gh-159111: Update vendored mimalloc to v2.5.2 by corona10 · Pull Request #159112 · python/cpython

corona10 · 2026-10-10T11:24:38Z

Issue: Upgrade mimalloc version from 2.1.2 to 2.5.2 #159111

bedevere-bot · 2026-10-10T11:25:18Z

🤖 New build scheduled with the buildbot fleet by @corona10 for commit 2649e2b 🤖

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Upgrade mimalloc from v2.1.2 to v2.5.2 as-is from upstream. v2.1.2: https://github.com/microsoft/mimalloc/releases/tag/v2.1.2 v2.5.2: https://github.com/microsoft/mimalloc/releases/tag/v2.5.2

bedevere-bot · 2026-10-10T11:30:47Z

🤖 New build scheduled with the buildbot fleet by @corona10 for commit f81e6c2 🤖

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If you want to schedule another build, you need to add the 🔨 test-with-refleak-buildbots label again.

corona10 · 2026-10-10T11:34:29Z

@mpage, could you trigger a free-threading benchmark for this change? :)

read-the-docs-community · 2026-10-10T11:34:59Z

Documentation build overview

📚 cpython-previews | 🛠️ Build #35145094 | 📁 Comparing 09196b1 against main (88fd5fe)

🔍 Preview build

85 files changed · ± 85 modified

± Modified

bedevere-bot · 2026-10-10T11:50:50Z

🤖 New build scheduled with the buildbot fleet by @corona10 for commit 222601b 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F159112%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-refleak-buildbots label again.

corona10 · 2026-10-10T11:59:00Z

I will fix MSAN later:

==11983==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x55e368c48377 in _mi_random_next /home/runner/work/cpython/cpython/Objects/mimalloc/random.c:158:3
    #1 0x55e368c48377 in _mi_heap_random_next /home/runner/work/cpython/cpython/Objects/mimalloc/heap.c:281:10
    #2 0x55e368c48377 in mi_heap_main_init /home/runner/work/cpython/cpython/Objects/mimalloc/init.c:220:29
    #3 0x55e368c49eaf in _mi_auto_process_init /home/runner/work/cpython/cpython/Objects/mimalloc/init.c:568:3
    #4 0x7f8aa762aa53 in __libc_start_main (/usr/lib/x86_64-linux-gnu/libc.so.6+0x2aa53) (BuildId: 4ec69afdf3da96ce7388640dcd27c6d0c1b256ef)
    #5 0x55e36886ee24 in _start (/home/runner/work/cpython/cpython/Programs/_freeze_module+0xb4e24) (BuildId: 5e5dc085937d530e3b49b31b09d53deb8664a51d)

  Uninitialized value was stored to memory at
    #0 0x55e368c8b8a0 in chacha_block /home/runner/work/cpython/cpython/Objects/mimalloc/random.c:67:20
    #1 0x55e368c47beb in chacha_next32 /home/runner/work/cpython/cpython/Objects/mimalloc/random.c:83:5
    #2 0x55e368c47beb in _mi_random_next /home/runner/work/cpython/cpython/Objects/mimalloc/random.c:154:22
    #3 0x55e368c47beb in _mi_heap_random_next /home/runner/work/cpython/cpython/Objects/mimalloc/heap.c:281:10
    #4 0x55e368c47beb in mi_heap_main_init /home/runner/work/cpython/cpython/Objects/mimalloc/init.c:220:29
    #5 0x55e368c49eaf in _mi_auto_process_init /home/runner/work/cpython/cpython/Objects/mimalloc/init.c:568:3
    #6 0x7f8aa762aa53 in __libc_start_main (/usr/lib/x86_64-linux-gnu/libc.so.6+0x2aa53) (BuildId: 4ec69afdf3da96ce7388640dcd27c6d0c1b256ef)
    #7 0x55e36886ee24 in _start (/home/runner/work/cpython/cpython/Programs/_freeze_module+0xb4e24) (BuildId: 5e5dc085937d530e3b49b31b09d53deb8664a51d)

  Uninitialized value was stored to memory at
    #0 0x55e368c5d150 in chacha_init /home/runner/work/cpython/cpython/Objects/mimalloc/random.c:109:23
    #1 0x55e368c5d150 in mi_random_init_ex /home/runner/work/cpython/cpython/Objects/mimalloc/random.c:201:3
    #2 0x55e368c47b57 in _mi_random_init /home/runner/work/cpython/cpython/Objects/mimalloc/random.c:205:3
    #3 0x55e368c47b57 in mi_heap_main_init /home/runner/work/cpython/cpython/Objects/mimalloc/init.c:218:7
    #4 0x55e368c49eaf in _mi_auto_process_init /home/runner/work/cpython/cpython/Objects/mimalloc/init.c:568:3
    #5 0x7f8aa762aa53 in __libc_start_main (/usr/lib/x86_64-linux-gnu/libc.so.6+0x2aa53) (BuildId: 4ec69afdf3da96ce7388640dcd27c6d0c1b256ef)
    #6 0x55e36886ee24 in _start (/home/runner/work/cpython/cpython/Programs/_freeze_module+0xb4e24) (BuildId: 5e5dc085937d530e3b49b31b09d53deb8664a51d)

  Uninitialized value was created by an allocation of 'key' in the stack frame
    #0 0x55e368c5cbad in mi_random_init_ex /home/runner/work/cpython/cpython/Objects/mimalloc/random.c:181:3

SUMMARY: MemorySanitizer: use-of-uninitialized-value /home/runner/work/cpython/cpython/Objects/mimalloc/random.c:158:3 in _mi_random_next
Exiting

Most of the previous downstream changes were upstreamed in mimalloc v2.1.3 - v2.1.7 (heap tags, _mi_heap_init/_mi_tld_init, MI_PRIM_THREAD_ID, sub-processes, heap walk optimizations, platform fixes) and are dropped. Remaining CPython-specific changes, all marked with a "CPython" comment: - QSBR delayed page freeing for free-threaded builds (pythongh-115103, pythongh-145615): mi_page_t/mi_heap_t fields and _PyMem_mi_* hooks in page.c, heap.c and segment.c. - debug_offset: preserve the first bytes of a PyObject (ob_tid, refcount fields) when filling freed or uninitialized memory in debug builds (pythongh-115491). - Atomic mi_block_t.next loads/stores to avoid TSAN reports (pythongh-129748). - Relax the owning thread assertion in _mi_free_delayed_block for free-threaded builds, since the GC traverses heaps of other threads. - Call _mi_heap_delayed_free_partial() in mi_heap_visit_blocks() (pythongh-112532). - Only count reclaimed pages up to MI_MEDIUM_OBJ_SIZE_MAX as a "right page" in mi_segment_reclaim (pythongh-115103). - Rename the weak _ZSt15get_new_handlerv symbol and disable the mi_is_in_heap_region debug warning. - Zero-initialize _mi_page_empty instead of a positional initializer.

bedevere-bot · 2026-10-13T12:46:51Z

🤖 New build scheduled with the buildbot fleet by @corona10 for commit 8b9cb49 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F159112%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-refleak-buildbots label again.

mpage · 2026-10-13T15:43:40Z

@corona10 - Kicked off another run. Fingers crossed this one works.

mpage · 2026-10-13T18:01:12Z

@corona10 - New results

corona10 · 2026-10-13T21:55:04Z

@mpage

Okay benchmark result now looks good to me since there is no significant change

fork: corona10
ref: gh_159111
machine: linux-x86_64
commit hash: 8b9cb49
commit date: 2026-10-13
overall geometric mean: 1.003x faster
HPT reliability: 97.78%
HPT 99th percentile: 1.00x faster
Memory change: 1.00x

Would you like to review this PR?

corona10 · 2026-10-22T00:00:08Z

+* Update the bundled `mimalloc library`_ from version 2.1.2 to 2.5.2.
+  (Contributed by Donghee Na in :gh:`159111`.)
+
+  .. _mimalloc library: https://github.com/microsoft/mimalloc/
+


Suggested change

* Update the bundled `mimalloc library`_ from version 2.1.2 to 2.5.2.

(Contributed by Donghee Na in :gh:`159111`.)

.. _mimalloc library: https://github.com/microsoft/mimalloc/

Because for the main branch, we will target v3.

corona10 · 2026-10-22T00:00:38Z

cc @DinoV

DinoV

This looks good!

miss-islington-app · 2026-10-22T00:41:11Z

Thanks @corona10 for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14, 3.15.
🐍🍒⛏🤖

miss-islington-app · 2026-10-22T00:41:14Z

Sorry, @corona10, I could not cleanly backport this to 3.15 due to a conflict.

Please backport manually with cherry_picker, see the devguide for more information.

cherry_picker aff7a2e23bd6ed68fa15c1ceda94a2fdd7f8cf7f 3.15

miss-islington-app · 2026-10-22T00:41:19Z

Sorry, @corona10, I could not cleanly backport this to 3.14 due to a conflict.

Please backport manually with cherry_picker, see the devguide for more information.

cherry_picker aff7a2e23bd6ed68fa15c1ceda94a2fdd7f8cf7f 3.14

…159112) * Update vendored mimalloc to v2.5.2 Upgrade mimalloc from v2.1.2 to v2.5.2 as-is from upstream. v2.1.2: https://github.com/microsoft/mimalloc/releases/tag/v2.1.2 v2.5.2: https://github.com/microsoft/mimalloc/releases/tag/v2.5.2 * Re-apply CPython-specific changes to mimalloc v2.5.2 Most of the previous downstream changes were upstreamed in mimalloc v2.1.3 - v2.1.7 (heap tags, _mi_heap_init/_mi_tld_init, MI_PRIM_THREAD_ID, sub-processes, heap walk optimizations, platform fixes) and are dropped. Remaining CPython-specific changes, all marked with a "CPython" comment: - QSBR delayed page freeing for free-threaded builds (pythongh-115103, pythongh-145615): mi_page_t/mi_heap_t fields and _PyMem_mi_* hooks in page.c, heap.c and segment.c. - debug_offset: preserve the first bytes of a PyObject (ob_tid, refcount fields) when filling freed or uninitialized memory in debug builds (pythongh-115491). - Atomic mi_block_t.next loads/stores to avoid TSAN reports (pythongh-129748). - Relax the owning thread assertion in _mi_free_delayed_block for free-threaded builds, since the GC traverses heaps of other threads. - Call _mi_heap_delayed_free_partial() in mi_heap_visit_blocks() (pythongh-112532). - Only count reclaimed pages up to MI_MEDIUM_OBJ_SIZE_MAX as a "right page" in mi_segment_reclaim (pythongh-115103). - Rename the weak _ZSt15get_new_handlerv symbol and disable the mi_is_in_heap_region debug warning. - Zero-initialize _mi_page_empty instead of a positional initializer. * Update the free-threaded build to mimalloc v2.5.2 changes mimalloc v2.1.4 replaced the list of abandoned segments with per-arena bitmaps and introduced sub-processes (mi_subproc_t) to keep abandoned segments of separate interpreters apart. Use them instead of the CPython-specific mi_abandoned_pool_t: - Each interpreter owns a mi_subproc_t created in init_interpreter(). - tstate_mimalloc_bind() sets tld->segments.subproc and uses the upstream _mi_heap_init() signature. - The GC and sys.getallocatedblocks() visit abandoned segments with _PyMem_mi_visit_abandoned_blocks(), the upstream cursor API without the mi_option_visit_abandoned requirement, while STW. * Update NEWS.d * fix MSAN * fix TSAN * fix test_cext * Disable THP * Address code review * Address code review * Address code review * nit * Address code review * Address code review (cherry picked from commit aff7a2e) Co-authored-by: Donghee Na <donghee.na@python.org>

bedevere-app · 2026-10-22T04:19:59Z

GH-160178 is a backport of this pull request to the 3.15 branch.

bedevere-app · 2026-10-22T04:26:53Z

GH-160183 is a backport of this pull request to the 3.14 branch.

…160183)

…160178)

bedevere-app Bot added the type-feature A feature request or enhancement label Oct 10, 2026

bedevere-app Bot mentioned this pull request Oct 10, 2026

Upgrade mimalloc version from 2.1.2 to 2.5.2 #159111

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corona10 self-assigned this Oct 10, 2026

corona10 added the 🔨 test-with-refleak-buildbots Test PR w/ refleak buildbots; report in status section label Oct 10, 2026

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corona10 marked this pull request as ready for review October 10, 2026 11:25

corona10 requested review from a team, AA-Turner, FFY00, ZeroIntensity, emmatyping, ericsnowcurrently, erlend-aasland and itamaro as code owners October 10, 2026 11:25

bedevere-app Bot added the awaiting core review label Oct 10, 2026

corona10 requested a review from mpage October 10, 2026 11:25

Update vendored mimalloc to v2.5.2

4180f6d

Upgrade mimalloc from v2.1.2 to v2.5.2 as-is from upstream. v2.1.2: https://github.com/microsoft/mimalloc/releases/tag/v2.1.2 v2.5.2: https://github.com/microsoft/mimalloc/releases/tag/v2.5.2

corona10 force-pushed the gh-159111 branch from 2649e2b to f81e6c2 Compare October 10, 2026 11:29

corona10 added the 🔨 test-with-refleak-buildbots Test PR w/ refleak buildbots; report in status section label Oct 10, 2026

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corona10 force-pushed the gh-159111 branch from ceb117e to 5fb37d1 Compare October 10, 2026 13:42

bedevere-bot removed the 🔨 test-with-refleak-buildbots Test PR w/ refleak buildbots; report in status section label Oct 13, 2026

Merge remote-tracking branch 'upstream/main' into pythongh-159111

09196b1

corona10 assigned DinoV Oct 20, 2026

corona10 mentioned this pull request Oct 20, 2026

gh-131999: Add support for tvOS, visionOS and watchOS. #159944

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corona10 commented Oct 22, 2026

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DinoV approved these changes Oct 22, 2026

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bedevere-app Bot added awaiting merge and removed awaiting core review labels Oct 22, 2026

corona10 added needs backport to 3.14 bugs and security fixes needs backport to 3.15 bugs and security fixes labels Oct 22, 2026

corona10 merged commit aff7a2e into python:main Oct 22, 2026
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corona10 deleted the gh-159111 branch October 22, 2026 07:52

corona10 added a commit that referenced this pull request Oct 22, 2026

[3.14] gh-159111: Update vendored mimalloc to v2.5.2 (gh-159112) (gh-…

1bcc0d1

…160183)

corona10 added a commit that referenced this pull request Oct 22, 2026

[3.15] gh-159111: Update vendored mimalloc to v2.5.2 (gh-159112) (gh-…

86e3b9f

…160178)

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