[JSC] Return free memory sooner: idle worker threads, Atomics.wait - #768
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Included review availability: This review used your included allowance. 2 included reviews remain after this review. Your included PR review attempts over the past 7 days set your current allowance at 5 reviews per hour. WalkthroughThe change adds idle-thread FastMalloc scavenging paths for MIMALLOC builds, including external mimalloc support. It also changes ChangesIdle-thread memory release
IncrementalSweeper timer
Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to No confirmed issue currently blocks merging. The external-mimalloc linkage could not be verified from the available evidence. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
Review comments at @Source/WTF/wtf/AutomaticThread.cpp:
- Around line 254-255: Update the timeout waits around m_waitCondition so the
idle-release delay and subsequent wait together stay within m_timeout; after the
release, wait only for the remaining timeout rather than the full m_timeout.
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Source/JavaScriptCore/heap/IncrementalSweeper.cppSource/JavaScriptCore/runtime/WaiterListManager.cppSource/JavaScriptCore/shell/CMakeLists.txtSource/JavaScriptCore/shell/ExternalMimallocShims.cppSource/WTF/wtf/AutomaticThread.cppSource/WTF/wtf/FastMalloc.cppSource/WTF/wtf/FastMalloc.hSource/bmalloc/CMakeLists.txtSource/bmalloc/bmalloc/BPlatform.hSource/bmalloc/bmalloc/bmalloc.cppSource/bmalloc/bmalloc/bmalloc.h
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I also checked: the IncrementalSweeper guard (timeUntilFire() returns std::optional<Seconds>, so the test means "no timer pending", not "fires now"); the Atomics.wait notify race around the dropped list lock (a notify removes the waiter from the list and the loop re-tests isOnList() and termination under the lock before waiting again); and the link surface for USE_EXTERNAL_MIMALLOC consumers (mi_on_thread_idle is only referenced when the consumer's mimalloc is in use, and the in-repo executables get it from the new shim).
Extended reasoning...
The change touches the GC sweeper timer, WTF::AutomaticThread's idle loop, JSC's Atomics.wait sync path, and adds a bmalloc/WTF entry point plus a USE_EXTERNAL_MIMALLOC build flag; no security-sensitive surface. Inline findings are posted, so a human look is already indicated; this note only records what else was examined.
Points at the preview build of oven-sh/WebKit#768.
Only the owning thread can collect its mimalloc thread-local heap. An AutomaticThread that finishes its work and waits on its condition keeps every page it freed: retired pages, blocks other threads freed into its pages, and the free runs inside pages it still uses. Nothing touches them until the thread times out after 10 s and exits, or until it works again. For the wasm compiler threads this is about 10 MB each after one compile of a 4 MB module. With numberOfWasmCompilerThreads at cores - 1, a process that compiles wasm modules holds cores x 10 MB of RSS that no live object accounts for (oven-sh/bun#41438). The JIT worklist threads hold their B3 and Air temporaries the same way. When a thread has waited 100 ms without a notify, it now calls releaseFastMallocFreeMemoryForIdleThread() with the worklist lock dropped, then waits out the rest of its timeout. A thread that is notified within 100 ms, the usual case between tasks, pays nothing. The flag resets on each unit of work. On the consumer's mimalloc (USE_EXTERNAL_MIMALLOC) the release is mi_on_thread_idle(), which also discards the free runs inside still-used pages and hands the arena purge to mimalloc's scavenger thread. The vendored mimalloc does not have that entry point, so it falls back to a forced mi_theap_collect. The jsc shell and testFFI link the archives against the vendored mimalloc-obj, so they define mi_on_thread_idle in terms of the vendored API (shell/ExternalMimallocShims.cpp). libpas and system malloc are unchanged: the hook is compiled only under USE(MIMALLOC).
- notifyOne prefers a thread that is asleep over one that is releasing its free memory, which only sees the notification when it is done with that - the 100 ms before the release count towards the timeout of the thread - the fallback for the vendored mimalloc does not force the collect: that also purges what every other thread just freed
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Jarred-Sumner
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Points at the preview build of oven-sh/WebKit#768.
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The new commits address both points from my earlier pass, and this round found no new bugs; because the change alters wait/notify behavior in WTF::AutomaticThread and Atomics.wait, and the sweeper part is still being measured, a human look is still worthwhile.
What was reviewed:
- AutomaticThread idle split: a notify that lands while the lock is dropped is caught by the m_isWaiting re-check; m_isReleasingFreeMemory is only touched under m_lock; after one release the thread waits the full m_timeout without releasing again.
- WaiterListManager: the release only runs when the deadline is past the 100 ms mark; isOnList and the termination check are re-evaluated under the list lock before the final wait.
- IncrementalSweeper flag: timeUntilFire() is nullopt both when unscheduled and while the timer callback is running, so a collection landing mid-slice still reschedules; the flag is cleared on every path that cancels the timer.
- The mi_on_thread_idle declarations in bmalloc.cpp and the shell shim agree (extern "C", noexcept), and bmalloc/mimalloc.h is an installed forwarding header.
Extended reasoning...
The diff adds a mimalloc idle-release hook through bmalloc/WTF (scavengeThisThreadOnIdle, releaseFastMallocFreeMemoryForIdleThread), splits the idle wait in WTF::AutomaticThread and WaiterListManager::waitSyncImpl to drop the lock and release memory after 100 ms, adds a jsc-shell/testFFI shim for mi_on_thread_idle, and adds an IncrementalSweeper flag so eden collections do not push back a sweep that follows a full collection. It touches no auth, injection, or data-exposure surface. The four new commits implement the fixes I asked for previously, and the lock-drop paths re-check their wake conditions under the lock, so no new finding was reported. Approval is not appropriate because the author states the PR is not ready to merge (the sweeper change may be dropped) and the change alters wait/notify semantics of the thread primitive used by the JIT worklists and GC collector thread.
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Jarred-Sumner
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…ory (#44564) Bumps WebKit to oven-sh/WebKit#768. Nothing else is in the range. - JSC's worker threads (wasm and JIT compilers, GC helpers) release the free memory of their mimalloc heap after 100 ms idle instead of when they exit after 10 s. Fixes #41438. - A thread that blocks in Atomics.wait for over 100 ms does the same. It used to keep everything it had freed for the whole wait. The wasm test is from #41449.
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Two places where memory that is already free stays resident longer than it has to. Includes the commit of #567.
1. AutomaticThread: release the thread-local heap after 100 ms idle
The commit of #567, plus a commit for its review here:
notifyOneprefers a thread that is asleep over one that is in the release, the 100 ms count towards the timeout of the thread, and the fallback for the vendored mimalloc does not force the collect. See #567 for the problem (about 10 MB per wasm compiler thread stays resident until the thread exits after 10 s).It also matters for the GC heap. 2M objects of one shape of which 1 in 400 survives, a full collection, then the event loop stays busy allocating arrays (nothing retained,
setTimeout(tick, 0)between ticks). RSS in MB after so many seconds of that; Linux x64, release, same Bun and mimalloc:(An earlier version of this PR also changed the incremental sweeper, and credited this to that change. These numbers are without it.)
2. Atomics.wait: release the thread-local heap in a wait that takes over 100 ms
Problem. Only the owning thread can give back the free blocks inside the pages of its mimalloc heap, and it does that when it goes idle. Bun's event loop and thread pool tell mimalloc so; a thread that blocks in
Atomics.waitdoes not, so a worker that waits for work this way keeps everything it freed.Fix. The same as for
AutomaticThread, with the entry point that #567 adds: wait for 100 ms first, so a wait that is notified soon pays nothing, then release once with the list lock dropped, and wait for the rest. The waiter stays on the list meanwhile. A notification takes it off the list, and the loop tests for that (and for a termination request) under the lock before it waits again.100k strings of about 900 bytes of which 1 in 16 is kept, a full collection, then a wait of 500 ms.
heapStats().mimalloc.purge_callsover the wait:await Bun.sleep(500)Atomics.wait(..., 500)120 waits in a worker that are notified 96 to 107 ms after they start, so around the time the lock is dropped: all return
"ok".Under load
Next.js (pages router,
getServerSideProps) underoha -c 32, median of 6 runs of 6000 requests, and Express + EJS, median of 4 runs of 20000:Not in this PR: the incremental sweeper
startSweepingsets the timer to 100 ms from now at the end of every collection, also when it is pending, so with collections less than 100 ms apart it never fires. Not pushing back the sweep after a full collection (branchclaude/sweeper-after-full-gc) takes Next.js from 247 MB to 231 MB at the end of the load, for 43.3 instead of 32.0 faults per request. In a large interactive application it did nothing for RSS during a session, and RSS a minute after a session was more often the same as before this PR than with the two changes here alone (below 275 MB in 2 of 9 runs against 5 of 9; 1 of 9 before). So it is left out.Idle
A burst of allocation (and a worker that does the same), then nothing for 135 s, which includes Bun's idle collections after 10 s and 2 min. Two runs each: