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deps: mimalloc to the head of bun-dev3-v2 (oven-sh/mimalloc#47, #49, #50, #51): recently used large pages are left alone for a while - #42808
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I reviewed this PR and didn't find any bugs. Because it bumps the process-wide allocator pin — a change whose real diff lives in the mimalloc fork and touches every allocation path — a human look is still worthwhile.
What was reviewed:
- Pin bump:
MIMALLOC_COMMITconstant name preserved for the update-mimalloc workflow's sed; grepped the repo for the old hash — no other occurrences. - New test:
bunExe()/spreadbunEnv,port: 0,127.0.0.1,await using,stderr: "inherit"(no pipe deadlock), skip reasons commented; theBun.sleep(50)is commented as the deliberate idle input, not a wait-for-condition. /proc/<pid>/statparse handles spaces in comm vialastIndexOf(")"); index 7 after the paren isminflt— checked against proc(5).- Env knobs are live:
BUN_GC_TIMER_INTERVALis read insrc/jsc/GarbageCollectionController.rs;MIMALLOC_PURGE_HOLES_MIN_INTERVALis a mimalloc-side option. Threshold (20) sits well between the reported fixed (~2.4) and unfixed (~130) values.
Extended reasoning...
Overview
Two files: scripts/build/deps/mimalloc.ts moves the MIMALLOC_COMMIT pin from 69050cf3… to 6df7aa13… (head of oven-sh/mimalloc bun-dev3-v2, picking up fork PRs #47/#49/#50), and test/js/bun/jsc/heapStats-mimalloc.test.ts gains a Linux-only, non-ASAN regression test that spawns an echo Bun.serve child with a shortened sweep epoch, sends spaced 256 KiB requests, and asserts minor page faults per request stay under 20 (vs ~130 on the old pin). The Bun-repo diff is small; the effective change is allocator behavior — recently-used large pages are retained for up to 256 sweep epochs / ~4 MiB process-wide instead of being discarded after two sweeps.
Security risks
None identified. No new user-facing surface, no parsing of untrusted input in the Bun tree, no auth/crypto/permissions paths. The allocator change is a retention heuristic; the test only reads /proc/<pid>/stat for the process it spawned and hits a local 127.0.0.1 server on an ephemeral port.
Level of scrutiny
High, despite the tiny in-repo diff. mimalloc backs every heap allocation in the runtime; a behavioral change there (retention vs. purge timing, cross-thread epoch counting, the pthread-key one-shot on macOS from #49) is foundational and its effects are hard to bound from the pin bump alone. The substantive review belongs on the fork PRs, but the decision to ship that behavior in Bun still merits a human sign-off. Not approving an allocator bump automatically.
Other factors
The test follows repo conventions closely (appended to the existing mimalloc test file, same test.skipIf(!isLinux || isASAN) pattern already used twice in that file, imports already present). The /proc stat parsing correctly skips the parenthesized comm field and lands on minflt. BUN_GC_TIMER_INTERVAL is confirmed read in src/jsc/GarbageCollectionController.rs; MIMALLOC_PURGE_HOLES_MIN_INTERVAL is a mimalloc env option consumed by the vendored library. Warmup (6 requests) precedes measurement (12 requests) so JIT/first-touch faults don't skew the delta; total wall time is ~0.9 s. No CODEOWNERS entry covers either changed path, and the timeline shows no outstanding third-party objections. The old commit hash has no other occurrences in the tree, so the single-line bump is complete.
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Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review. WalkthroughThe mimalloc dependency pin changes to a new commit. A Linux non-ASAN regression test measures minor page faults during repeated idle HTTP requests with 256 KiB request bodies. Changesmimalloc regression
Suggested reviewers: Priority: ⬇️ Low Merge Risk: 🔵 Low · up to The Linux regression test can misread a fragmented port value and become flaky. This is a narrow test-only issue, so merge risk is low. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Actionable comments posted: 1
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Inline comments:
In `@test/js/bun/jsc/heapStats-mimalloc.test.ts`:
- Line 270: Update the stdout-reading logic around reader.read() to accumulate
chunks until a newline is received, then parse the complete newline-delimited
port value with Number(). Preserve the existing reader flow and use the complete
line for subsequent endpoint requests.
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scripts/build/deps/mimalloc.tstest/js/bun/jsc/heapStats-mimalloc.test.ts
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…is initialised as one) The Windows lanes compile mimalloc's static.c as C++14, where a plain 0 for an _Atomic(size_t) member of a static initializer needs std::atomic's deleted copy constructor. The fork's head has it as MI_ATOMIC_VAR_INIT(0).
Jarred-Sumner
changed the title
… collect after a garbage collection (#42853) ## What `MIMALLOC_COMMIT` 20146828 → eab09015, the head of `bun-dev3-v2`: one fork PR, oven-sh/mimalloc#52. Since #42808 the allocator's idle sweep leaves up to 4 MiB of recently used large pages (buffers of 96 to 512 KiB) alone, so that a server does not fault the buffers of every request in again. A page that stays with *no* buffer in use was freed all the same by the next collect that is not forced, and JavaScriptCore does one after every full garbage collection (`bmalloc::api::scavengeThisThread(false)`), on the thread that allocates the buffers. The next request faulted the whole page in again. oven-sh/mimalloc#52: such a page carries the mark of the thread that keeps it, and only the sweeps (after 256 epochs), a forced collect and the end of the thread take it. ## Numbers Release builds of this tree (`f5649a7edb4`), the mimalloc commit the only difference, Linux x64. `Bun.serve` that answers a request with its own body, one connection; minor faults of the server from `/proc/<pid>/stat` and kernel instructions from `perf stat -e instructions:k -p`, per request: | | main: faults | kernel instr | this PR: faults | kernel instr | |---|---|---|---|---| | 128 KiB, 5 requests a second | 1.9 | 256 K | 2.5 | 476 K | | 256 KiB, 5 a second | 2.5 | 305 K | 2.5 | 294 K | | 512 KiB, 5 a second | 6.0 | 775 K | 3.5 | 835 K | | 128 KiB, one request every 2 s | 12.7 | 2410 K | 6.7 | 897 K | | 256 KiB, one every 2 s | 35.1 | 1273 K | 10.7 | 776 K | | 512 KiB, one every 2 s | 61.0 | 1485 K | 14.8 | 1640 K | | `hello`, 5 a second | 1.1 | 121 K | 1.1 | 101 K | (20 s at 5 a second, 40 s and 20 requests at one every 2 s, so the slow rows are few requests each; the fork PR's own table, with more of them, has 144 → 3.8 for 512 KiB every 2 s and 12 of 25 → 0 of 25 bursts of 6 × 512 KiB that fault all of their 770 pages in again.) ## Cost What #42808 stated and did not pay, because that collect gave the pages back: a server that goes completely idle holds what the floor kept until a thread parks again. Anonymous RSS of the server in KiB, 10 s / 70 s after the load stopped: | after | main | this PR | |---|---|---| | 128 KiB bodies | 3188 / 2964 | 4084 / 3824 | | 256 KiB bodies | 3264 / 3092 | 5048 / 4868 | | 512 KiB bodies | 3520 / 3388 | 7376 / 7184 | | `hello` | 2824 / 2748 | 2892 / 2792 | 3.8 MB more at the most (the floor is 4 MiB). `MIMALLOC_PURGE_HOLES_LARGE_FLOOR=0` turns the floor off. A large bundled CLI application (a `--compile`d program of some 200 MB that sits at a prompt), anonymous + file-backed RSS in MB; 10 minutes at the first prompt, a scripted session of 20 turns, 10 minutes parked; two runs each, all four started together: | | main | this PR | |---|---|---| | first prompt + 30 s | 105+98 / 106+98 | 106+98 / 106+98 | | + 2 min | 89+45 / 89+45 | 91+45 / 90+45 | | + 5 min | 85+46 / 85+47 | 86+46 / 86+46 | | + 10 min | 85+45 / 85+45 | 86+46 / 86+46 | | after the session + 30 s | 250+92 / 275+92 | 292+92 / 254+91 | | + 2 min | 185+46 / 199+46 | 181+46 / 190+46 | | + 5 min | 159+47 / 164+47 | 154+47 / 164+47 | | + 10 min | 165+56 / 169+57 | 159+56 / 169+56 | 1 MB more at the prompt, the same after the session (that application moves the sweep's epochs by its own wakeups, about two a second, so what it keeps goes after two minutes). `test/js/bun/jsc/heapStats-mimalloc.test.ts` passes 8/8 three times with this pin (release build).
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What
MIMALLOC_COMMIT69050cf3 → 6df7aa13, the head ofbun-dev3-v2. Relative to main's pin that is three fork PRs:Bun.pprof.heap: sampling native heap profiles in pprof format #42626 (Bun.pprof.heap) bumps the pin to that commit for its own sake. Whichever of the two PRs lands second keeps the newer commit, which is this one.Since #42668 the idle sweep hands back the free blocks of the allocator's 4 MiB pages (buffers of 96 KiB and up) two sweeps after the page was last allocated from, which is 100–200 ms. A server sits idle between two requests for longer than that, so it took the buffers of every request out of discarded memory again: a page fault for each 4 KiB of them, on every request. About half of that was free blocks in a page that still had a buffer in use; the other half was pages with no buffer in use at all, which were freed outright and made anew for the next request.
oven-sh/mimalloc#50: a large page (its free blocks, or all of it) is left alone until it has not been allocated from for 256 epochs of the sweep, for up to 4 MiB process-wide, the pages that were used last first. There is no timer in this: an epoch ends only when some thread is swept.
MIMALLOC_PURGE_HOLES_LARGE_FLOOR=0is the behaviour before (a bare number there is bytes: write4MiB, not4096).What it costs
Numbers
Linux x64, release builds of this tree, the mimalloc commit the only difference.
Bun.servethat answers a request with its own body, one connection, 5 requests a second for 20 s. Minor faults of the server (/proc/<pid>/stat) and kernel instructions (perf stat -e instructions:k -p), per request:hello(No latencies: the machine was busy with other work and the median of one binary moved more from run to run than the two differ. At 512 KiB it shows through: 2.60 ms → 2.08 ms.)
Idle afterwards, anonymous RSS of the server in KiB at +10 s / +70 s: 128 KiB 3128 / 2908 → 4044 / 2940; 256 KiB 3248 / 3056 → 3280 / 3044; 512 KiB 3564 / 3356 → 6624 / 3376;
hello2936 / 2808 → 2784 / 2780. So it holds up to 3 MB more ten seconds after the load and the same a minute after.A large
--compiled CLI application that sits at a prompt (anonymous + file-backed RSS in MB; 10 minutes at the first prompt, a scripted session of 20 turns, 10 minutes parked; two runs each, all four started together):At the prompt that is 3 MB more for the first minute, 1 MB at two minutes and after. After the session these four runs are 5–10 MB apart; the same comparison with one binary and the floor switched by the environment (
…LARGE_FLOOR=0) came out at 171 / 161 against 168 / 173 at + 5 min, so what the floor itself keeps there (3–4 MB for two minutes) is inside what two runs of one configuration differ by.Instructions (
perf stat -e instructions:u, pinned, interleaved, median [range]):bun -e 1(9 runs)JSON.stringify+JSON.parseloop (5)malloc/freemalloc(128 KiB)+free(The allocation loop moves by that much between runs of one binary; its ranges overlap.)
Test
test/js/bun/jsc/heapStats-mimalloc.test.ts: aBun.servechild that echoes 256 KiB bodies, requests 50 ms apart with the sweep's epoch shortened to 10 ms and the collector looking every 100 ms, minor faults per request read from/proc. 2.3–2.4 with this pin, 130–131 with main's (the bound is 20). Linux only, not ASAN.