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GeezerVison is showing github.com — gh-159137: Untrack tuples and count survivors in move_unreachable() by pablogsal · Pull Request #159154 · python/cpython

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@pablogsal

@pablogsal pablogsal commented Oct 10, 2026 •
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After finding the surviving objects, the collector walks them two more times: once in untrack_tuples() and once in gc_list_size() just to count them. move_unreachable() already visits each survivor, so this does both there and drops the two walks.

Benchmarks

GC time from gc.get_stats(). Medians of four runs per executable (nine for the AST heap), in baseline/candidate/candidate/baseline order.

Workload GC before GC after Less GC time
Sphinx building the CPython docs 27.956 s 20.387 s 27.1%
Pylint on five stdlib packages 2.740 s 1.607 s 41.4%
Parso parsing the standard library 1.397 s 0.860 s 38.4%
sqlglot parsing and transpiling 4,000 queries 4.248 s 2.900 s 31.7%
Beautiful Soup parsing 90 HTML pages 2.122 s 1.485 s 30.0%
Black checking three stdlib packages 1.643 s 1.460 s 11.1%
Full collection of 720,000 live AST objects 292.4 ms 153.0 ms 47.7%
Geometric mean 33.3%

Collector cycles on the Pylint run. Width is the share before the change, blue frames take fewer cycles after it:

pylint

  • Issue: Speed up garbage collector #159137

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@pablogsal
pythongh-159137: Untrack tuples and count survivors in move_unreachab…
4a47f38 
…le()
@bedevere-app bedevere-app Bot mentioned this pull request Oct 10, 2026
Speed up garbage collector #159137
Open
@pablogsal pablogsal mentioned this pull request Oct 10, 2026
gh-159137: gc: Count GC survivors in untrack_tuples() instead of walking the young generation again #159149
Closed
sergey-miryanov
sergey-miryanov reviewed Oct 10, 2026
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Comment thread Python/gc.c Resolved

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Comment thread Python/gc.c Resolved

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@pablogsal
pablogsal marked this pull request as ready for review October 10, 2026 22:45
@bedevere-app bedevere-app Bot added the awaiting core review label Oct 10, 2026
@pablogsal
pablogsal requested a review from nascheme October 10, 2026 22:45
@pablogsal
Check for an untracked tuple only in the tuple branch and add a NEWS …
6c50d7b 
…entry
@pablogsal pablogsal mentioned this pull request Oct 11, 2026
gh-159137: Restore known-root marking for the GIL garbage collector #159162
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sergey-miryanov
sergey-miryanov approved these changes Oct 11, 2026
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Comment thread Python/gc.c Resolved

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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting core review labels Oct 11, 2026
@pablogsal @sergey-miryanov
Update Python/gc.c
a75955d 
Co-authored-by: Sergey Miryanov <[email protected]>
@pablogsal
pablogsal enabled auto-merge (squash) October 11, 2026 15:44
@pablogsal
pablogsal merged commit e878e1e into python:main Oct 11, 2026
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@pablogsal
pablogsal deleted the gh-159137-gc-fold-survivor-walks branch October 11, 2026 16:01
@bedevere-app bedevere-app Bot removed the awaiting merge label Oct 11, 2026
orsenthil pushed a commit to orsenthil/cpython that referenced this pull request Oct 11, 2026
@pablogsal @orsenthil
pythongh-159137: Untrack tuples and count survivors in move_unreachab…
6a32de1 
…le() (python#159154)
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@sergey-miryanov sergey-miryanov sergey-miryanov approved these changes
@nascheme nascheme Awaiting requested review from nascheme

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@pablogsal @sergey-miryanov
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