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

Issue: Upgrade mimalloc version from 2.1.2 to 2.5.2 #159111

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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

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

Documentation build overview

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

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85 files changed · ± 85 modified

± Modified

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

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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.

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

Results will be shown at:

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

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

@corona10 - New results

@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?

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.

cc @DinoV

This looks good!

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

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

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>

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

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

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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

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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

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

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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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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)

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

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gh-159111: Update vendored mimalloc to v2.5.2 - #159112

gh-159111: Update vendored mimalloc to v2.5.2#159112corona10 merged 16 commits intopython:mainpython/cpython:mainfrom corona10:gh-159111corona10/cpython:gh-159111Copy head branch name to clipboard

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gh-159111: Update vendored mimalloc to v2.5.2#159112
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