[JSC] Decommit a MarkedBlock's dead pages on its first sweep after a full collection, whichever collection finished last - #708
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…full collection, whichever collection finished last MarkedBlock::Handle::decommitUnusedPages() returns the pages of a block that hold no live cell to the OS when the incremental sweeper reaches a block a full collection left partly dead. It decided that by Heap::lastCollectionScope() at sweep time. An embedder that requests an eden collection while a full one is finishing (or right after it) flips that to Eden before the sweeper's first timer slice, and then nothing the full collection left behind is decommitted until some later full collection happens not to be followed by an eden one. In an application idling at a prompt with a 1 s GC timer that was 12-14 MB of dead pages staying committed after one idle collection in four. Each block handle now remembers MarkedSpace::markingVersion() as of its last sweep; only a full collection moves that version on, so "this is the block's first sweep since a full collection began" is a per-block fact that a later eden collection does not disturb. Same call sites (SweepOnly sweeps only), same exclusions, same recommit path.
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Beyond the inline finding, I also checked: the comment's premise holds (only the CollectionScope::Full arm of MarkedSpace::beginMarking advances m_markingVersion, MarkedSpace.cpp:400-428); the decommitUnusedMarkedBlockPagesAfterEdenCollections=true path short-circuits exactly as before; and m_markingVersionAtLastSweep is declared above m_alignedMemoryAllocator in MarkedBlock.h, so the initializer order matches and heap.objectSpace() is already constructed when the handle is built.
Extended reasoning...
The inline finding covers the concurrent-marking case (a SweepOnly sweep reached during marking stamps the already-advanced version, so the post-collection sweep of that block sees "not first" and skips decommit). This note only records what else was looked at and ruled out from the diff and MarkedSpace::beginMarking: the markingVersion-only-on-full claim, the unchanged behavior of the eden option path, and the member declaration/initializer order in the Handle constructor. It is informational and not a correctness guarantee; the GC heap code still warrants a human look given the open inline finding.
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ChangesMarked block sweep handling
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In `@Source/JavaScriptCore/heap/MarkedBlock.cpp`:
- Line 642: Update the sweep bookkeeping around isFirstSweepSinceFullCollection
in SweepToFreeList so m_markingVersionAtLastSweep is compared and updated only
when sweepMode == SweepOnly; keep the marker false for other sweep modes so the
later SweepOnly pass can decommit pages left dead by the full collection.
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Source/JavaScriptCore/heap/MarkedBlock.cppSource/JavaScriptCore/heap/MarkedBlock.h
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…st sweep after the full collection The mutator sweeps blocks to a free list while a full collection is still marking; such a sweep goes by the previous cycle's marks. Stamping the block with the version beginMarking() had already advanced made the block's real first sweep after that collection read as a later one and skip the decommit the old lastCollectionScope() test performed. Leave the stamp alone while marking is in progress.
…s during eden marking count as first sweeps During an eden collection's concurrent marking the marking version is unchanged and the old blocks' marks are current, so the sweeper reaching such a block then is a genuine first sweep after the last full collection and should decommit; the previous commit excluded it and the block is not queued again until the next full collection.
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…k backs off under timer chatter (#43681) ## 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 (#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 — oven-sh/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 #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-#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`. --------- Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
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MarkedBlock::Handle::decommitUnusedPages()gives the pages of a block that hold no live cell back to the OS when the incremental sweeper reaches a block that a full collection left partly dead. Whether a sweep qualifies was decided byHeap::lastCollectionScope()at sweep time.An embedder that requests an eden collection while a full one is finishing, or right after it, flips that to
Edenbefore the sweeper's first timer slice, and then nothing the full collection left behind is decommitted until some later full collection happens not to be followed by an eden one. Measured in an application idling at a prompt whose embedder runs a 1 s GC timer: 12–14 MB of dead pages stayed committed after roughly one idle full collection in four (17 of 65 observed), for minutes.Each block handle now remembers
MarkedSpace::markingVersion()as of its last sweep. Only a full collection moves that version on, so "this is the block's first sweep since a full collection began" is a per-block fact that a later eden collection does not disturb. Same call sites (SweepOnly sweeps, i.e. the incremental sweeper; never the allocator's sweep-to-free-list), same exclusions (Structure spaces, shutdown, mostly-full blocks), same recommit path.One
HeapVersionperMarkedBlock::Handle.