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[3.14] GH-139951: Fix major GC performance regression (GH-140262)#140423

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miss-islington wants to merge1 commit intopython:3.14frommiss-islington:backport-0c01090-3.14

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[3.14] GH-139951: Fix major GC performance regression (GH-140262)#140423

miss-islington wants to merge1 commit intopython:3.14frommiss-islington:backport-0c01090-3.14

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miss-islington commentedOct 21, 2025•
edited by bedevere-appbot
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Count number of actually tracked objects, instead of trackable objects. This ensures that untracking tuples has the desired effect of reducing GC overhead
Do not track most untrackable tuples during creation. This prevents large numbers of small tuples causing execessive GCs.
(cherry picked from commit0c01090)

Co-authored-by: Mark Shannonmark@hotpy.org

Issue:Potentially serious regression in garbage collector performance in Python 3.14 #139951

pythonGH-139951: Fix major GC performance regression (pythonGH-140262)

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* Count number of actually tracked objects, instead of trackable objects. This ensures that untracking tuples has the desired effect of reducing GC overhead* Do not track most untrackable tuples during creation. This prevents large numbers of small tuples causing execessive GCs.(cherry picked from commit0c01090)Co-authored-by: Mark Shannon <mark@hotpy.org>

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Oct 21, 2025

Potentially serious regression in garbage collector performance in Python 3.14#139951

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Oct 21, 2025

bedevere-appbot mentioned this pull request

Oct 21, 2025

GH-139951: Fix major GC performance regression#140262

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sergey-miryanov commentedOct 21, 2025•
edited by hugovk
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The following change fixes free-threading tests (mainly-1386,10 +1385,6 fixes tests, other is a cleanup):

diff --git a/Lib/test/test_gc.py b/Lib/test/test_gc.pyindex db643330894..d7a4c066437 100644--- a/Lib/test/test_gc.py+++ b/Lib/test/test_gc.py@@ -3,8 +3,7 @@ from test import support from test.support import (verbose, refcount_test,                           cpython_only, requires_subprocess,-                          requires_gil_enabled,-                          Py_GIL_DISABLED)+                          requires_gil_enabled) from test.support.import_helper import import_module from test.support.os_helper import temp_dir, TESTFN, unlink from test.support.script_helper import assert_python_ok, make_script, run_test_script@@ -1386,10 +1385,6 @@ def callback(ignored):         junk = []         i = 0         detector = GC_Detector()-        if Py_GIL_DISABLED:-            # The free-threaded build doesn't have multiple generations, so-            # just trigger a GC manually.-            gc.collect()         assert not detector.gc_happened         while not detector.gc_happened:             i += 1@@ -1459,10 +1454,6 @@ def __del__(self):         detector = GC_Detector()         junk = []         i = 0-        if Py_GIL_DISABLED:-            # The free-threaded build doesn't have multiple generations, so-            # just trigger a GC manually.-            gc.collect()         while not detector.gc_happened:             i += 1             if i > 50000: