An idle collection finishes while the JS thread is parked; the GC tick backs off under timer chatter

[Jarred-Sumner]

What was wrong

Two things in GarbageCollectionController at an idle prompt (a TUI sitting at its input line, a server after a burst):

1. The 1 s GC tick never backed off. It drops to its 30 s cadence after 30 ticks without heap growth, but that test had no slack, while the idle-collection logic next to it allows 2 MB for timer chatter. An application whose timers allocate a few blocks a second never left the 1 s tick: one eden request and one wakeup per second for as long as it sat there.

2. An idle full collection could not finish while the program was parked. It is requested, not run, and a requested collection only advances at the mutator's safepoints while the mutator holds the collector's conn — which in Bun it always did, because the JS thread keeps heap access across its wait in the event loop (Use stopIfNecessary() instead of heap.{acquireAccess,releaseAccess} #21598). A parked program has no safepoints, so the controller re-armed 30 fast ticks after every idle collection to provide some; the collection still took a second or more of wall time, in a program that allocates nothing its concurrent phase never ended at all, and the eden requests those ticks made raced the sweep that decommits the dead pages a full collection leaves behind (JSC skips that when the last collection to finish was an eden one — [JSC] Decommit a MarkedBlock's dead pages on its first sweep after a full collection, whichever collection finished last WebKit#708 is the JSC half of that).

What this does

• The back-off uses the same 2 MB slack as the idle collections.
• While an idle collection the controller requested is unfinished (JSVMClientData::idleCollectionsPending), the JS thread gives up JSC heap access across the epoll/kqueue wait (Bun__JSC_onBeforeWait reports it through an out-parameter) and takes it back before anything runs (Bun__JSC_acquireHeapAccessAfterWait in us_loop_run_bun_tick). JSC hands the conn to the collector thread, which runs the collection while the JS thread sleeps. The request's didFinishEndPhase hook clears the flag and wakes the loop once, so the JS thread runs the epilogue (the sweep of precise allocations) right away and a burst's memory is back within the second rather than whenever the program next does something. No extra ticks, no extra requests, nothing changes while the program is busy. libuv dispatches callbacks inside its poll, so on Windows the ticks stay as they were.

Numbers

Release build, Linux x64.

	main	this PR
bun -e 'setInterval(()=>{}, 60000)', BUN_IDLE_GC_SECONDS=2: voluntary context switches over 5 s, well after the idle collection	5	5
same: does that idle full collection ever finish?	no (stays in its concurrent phase)	yes
timers allocating 4 KB every 20 ms for 3 s, BUN_GC_TIMER_INTERVAL=20: collections	149	30
350 MB burst then parked (gc-controller-cadence "a burst's garbage is given back"): RSS 2 s after the idle collection	103 MB	104 MB
a TUI application, 20 scripted requests then 3 min idle at its prompt (2–3 runs each): CPU consumed between +60 s and +180 s	660–780 ms	370–410 ms
same: voluntary context switches in that window	856–1376	408–468
same: collections logged over the whole run	206	82
same: anonymous RSS at +30 / +60 / +180 s	185–187 / 185–187 / 147–152 MB	185–190 / 184–189 / 148–152 MB
same: seconds until the 10 s / 2 min idle collections show in RSS	+10–11 / +121–122	+10–12 / +132–135
same: first request after the idle prompt, wall / CPU (median of 7 / 9 runs; single runs swing ±150 ms on this machine)	997 / 799 ms	875 / 791 ms

test/js/bun/gc/gc-controller-cadence.test.ts: 18 pass on the release build (the two FTL-aging tests time out on a loaded machine on main and here alike in debug). Two new tests, both failing on main: "the tick backs off while timers allocate a trickle" (80–149 collections vs < 55) and "an idle collection finishes while the program is parked" (150 MB live heap, parked, no courtesy safepoints: on main the idle collection's log has a START and no END).

Notes for review

• The heap-access release only happens on a park that will actually idle (will_idle_inside_event_loop) and only while such a request is pending — a few parks per idle period; everything else keeps today's "hold access, stopIfNecessary() for a few polls after JS ran" policy.
• Taking access back can run a finished collection's epilogue between the wait and dispatch; current_ready_poll is now reset right after the wait so anything that stops a poll of the fresh batch scrubs it.
• With the conn handed over, the collection's stop-the-world phases — including the end phase (sweepArrayBuffers, weak-reference reconciliation, deleteUnmarkedCompiledCode, finalizeUnconditionally) — run on the collector thread while the JS thread is parked or blocked in acquireAccess(). That is JSC's contract (and was routine in Bun before Use stopIfNecessary() instead of heap.{acquireAccess,releaseAccess} #21598). Two places that had come to assume the JS thread are handled here: Error stack formatting from ErrorInstance::finalizeUnconditionally looked up the global through a thread-local (defaultGlobalObject()), which is null on the collector thread and silently skipped source-map remapping — it now resolves the VM's default global from JSVMClientData (defaultGlobalObject(VM&)); and bun:ffi's toArrayBuffer/toBuffer deallocator callbacks, which follow JSC's JSTypedArrayBytesDeallocator contract, are now documented as possibly running on a GC thread. NAPI external-buffer finalizers already route through the VM handle when off-thread. Cell destructors still run at sweep, on the JS thread.
• On the pre-Use stopIfNecessary() instead of heap.{acquireAccess,releaseAccess} #21598 hang (acquire under JSC's SIGPWR thread suspension while nothing held access): the epoll wait is epoll_pwait2 with an empty signal mask and EINTR retry, so the suspend signal is delivered while parked; the top-level-await-on-stdin shape (prompt waits under TLA, idle collection runs on the collector thread, input arrives) completes normally in testing. Release happens only while an idle collection is pending, so nothing is added to the per-park path of a busy server.
• In the TUI runs the 2-minute collection's memory shows up ~10 s later than on main (the 10-second one is on time; a standalone burst-then-park script returns its memory as fast as main). End state is identical; I have not pinned down where those seconds go — most likely the coarser quiet accounting once the tick is slow — and left it rather than add machinery for it.
• A full collection that was already queued ahead of the idle request and ends on the collector thread has no wake of its own; its epilogue then waits for the next timer tick (1 s, or 30 s once slow) before the idle request proceeds. JSC's StopIfNecessaryTimer would cover that but is disabled in Bun outside --smol.

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Walkthrough

Changes

The change coordinates idle full collections with event-loop waits. It updates GC timer cadence, tracks pending collections, wakes the VM after collection completion, and releases and reacquires JSC heap access around idle waits.

Idle collection coordination

Layer / File(s)	Summary
GC timer cadence src/jsc/GarbageCollectionController.rs, test/js/bun/gc/gc-controller-cadence.test.ts	Timer progression now uses heap growth beyond interval-scaled slack. Windows idle full collections retain fast ticks. Tests cover trickle allocation and parked-program collection completion.
Idle collection state and VM wake-up src/jsc/bindings/BunClientData.h, src/jsc/VmHandle.rs, src/jsc/bindings/bindings.cpp	Idle collection requests increment idleCollectionsPending. The end phase decrements the counter and wakes the VM when the API lock is not held.
Heap access around event-loop waits packages/bun-usockets/src/eventing/epoll_kqueue.c, packages/bun-usockets/src/eventing/libuv.c, src/jsc/bindings/BunJSCEventLoop.cpp	The event loop releases heap access before an idle wait and reacquires it afterward. The wait-hook signature is updated for both event-loop implementations. Ready-poll state is reset before dispatch.
VM-specific global-object lookup src/jsc/bindings/ZigGlobalObject.h, src/jsc/bindings/ZigGlobalObject.cpp, src/jsc/bindings/FormatStackTraceForJS.cpp	A VM-aware defaultGlobalObject lookup uses the registered WebCore global. Stack-trace formatting passes the active VM.
GC-thread callback contract docs/runtime/ffi.mdx	The FFI documentation states that native-buffer deallocator callbacks can run on a garbage-collector thread and must avoid JavaScript calls and thread-local assumptions.

Suggested reviewers: cirospaciari

Priority: ➖ Normal

🚥 Pre-merge checks | ✅ 4

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Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes both primary changes: parked idle collections can finish, and GC tick cadence backs off during low-rate timer allocations.
Description check	✅ Passed	The description explains the problem, implementation, verification results, tests, and review considerations. It uses custom headings instead of the template headings, but it provides the required inf…

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^{Updated 3:35 PM PT - Sep 21st, 2026}

@autofix-ci[bot], your commit 3081e1a is building: #119419

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Actionable comments posted: 1

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Inline comments:
In `@src/jsc/bindings/BunJSCEventLoop.cpp`:
- Around line 135-140: Update the conditional guarding
vm->heap.stopIfNecessary(), vm->heap.releaseAccess(), and the releasedHeapAccess
assignment to also require previouslyHadAccess, ensuring the branch only
releases heap access that the caller held on entry.

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• src/jsc/bindings/BunJSCEventLoop.cpp

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[claude]

Beyond the inline findings, I also checked the release/acquire pairing in us_loop_run_bun_tick (packages/bun-usockets/src/eventing/epoll_kqueue.c): the num_polls == 0 return precedes Bun__JSC_onBeforeWait, and there is no exit between the kernel wait and Bun__JSC_acquireHeapAccessAfterWait, so access is taken back on every path through the tick; mi_on_thread_idle_end() runs before it, so a finished collection's epilogue never allocates while the mimalloc heap is handed off.

Extended reasoning...

Findings were reported and the hunt was cut off at its bug cap, so this is not an approval. This note only records one concrete check made against the diff that is not covered by the inline comments: the heap-access release in Bun__JSC_onBeforeWait and the re-acquire in the epoll/kqueue tick are straight-line with no intervening return, and the ordering relative to the mimalloc idle handoff is correct. Remaining questions (off-thread end phase running FFI/ArrayBuffer finalizers, source-mapped stack materialization, the fixed 6 s wait in the new test, uncommented weakened orderings) are in the inline findings.

Findings marked 🟡 are optional suggestions and need no follow-up push.

[claude]

Beyond the inline findings, I also checked the releaseAccess/acquireAccess pairing across the wait in us_loop_run_bun_tick: the only early return (num_polls == 0) is before Bun__JSC_onBeforeWait, the new release block is the last statement of that function so no branch above it skips the flag, and the libuv loop passes NULL so it never releases — no path parks without access and then dispatches before Bun__JSC_acquireHeapAccessAfterWait.

Extended reasoning...

This run confirmed two findings (posted inline) and a prior run already covered the off-thread end-phase consequences, the FFI/NAPI finalizer thread affinity, the atomic ordering comments, and the 6 s fixed-wait test. The one additional concern examined here was the acquisition/release pairing the repo's review rules call out: Bun__JSC_onBeforeWait sets *releasedHeapAccess only in its final block after all earlier branches, us_loop_run_bun_tick has no return or goto between that call and Bun__JSC_acquireHeapAccessAfterWait, and the Windows libuv twin passes NULL. The idleCollectionsPending increment is balanced by the didFinishEndPhase task, which JSC runs for every request carrying the hook. Nothing new beyond the inline comments rises to a bug, so this note is informational only.

Findings marked 🟡 are optional suggestions and need no follow-up push.

Still open from earlier reviews (4):

• 🔴 src/jsc/bindings/BunJSCEventLoop.cpp:38 — Users can get un-source-mapped line numbers in err.stack after merging, for a retained Error whose stack is first mater…
• Also unresolved: 3 minor or pre-existing.

If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.

[claude]

Still open from earlier reviews (1):

• Unresolved: 1 minor or pre-existing.

[claude]

I re-reviewed the latest push and found no new bugs; since this hands the collector's conn to another thread across the event-loop wait, a human look at the heap-access handoff is still worthwhile.

What was reviewed:

• The new push addresses the earlier inline findings: setDefaultGlobalObject(vm, ...) now keeps JSVMClientData::defaultGlobalObject in sync at both Zig__GlobalObject__create and Zig__GlobalObject__createForTestIsolation, the slack is derived from named constants, idleCollectionsPending uses default seq_cst, and the bun:ffi deallocator thread contract is stated in docs/runtime/ffi.mdx.
• Release/re-acquire pairing in us_loop_run_bun_tick: no exit path between Bun__JSC_onBeforeWait and Bun__JSC_acquireHeapAccessAfterWait; current_ready_poll is reset before the re-acquire so a finished collection's epilogue cannot leave a stale index into the fresh batch.
• Counter balance: fetch_add happens under the API lock before collectAsync, and the didFinishEndPhase hook is per-request (a request carrying it is not coalesced), so the count returns to zero; the useGC() early return happens before the increment.
• Ruled out: a spurious extra wake when the JS thread itself finishes the collection at a park (one wake per idle collection, harmless), and the epilogue of a large parked heap landing on the next timer tick rather than immediately (a latency property of the design the description already documents, not a correctness issue).

Extended reasoning...

The change touches the JSC heap-access policy in the event loop (BunJSCEventLoop.cpp, epoll_kqueue.c, libuv.c), adds a per-VM atomic counter and default-global pointer on JSVMClientData, makes the GC controller's back-off tolerate timer chatter, and moves Error stack formatting to a VM-scoped global lookup so it works on the collector thread. It touches no auth, crypto, or input-parsing surface. The latest commit addresses every inline finding from prior runs (the --isolate default-global desync, named constants, memory orderings, ffi docs), and this run's hunt ended by dry streak with no findings. It is not approved outright because the design decision to give up heap access while parked, and the resulting off-JS-thread end phase, is a runtime-wide behavioral change owned by the core team that merits a human sign-off; the Windows path deliberately keeps the old tick-driven behavior.

[claude]

Code review completed

Nothing new to post: everything this review found is already covered by existing comments on this pull request or didn't merit a separate one.

[claude]

Findings marked 🟡 are optional suggestions and need no follow-up push.

The last 3 reviews of this pull request each found new blocking issues. Where they share a root cause, one fix may close them together.

[claude]

Code review completed

Nothing new to post: everything this review found is already covered by existing comments on this pull request or didn't merit a separate one.

Still open from earlier reviews (1):

• Unresolved: 1 blocking on lines changed since (possibly already fixed).

If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.

[Jarred-Sumner]

@claude review