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Merged
merged 8 commits into from
Dec 2, 2025
Merged

gh-142048: Fix quadratically increasing GC delays #142051

merged 8 commits into from
Dec 2, 2025
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@kevmo314
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@kevmo314 kevmo314 commented Nov 28, 2025 •
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Fixes quadratically increasing GC delays in the free-threaded build. This is done by updating the local to global allocation count propagation to require the global count to be non-negative. This matches the existing behavior in non-free-threaded gc.c. Additionally, adds a flush that I found was causing allocations to be lost.

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

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sergey-miryanov commented Nov 28, 2025

cc @nascheme @colesbury

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kevmo314 commented Nov 28, 2025 •
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Thinking about this solution a little more, perhaps an alternative strategy that's more aligned with the original intent would be to only update the global counter to be non-negative, allowing the local ones to be negative. I'm not sure how to do that with the atomics though.

I think I have a better intuition about the core bug though: the compounding happens because garbage collection first resets the global count to zero, then frees objects. Each freed object calls record_deallocation(), driving local counts negative, which eventually get flushed to the global counter. Thus the global counter keeps getting more and more negative.

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bedevere-app bot commented Nov 30, 2025

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

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@kevmo314
Fix allocation count decrement and GC state update
9388518 
Adjust allocation count handling to prevent negative values and update GC state accordingly.
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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

@kevmo314 kevmo314 changed the title gh-142048: Reset allocation counts in gc thread-local buffers gh-142048: Enforce global allocation count to be non-negative Dec 1, 2025
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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

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colesbury
colesbury reviewed Dec 1, 2025
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@kevmo314 @colesbury
Update Python/pystate.c
795e703 
Co-authored-by: Sam Gross <colesbury@gmail.com>
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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

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colesbury commented Dec 1, 2025

Please add a NEWS blurb. You can either use the blurb-it webapp mentioned above or the blurb command. (I find the blurb command via uvx blurb to be the easiest). It should be in the "Core and Builtins" sections. I think the important thing here is that it fixes quadratic behavior in the GC for free threaded Python, not really whether the count can be negative.

@kevmo314 kevmo314 changed the title gh-142048: Enforce global allocation count to be non-negative gh-142048: Fix quadratically increasing GC delays Dec 1, 2025
@kevmo314 kevmo314 requested a review from colesbury December 1, 2025 20:43
colesbury
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Remove local GC allocation count flush in pystate.c
8be49b5 
Removed flushing of thread's local GC allocation count before thread state deletion.
@kevmo314
Merge branch 'main' into fix-gc-alloc-count-negative
f202991
@kevmo314 kevmo314 requested a review from colesbury December 1, 2025 23:40
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Thanks!

@colesbury colesbury added needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes awaiting merge and removed awaiting merge labels Dec 1, 2025
@colesbury colesbury merged commit eb89286 into python:main Dec 2, 2025
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miss-islington-app bot commented Dec 2, 2025

Thanks @kevmo314 for the PR, and @colesbury for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

miss-islington pushed a commit to miss-islington/cpython that referenced this pull request Dec 2, 2025
@kevmo314 @miss-islington
pythongh-142048: Fix quadratically increasing GC delays (pythongh-142051
406cda9 
)

The GC for the free threaded build would get slower with each collection due
to effectively double counting objects freed by the GC.
(cherry picked from commit eb89286)

Co-authored-by: Kevin Wang <kevmo314@gmail.com>
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miss-islington-app bot commented Dec 2, 2025

Sorry, @kevmo314 and @colesbury, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker eb892868b31322d7cf271bc25923e14b1f67ae38 3.13

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bedevere-app bot commented Dec 2, 2025

GH-142166 is a backport of this pull request to the 3.14 branch.

@bedevere-app bedevere-app bot removed the needs backport to 3.14 bugs and security fixes label Dec 2, 2025
colesbury pushed a commit to colesbury/cpython that referenced this pull request Dec 2, 2025
@kevmo314 @colesbury
[3.13] pythongh-142048: Fix quadratically increasing GC delays (pytho…
4303d74 
…ngh-142051)

The GC for the free threaded build would get slower with each collection due
to effectively double counting objects freed by the GC.
(cherry picked from commit eb89286)

Co-authored-by: Kevin Wang <kevmo314@gmail.com>
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bedevere-app bot commented Dec 2, 2025

GH-142167 is a backport of this pull request to the 3.13 branch.

@bedevere-app bedevere-app bot removed the needs backport to 3.13 bugs and security fixes label Dec 2, 2025
colesbury pushed a commit that referenced this pull request Dec 2, 2025
@miss-islington @kevmo314
[3.14] gh-142048: Fix quadratically increasing GC delays (gh-142051) (g…
5c5670e 
…h-142166)

The GC for the free threaded build would get slower with each collection due
to effectively double counting objects freed by the GC.
(cherry picked from commit eb89286)

Co-authored-by: Kevin Wang <kevmo314@gmail.com>
colesbury added a commit that referenced this pull request Dec 2, 2025
@colesbury @kevmo314
[3.13] gh-142048: Fix quadratically increasing GC delays (gh-142051) (#…
01393ff 
…142167)

The GC for the free threaded build would get slower with each collection due
to effectively double counting objects freed by the GC.
(cherry picked from commit eb89286)

Co-authored-by: Kevin Wang <kevmo314@gmail.com>
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colesbury commented Dec 2, 2025

I think the buildbot failure is unrelated. The change only affects the free threaded build and the buildbot isn't for free threaded Python.

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@colesbury colesbury colesbury approved these changes

@ericsnowcurrently ericsnowcurrently Awaiting requested review from ericsnowcurrently

@ZeroIntensity ZeroIntensity Awaiting requested review from ZeroIntensity

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