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GeezerVison is showing github.com — gh-161420: Process mimalloc's delayed frees on every generic malloc again by maikschulze · Pull Request #161533 · python/cpython

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@maikschulze

@maikschulze maikschulze commented Nov 7, 2026 •
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Since mimalloc 2.5.2 (gh-159111), _mi_malloc_generic() processes delayed frees from other threads only every 100th call, so blocks over 64 KiB that other threads freed can pile up, up to about 100 per heap. This processes them on every generic malloc again, as before.

Peak RSS of the example from the issue, free-threaded release builds:

Peak RSS main this PR
macOS 5,900–8,000 MiB 410–500 MiB
Linux 9,500–11,000 MiB 420–540 MiB
Windows 8,000–8,200 MiB 430–450 MiB

No measurable difference for small objects (pyperf, macOS and Linux).

I used an AI assistant to investigate this and draft the patch; I reviewed and tested the changes myself.

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@maikschulze
pythongh-161420: Process mimalloc's delayed frees on every generic ma…
9ba8532 
…lloc again

Since the update to mimalloc 2.5.2 (pythongh-159111), _mi_malloc_generic() processes the heap's delayed
frees only every 100 calls. A block over MI_MEDIUM_OBJ_SIZE_MAX has a page of its own, so a free of
it from another thread is a delayed free, and every allocation of such a block is a generic malloc:
each thread could keep up to about 100 large blocks that other threads had freed. Process the
delayed frees on every generic malloc again, as mimalloc 2.1 did; the deferred-free callback and
the periodic collection stay batched.
@bedevere-app bedevere-app Bot added the awaiting review label Nov 7, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Nov 7, 2026
Free-threaded 3.14.10 and 3.15.2 keep memory freed by other threads: 10 GB peaks since the mimalloc 2.5.2 update #161420
Closed
@corona10
corona10 self-requested a review November 7, 2026 06:22
@corona10 corona10 self-assigned this Nov 7, 2026
@corona10 corona10 added needs backport to 3.14 bugs and security fixes needs backport to 3.15 bugs and security fixes labels Nov 7, 2026
corona10
corona10 reviewed Nov 7, 2026
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Comment thread Objects/mimalloc/page.c Resolved

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corona10
corona10 reviewed Nov 7, 2026
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Comment thread Misc/NEWS.d/next/Core_and_Builtins/2026-11-07-09-30-00.gh-issue-161420.mIdFre.rst Resolved

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corona10
corona10 requested changes Nov 7, 2026
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Please write descption to be concisely as possible.

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@bedevere-app bedevere-app Bot added awaiting changes and removed awaiting review labels Nov 7, 2026
@bedevere-app

bedevere-app Bot commented Nov 7, 2026

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A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.
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@maikschulze @corona10
Apply batched suggestions from code review
b4fe0c0 
Co-authored-by: Donghee Na <[email protected]>
@python-cla-bot

python-cla-bot Bot commented Nov 7, 2026 •
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All commit authors signed the Contributor License Agreement.

CLA signed
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@maikschulze

maikschulze commented Nov 7, 2026

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I have made the requested changes; please review again
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@bedevere-app bedevere-app Bot added awaiting change review and removed awaiting changes labels Nov 7, 2026
@bedevere-app

bedevere-app Bot commented Nov 7, 2026

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Thanks for making the requested changes!

@corona10: please review the changes made to this pull request.
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@bedevere-app
bedevere-app Bot requested a review from corona10 November 7, 2026 07:01
corona10
corona10 approved these changes Nov 7, 2026
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lgtm!

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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting change review labels Nov 7, 2026
@corona10
corona10 enabled auto-merge (squash) November 7, 2026 07:12
@corona10
corona10 merged commit b8dd06f into python:main Nov 7, 2026
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@bedevere-app bedevere-app Bot removed the awaiting merge label Nov 7, 2026
@miss-islington-app

miss-islington-app Bot commented Nov 7, 2026

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Thanks @maikschulze for the PR, and @corona10 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14, 3.15.
🐍🍒⛏🤖
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@bedevere-app

bedevere-app Bot commented Nov 7, 2026

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GH-161537 is a backport of this pull request to the 3.15 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label Nov 7, 2026
@bedevere-app

bedevere-app Bot commented Nov 7, 2026

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GH-161538 is a backport of this pull request to the 3.14 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Nov 7, 2026
corona10 pushed a commit that referenced this pull request Nov 7, 2026
@miss-islington @maikschulze
[3.14] gh-161420: Process mimalloc's delayed frees on every generic m…
e03963c 
…alloc again (gh-161533) (gh-161538)

gh-161420: Process mimalloc's delayed frees on every generic malloc again (gh-161533)
(cherry picked from commit b8dd06f)

Co-authored-by: Maik Schulze <[email protected]>
corona10 pushed a commit that referenced this pull request Nov 7, 2026
@miss-islington @maikschulze
[3.15] gh-161420: Process mimalloc's delayed frees on every generic m…
d52dd4a 
…alloc again (gh-161533) (gh-161537)

gh-161420: Process mimalloc's delayed frees on every generic malloc again (gh-161533)
(cherry picked from commit b8dd06f)

Co-authored-by: Maik Schulze <[email protected]>
@vstinner

vstinner commented Nov 7, 2026

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@corona10: Should we keep track of mimalloc 2.5 downstream changes made in Python? Should the change be reported and fixed to mimalloc upstream?
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@corona10

corona10 commented Nov 7, 2026

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It’s already reported in microsoft/mimalloc#1117.

And comments starts with “CPython:” in mimalloc of CPython repo means that it’s our downstream modification, so it’s already tracked.
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@maikschulze @vstinner @corona10
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