builds, matching its
implementation and state.
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Thanks — the follow-up commit addresses all three points from the earlier round: the allocation snapshot now lives in its own wrap-safe uint32_t m_leaseStartAllocatedMB (so no float-precision drift), both stress tests are gated with //@ skip if !$isFTLPlatform, and the idle windows were cut from 0.2 s to 0.04 s with fewer rounds. I re-read the updated logic and didn't find further bugs; a human look is still worth having since this changes the jettison policy for the hottest tier and picks new default thresholds (1 MB / 30 s).
What was reviewed:
- Wrapping arithmetic in
shouldJettisonDueToOldAge—uint32_t(allocatedMB - m_leaseStartAllocatedMB)gives correct distance across the 2^32-MB wrap, and the field-still-0 first check falls into the renew branch as the comment says. Heap::totalBytesAllocated()is private butCodeBlockis already afriendofHeap, so the call compiles.updateActivity()early-return for quiet-regime blocks leaves the floatm_previousCounteruntouched, so a DFG block flipping between regimes (viam_didFailFTLCompilation) can't misread one snapshot as the other now that they're separate fields.
Extended reasoning...
Overview
The follow-up commit reworked the allocation-snapshot storage from the shared float m_previousCounter to a dedicated uint32_t m_leaseStartAllocatedMB in CodeBlock.h, with the comparison in CodeBlock.cpp:1387-1391 done as unsigned wrapping subtraction of whole-MB values. It also introduced optimizedCodeAgingQuietSeconds (default 30 s, capped at the tier TTL under eager timing) so the quiet window is decoupled from timeToLive, and updated both new stress tests to skip on non-FTL platforms and use 0.04 s idle windows with 3 rounds instead of 0.2 s × 5.
Security risks
None identified. The change is entirely inside #if USE(BUN_JSC_ADDITIONS) guards, adds a monotonic counter and two tunable options, and only affects when already-compiled optimized code is discarded. No new external inputs, no parsing, no privilege boundaries.
Level of scrutiny
Moderate-to-high. Jettisoning FTL code is a performance-and-correctness-adjacent heuristic: getting the thresholds wrong could cause recompile churn under load or hold memory at idle, and shouldJettisonDueToOldAge runs during GC visiting. The logic itself is small and self-contained, but the choice of defaults (1 MB allocation delta, 30 s quiet window) and the interaction with Bun's idle-GC schedule are design decisions a maintainer should confirm rather than an automated reviewer.
Other factors
All three inline concerns from the previous run were substantively addressed by commit 884b98e. The wrapping-uint32 approach is sound (distance is correct as long as < 2^32 MB is allocated between two checks of the same block, which is always true given collection frequency). The tests still hard-code 1e6 warm-up iterations rather than testLoopCount and the on-path test's total wall time (~0.24 s of busy-wait plus warm-ups and 3×4 MB allocations) may still land a bit above the 200 ms JSTests guideline, but that was flagged as optional and the author made a good-faith reduction; not worth re-raising. The PR description's mention of m_previousCounterHoldsAllocation is stale relative to the current code — the separate field made that bit unnecessary.
…ity bit-field so the MSVC ABI packs it with the adjacent bools (it was costing 8 bytes there); guard the Bun-only agesByMutatorQuietness() declaration
No-Verification-Needed: dependency version bump only
…t counter-less optimized code go (#558) Follow-up to #557. FTL code (and DFG code without a tier-up counter) aged out in any full collection that found the mutator quiet for optimizedCodeAgingQuietSeconds. A program that forces full collections on a timer while running hot, non-allocating code between them (setInterval(() => Bun.gc(true), 30_000) with a pooled-buffer server, say) satisfies that and would re-jettison and re-tier its FTL code on every such collection. Add GCRequest::isIdle / Heap::isIdleCollection(): an embedder sets it on the collection it requests because the application went idle. Any full collection may still renew a block's lease when it sees allocation, but only an idle one may expire it, so allocation-paced, forced and memory-pressure collections never drop such code. An idle request is not subsumed by a pending ordinary one. The jsc shell gets idleFullGC() and the stress tests now check both that idleFullGC() ages the code out and that plain fullGC() under the same conditions does not.
…41297) ### What WebKit → `40e43a82a755` (oven-sh/WebKit#557 + #558). That change lets JSC age out FTL code (and DFG code compiled without tier-up checks) once the mutator has gone quiet. Such code has no execution counter, so since #41083 / WebKit#542 it never aged, and each block pinned its baseline alternative plus every baseline CodeBlock it inlined. Now a full collection lets it go once ≤ 1 MB has been allocated since the previous full collection looked at it for ≥ 30 s — which is exactly what the idle collections `GarbageCollectionController` already schedules (`BUN_IDLE_GC_SECONDS=10,65,65`) produce. Busy processes are unaffected: their full collections are allocation-driven and always renew the lease. Bun side: `GarbageCollectionController` now issues those idle full collections through `JSC__VM__collectAsyncIdle`, which tags the request (`GCRequest::isIdle`, WebKit#558). JSC lets such code go **only** in a tagged collection, so a program that forces `Bun.gc()` on a timer while running hot, non-allocating code keeps its optimized code; allocation-paced and memory-pressure collections never drop it either. ### Numbers express / hono with a library-heavy handler (ajv, zod, markdown-it+highlight.js, js-yaml, protobufjs, graphql, handlebars, sucrase, …), 5,000-request burst at c=50, then idle. Same Bun commit, WebKit#557 option off vs on, 3 runs each: | idle +120 s | off | on | |---|---|---| | express anon RSS | 148 MB | **127 MB** | | hono anon RSS | 137 MB | **117 MB** | | surviving `FunctionCodeBlock`s (express) | ~2,400 | ~200 | Nothing changes at +45 s (before the second idle collection) or under load; a 2-minute sustained-load run compiled the same number of FTL functions with the option on and off. For context, idle +240 s RSS across versions on the same apps (5 runs, medians): express 1.3.14 245 / 1.4.0 224 / canary 192 → ~171 with this; hono 228 / 207 / 173 → ~152; elysia 230 / 203 / 170 → ~149; next 339 / 296 / 244 → ~223. ### Tests `test/js/bun/gc/gc-controller-cadence.test.ts` gains two cases (eager JIT TTLs so it runs in seconds): the idle collections take a 40-function FTL workload from ~90 `FunctionCodeBlock`s to a handful; `BUN_IDLE_GC_SECONDS=0` plus `Bun.gc(true)` every 0.7 s leaves them in place. Both fail on current bun. The engine side is `JSTests/stress/codeblock-aging-ftl-idle*.js` in WebKit#557/#558.
… optimized code against the last active collection (#560) * [JSC] CodeBlock aging: drop the per-allocation byte counter; age idle optimized code against the last collection that saw allocation #557 added a running total in Heap::didAllocate and a per-CodeBlock MB snapshot to tell whether the mutator had been allocating between full collections. With #558's idle tag that is more machinery than the decision needs: a collection already knows how much was allocated since the previous one (totalBytesAllocatedThisCycle()). - Heap stamps m_lastActiveCollectionTime at the start of any collection that finds more than optimizedCodeAgingQuietAllocationMB allocated since the previous one. Nothing is added to didAllocate(). - CodeBlock::shouldJettisonDueToOldAge for FTL / counter-less DFG code: only an idle-tagged collection may drop it, and only once neither the block's compilation nor an active collection has happened for optimizedCodeAgingQuietSeconds. No per-block state is written; m_leaseStartAllocatedMB is gone (CodeBlock is 4 bytes smaller again). - An embedder's periodic no-op collections while idle (Bun ticks collectAsync() every 30 s) don't see allocation and so don't hold the code alive; a forced or allocation-paced collection still can never drop it. Same behaviour end to end (Bun, express app, idle +120 s: 125 MB anon before and after; idle probe 89 -> 2 CodeBlocks, forced-GC probe 89 -> 84); the stress test additionally checks that an idle collection right after an active one keeps the code. * Review: accumulate per-cycle allocation across collections for the active stamp (a steadily working mutator collected in small cycles still reads as active); guard the test's back-to-back-collections assertion on the measured gap
…erfly slots through libc memset (oven-sh/WebKit#564) (#41407) ### What Bumps WebKit from `fbd894680a6c` to [`2e2aa2290fac`](oven-sh/WebKit@2e2aa22), the oven-sh/WebKit#564 merge commit on `main`. The range contains two changes: **oven-sh/WebKit#564 — concurrent marker could read a torn JSValue on musl builds.** `JSArray::setLength` and `JSArray::shiftCountWithAnyIndexingType` — the Int32/Contiguous paths behind `array.length = n`, `Array.prototype.splice` and `shift` — move elements with `gcSafeMemmove()` and then cleared the vacated butterfly slots with a `WriteBarrier::clear()` loop, which clang compiles to a call to libc `memset()`. The GC's parallel marker scans that same butterfly on a helper thread without a lock, and libc makes no promise about store width: musl's `memset` writes the head and tail of the range with 1-, 2- and 4-byte and unaligned 8-byte stores (glibc happens to use 8-byte-or-wider stores for these sizes). So on `*-musl` builds (Alpine images, musl `--compile` targets) a marker thread could load a half-cleared slot — the low byte, or the low five bytes, of the pointer that was there — treat it as a `JSCell*`, and: - SIGSEGV on a `HeapHelper` thread in `JSObject::visitChildren` / `SlotVisitor::appendHiddenSlow` at `<garbage & ~0x3fff> + 0x20` (e.g. `0x20`, `0x8f6c954020`), or - SIGSEGV at `0xd0` in `SlotVisitor::drain` after the torn pointer was marked and pushed, or - a mark bit / cell-state byte written at a non-cell offset in an unrelated block (later crashes anywhere), or - a hung process with a marker parked in `MarkedBlock::aboutToMarkSlow` → `CountingLock::lockSlow` while the mutator waits in `Heap::runFixpointPhase`. A 25-line script (64 arrays of 3000 objects, `a.splice(N - 6, 3)` + refill in a loop) crashes `oven/bun:1.4.1-alpine` within seconds with default options, 6/6 runs; the glibc image and the same musl image with an `LD_PRELOAD` memset that only does 8-byte stores both run clean. The clears now use `gcSafeZeroMemory()`, and the unlocked in-place ArrayStorage grow path in `JSObject::increaseVectorLength` fences before publishing the larger vector length. **oven-sh/WebKit#560 — CodeBlock aging** drops the per-allocation byte counter added in oven-sh/WebKit#557 and ages idle optimized code against the last collection that saw allocation instead. ### Related reports - Fixes #19895 — musl x64, `HeapHelper` thread in `JSObject::visitChildren` → `visitButterfly` under `SlotVisitor::drain`; the fault address `0x7623000020` is a butterfly pointer with bytes 0–2 and 5–6 zeroed, which is exactly musl `memset`'s intermediate state when clearing a single 8-byte slot. - Fixes #17792 — musl x64-baseline, long-running `next start`, parallel marker `SlotVisitor::drain` → `visitChildren` faulting at `0xd0`, i.e. a torn pointer that was marked and then visited. - Not this bug, deliberately not marked: the glibc reports with marker-thread stacks (#32109, #30205, #30191, #34476, #26678, #28990) — glibc's `memset` does not tear these slots, and their fault addresses (`0x703b1a408020`, `0x800000020`, `0xe00000020`, …) point at a stale block or an `IndexingHeader` read rather than a torn pointer.
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Follow-up to #542 / #546. Those let LLInt, Baseline and DFG CodeBlocks age out once their execution counters stop moving, but exempted FTL code (and DFG code compiled without tier-up checks) because it has no counter to prove idleness with. Measured on a Bun HTTP server after a burst of work: at idle the Output constraint still holds ~480 optimizing CodeBlocks, and through their alternatives and inlined frames they pin ~1,350 baseline CodeBlocks plus metadata/JITData — most of what
deleteAllCodewould free. LLInt/Baseline aging itself works (8 self-rooted survivors).What
Optimizing code with no execution counter now ages against the mutator as a whole instead of never:
Heapkeeps a monotonictotalBytesAllocated()(one add inHeap::didAllocate).CodeBlock::shouldJettisonDueToOldAge, such a block (agesByMutatorQuietness(): FTL, or DFG without a usable tier-up counter) compares that total with the value recorded when its lease last started. More thanoptimizedCodeAgingQuietAllocationMB(1 MB) allocated since ⇒ the mutator has been working, renew the lease from now. Otherwise, once the lease is older thanoptimizedCodeAgingQuietSeconds(30 s; the tier's eager TTL underuseEagerCodeBlockJettisonTiming), the block is let go for old age and, via the existing [JSC] Return unused MarkedBlock pages after sweeping; let idle optimized code age out #542/[JSC] CodeBlock aging: make one full GC after the lease sufficient #546 path, takes its baseline alternative and inlinees with it.m_previousCounterwith the execution-count path; am_previousCounterHoldsAllocationbit says which kind of value it holds so a block that moves between regimes (e.g.m_didFailFTLCompilationflipping) never misreads one as the other, andupdateActivity()(unlinked-code jettisoning) leaves quiet-regime blocks' snapshots alone.optimizedCodeAgingQuietAllocationMB=0restores the previous behaviour.No change to DFG/FTL code generation.
Why this signal
Full collections in a busy mutator are allocation-driven, so two of them never see < 1 MB allocated in between and FTL code is kept exactly as before; an embedder's idle-time collections (Bun issues two once the heap has been quiet for 30 s and ~95 s) do, and release it. A VM-entry count was tried first and rejected: real servers run housekeeping timers (e.g. the HTTP server's 30 s connection check) so they are never entry-idle, and a long single-entry compute would have looked idle.
Numbers (Bun, express + a library-heavy handler, 5,000-request burst then idle; same binary, option off → on)
Sustained load (2 min, 30 connections, side by side): FTL compiles 1007 (off) vs 1003 (on), same requests/s — no jettison/recompile churn while the mutator is allocating.
Tests
JSTests/stress/codeblock-aging-ftl-idle.js: with eager timing an FTL function ages out across two full GCs with nothing allocated in between, survives five rounds of GCs with 4 MB allocated between each, then ages out once allocation stops.codeblock-aging-ftl-idle-off.js: with the option at 0 it never ages.codeblock-aging-execution-count.jsstill passes, also with--useUnlinkedCodeBlockJettisoning=1.