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…ease their mimalloc heap Each wasm compiler thread kept about 10 MB of freed compile temporaries after a large compile until it timed out and exited 10 s later. Only the owning thread can collect its mimalloc thread-local heap, and an idle AutomaticThread never did. RSS grew with the core count, not with the number of live modules. oven-sh/WebKit#567 makes an AutomaticThread release its heap after 100 ms without work. This pins that PR's preview build and adds a test that compiles a tree-sitter shaped module 8 times on 8 compiler threads and checks that RSS comes back while the threads are still alive.
WalkthroughChangesWebAssembly compiler RSS regression
Merge Risk: 🟡 Moderate · up to This change adds RSS validation for WebAssembly compiler-thread heap reclamation, but the Darwin measurement path can fail to produce usable metrics and the remaining test robustness concerns could cause unreliable CI results. Resolve these issues before merge. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
In `@test/js/bun/wasm/compile-rss-fixture.mjs`:
- Line 94: Validate targetMiB immediately after it is parsed and reject missing,
non-numeric, or otherwise invalid values before entering the poll loop, with a
clear error message. Preserve the existing loop behavior for valid targetMiB
values.
In `@test/js/bun/wasm/compile-rss.test.ts`:
- Around line 14-32: Set an explicit timeout for the test containing the
Bun.spawn invocation, long enough to cover process startup, repeated
multi-megabyte compilation, and the fixture’s fixed 3000 ms polling period. Keep
the existing assertions and test behavior unchanged.
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Addressed the review: the fixture now rejects a missing or non-numeric target (658d8a7). The explicit test timeout is not added: tests in this repo do not set timeouts, and the passing path takes about 0.4 s because the fixture stops polling once RSS is under the target. |
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Inline comments:
In `@test/js/bun/wasm/compile-rss-fixture.mjs`:
- Line 95: Update the target argument validation before the Number conversion in
the fixture setup so blank or whitespace-only process.argv[2] values are
rejected, while preserving acceptance of valid finite numeric targets including
zero.
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On the blank-argument finding: a blank argv[2] gives a target of 0, and the fixture then polls for the full 3 s and reports the true minimum. That is a valid run (it is how the floor was measured by hand), so the check stays as it is. |
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| * From https://github.com/oven-sh/WebKit releases. | ||
| */ | ||
| export const WEBKIT_VERSION = "2e2aa2290fac856d6f451ceacb58f7f5b44dd057"; | ||
| export const WEBKIT_VERSION = "autobuild-preview-pr-567-e62d118e"; |
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🔴 WEBKIT_VERSION is pinned to autobuild-preview-pr-567-e62d118e, an ephemeral preview tag; if merged as-is, clean builds break once oven-sh/WebKit#567 merges or closes and the tag is garbage-collected. Fix: before merge, swap the pin to the merged upstream oven-sh/WebKit main SHA and verify prebuilt artifacts exist for every platform × flavor (per .claude/docs/landing-prs.md §Dependencies & vendoring). The PR description already commits to this; noting it as a hard merge blocker.
Extended reasoning...
prebuiltUrl() (scripts/build/deps/webkit.ts:73) resolves the download URL from WEBKIT_VERSION; autobuild-preview-pr-* release tags on oven-sh/WebKit are ephemeral and are deleted once the associated PR is merged or closed. On the base branch the pin is a merged main SHA (2e2aa2290fac…) whose release is permanent. After this change merges, once WebKit#567 lands and the preview tag is pruned, every fresh checkout / CI cache miss fails at the WebKit prebuilt fetch step with a 404, blocking all builds until someone repins. .claude/docs/landing-prs.md explicitly forbids merging pins to preview tags or unmerged-PR builds. The author states the pin will move before landing; this comment tracks that requirement.
Verification: normal — acknowledged in diff: the PR description states "Before landing, oven-sh/WebKit#567 has to merge and the pin has to move to the resulting main sha", which is accurate and matches the repository rule; the finding correctly restates it as a hard merge blocker rather than resolved. scripts/build/deps/webkit.ts:6 changes WEBKIT_VERSION from "2e2aa2290fac856d6f451ceacb58f7f5b44dd057"…
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Agreed, and the PR body says the same: this does not land on the preview tag. Once oven-sh/WebKit#567 merges, the pin moves to the resulting main sha and CI runs again. The thread stays open until then.
…he compiles grew memory On macOS mimalloc returns memory with MADV_FREE_REUSABLE and RSS keeps counting it, so the test read +41 MB on a fixed build there. Use Bun.unsafe.memoryFootprint on Darwin like the other RSS tests do. The baseline now waits until memory stops falling instead of a fixed 50 ms sleep, and the test asserts that the compiles grew memory by at least twice the target before it checks that the growth came back.
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Pushed 7e65d35 for the review and the macOS CI failure. On Darwin mimalloc returns memory with MADV_FREE_REUSABLE, and RSS keeps counting it, so the fixture read +41 MB on a fixed build there. It now measures Bun.unsafe.memoryFootprint (phys_footprint) on Darwin, like the other RSS tests. The baseline waits until memory stops falling instead of a fixed sleep, and the test asserts that the compiles grew memory before it checks that the growth came back. The preview pin stays until oven-sh/WebKit#567 merges, then it moves to the main sha. |
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Inline comments:
In `@test/js/bun/wasm/compile-rss-fixture.mjs`:
- Line 100: Update each memory footprint measurement in the RSS fixture to use
process.memoryUsage.rss() when Bun.unsafe.memoryFootprint() returns undefined,
preserving numeric values for the later RSS arithmetic and assertions.
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On the memoryFootprint fallback finding: task_info on the calling process does not fail in practice, and the fixture uses the same Darwin selector as the other RSS tests in the repo (password.test.ts, filesystem_router.test.ts). A broken measurement would fail the numeric assertion loudly, so no fallback is added. |
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Updated 1:07 AM PT - Sep 6th, 2026
❌ @robobun, your commit aafb03b has 1 failures in
🧪 To try this PR locally: bunx bun-pr 41449That installs a local version of the PR into your bun-41449 --bun |
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The Darwin RSS finding refers to the fixture before 7e65d35. Since that commit the fixture measures Bun.unsafe.memoryFootprint on Darwin, and the test passed on both macOS lanes in the last CI run. The pin thread stays open until oven-sh/WebKit#567 merges. |
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CI state: the new test passes on every lane, including both macOS lanes and Windows. The remaining red is unrelated to this diff: the napi test_object worker crash (already reported to main) and install tests that passed on retry. This PR waits for oven-sh/WebKit#567 to merge, then the pin moves to the main sha. |
… on failure Folds in the parts of #41452 that this test lacked: the memoryFootprint() undefined fallback and the failure output that carries every number. Six compiles put the unfixed build at 35 to 39 MiB against the 10 MiB bound (was 18 to 27) for about 60 ms more.
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Pushed aafb03b. #41452 was the same pin (oven-sh/WebKit#567, tag autobuild-preview-pr-567-e62d118e) with a heavier test, so it is closed and this PR continues. Folded in from it: the memoryFootprint() undefined fallback and a failure output that prints the whole measurement. The fixture now compiles 6 times instead of 3. On a release build of main that puts the idle delta at 35 to 39 MiB against the 10 MiB bound (was 18 to 27), and the fixed build still settles at or below the baseline. The test takes about 0.7 s. The PR body has the numbers and notes that the preview tag also carries oven-sh/WebKit#566. |
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Code review found no new issues
No new issues were found in this update; 1 finding from earlier reviews is still open above.
Still open from earlier reviews (1):
- 🔴
scripts/build/deps/webkit.ts:6—WEBKIT_VERSION is pinned to autobuild-preview-pr-567-e62d118e, an ephemeral preview tag; if merged as-is, clean builds…
…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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Landed in #44564: the WebKit change went in through oven-sh/WebKit#768, and the test and fixture from this PR are included there. |
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Problem
WebAssembly.compileof a 4 MB module leaves about 10 MB of RSS per wasm compiler thread behind, and the memory does not come back when theWebAssembly.Moduleis collected. The issue's script ends at +150 MB on 16 cores and +340 MB on 32 cores (WebAssembly.compile retains ~10 MB per wasm compiler thread; RSS grows with core count and is never released #41438). It all returns at the 10 s mark, when the idle threads time out and exit.AutomaticThreadthat finishes a compile and waits on its condition keeps every page it freed until it exits or works again. The wasm compiler threads, the DFG/FTL worklist threads and the GC helpers all wait this way.Fix
AutomaticThreadhas waited 100 ms without a notify, it releases its thread-local heap (mi_on_thread_idle(), the same hook Bun's event loop and thread pool call) with the worklist lock dropped, then waits out the rest of its timeout. A thread notified within 100 ms pays nothing.WEBKIT_VERSIONat that PR's preview build (autobuild-preview-pr-567-e62d118e) so CI runs Bun against it. The tag is two commits past the current2e2aa2290facpin: [JSC] CodeBlock aging: refresh the execution-counter snapshot on every look, not only past the TTL WebKit#566 (CodeBlock aging refreshes its execution-counter snapshot on every look) and the Slow down when useawait#567 commit. Before landing, [WTF] AutomaticThread: release the thread-local heap after 100 ms idle WebKit#567 has to merge and the pin has to move to the resulting main sha.test/js/bun/wasm/compile-rss.test.tsspawnscompile-rss-fixture.mjswith 8 compiler threads. The fixture compiles a tree-sitter shaped module (35 functions, one giant, 2.2 MB) 6 times, drops the modules, then polls RSS for up to 3 s against a 10 MiB bound. Unfixed it stays at +35 to +39 MiB. Fixed it returns to the starting RSS within a few hundred ms, and the whole test takes about 0.7 s. Skipped on debug and ASAN builds: their JSC prebuilts do not allocate through mimalloc.memoryFootprint()fallback and its failure output that prints every number are folded in here.test/js/bun/wasm/,test/js/bun/jsc/,jsc-stress.test.ts,test/js/web/workers/. Self-reviewed: 1 concern raised (the jsc shell could not link the new symbol), addressed in the WebKit PR.Background
scripts/build/deps/webkit.tspins which build. An engine fix lands as a WebKit PR plus a pin bump here, andautobuild-preview-pr-*tags let the bump PR run Bun's suite against the WebKit PR before it merges.thread_freelist, and a page emptied by its owner is retired rather than unmapped. Both wait for the owner to run a collect. mimalloc's scavenger thread purges memory that is already free at the arena level, but it cannot walk another thread's heap while that thread may allocate.AutomaticThreadis WTF's worker thread with a lifetime: it polls for work under a shared lock, waits on a condition when there is none, and exits after 10 s of silence.Notes
Repro on Linux x64, 16 cores, bun 1.4.1 (the issue's script, 30 compiles of
tree-sitter-cpp.wasm):Scaling with
BUN_JSC_numberOfWasmCompilerThreads: 15 threads +132 MB, 8 +105 MB, 4 +66 MB, 2 +31 MB, 1 +18 MB.MIMALLOC_PURGE_DELAY=0gives +29 MB, which points at the allocator and not at the compiled code.Why the fixture's module is shaped like a tree-sitter parser: a uniform module of thousands of small functions does not show the retention. Its compile temporaries end up in fully free pages, which mimalloc's scavenger purges on its own. Large control-flow heavy functions leave pages that are partly used or that hold blocks the main thread frees later, and those need the owner.
Why the fixture compiles a trivial 16-function module after dropping the real ones: the main thread frees the dead modules' metadata into the compiler threads' pages. A thread hands that back the next time it works and goes idle. Without this step the fixed build kept 2 to 10 MB of late cross-thread frees, which made the threshold fragile.
The second preview build of the WebKit PR crashed Bun at 110 ms of idle: the first version re-polled after the release, and
JITWorklistThread::pollcounts one deactivation per Wait, so the extra poll trippedRELEASE_ASSERT(m_numberOfActiveThreads). The thread now stays in the waiting state through the release and polls again only if a notify arrived.Margin probe on Linux x64, 8 compiler threads, release builds, 3 s poll, lowest idle delta seen (MiB):
Each compile adds about 20 ms. The test uses 6.
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