gh-145685: Improve scaling of type attribute lookups by colesbury · Pull Request #145774 · python/cpython · GitHub

Avoid locking in the PyType_Lookup cache-miss path if the type's tp_version_tag is already valid.

Issue: Unexpected scaling bottleneck in _PyType_Lookup / _PyType_LookupStackRefAndVersion #145685

Avoid locking in the PyType_Lookup cache-miss path if the type's tp_version_tag is already valid.

🤖 New build scheduled with the buildbot fleet by @colesbury for commit 68c122c 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F145774%2Fmerge

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

Would it be worth adding a test case to the scaling benchmark?

Do we need to hold the type lock around this?

The operations in find_name_in_mro are individually thread-safe, although this allows for mutations to types to be interleaved with the MRO traversal. I think that's acceptable. There were already edge cases where that could happen due to ensure_shared_on_read() calls suspending the critical section during dictionary lookups in find_name_in_mro.

Importantly, if any type in the MRO chain is modified during lookup, the assigned version tag will be stale so any future cache lookups won't use it.

Would it be worth adding a test case to the scaling benchmark?

This improves the scaling behavior, but it's not enough to scale linearly so I don't want to add a benchmark to ftscalingbench.py yet.

Here's the benchmark I've occasionally used locally: https://gist.github.com/colesbury/7d43e7dde89c1cb5574b5a2c73cc52b1

Some of the remaining issues:

The code keeps getting re-specialized and the specialization code itself causes reference count contention. This seems fixable in follow up PRs
Seq-lock contention in the MRO cache. This is trickier.

Thanks @colesbury for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

…145774) Avoid locking in the PyType_Lookup cache-miss path if the type's tp_version_tag is already valid. (cherry picked from commit cd52172) Co-authored-by: Sam Gross <colesbury@gmail.com>

GH-145874 is a backport of this pull request to the 3.14 branch.

… (#145874) Avoid locking in the PyType_Lookup cache-miss path if the type's tp_version_tag is already valid. (cherry picked from commit cd52172) Co-authored-by: Sam Gross <colesbury@gmail.com>

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Ubuntu Shared 3.14 (tier-1) has failed when building commit cedff2d.

What do you need to do:

Don't panic.
Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1802/builds/838) and take a look at the build logs.
Check if the failure is related to this commit (cedff2d) or if it is a false positive.
If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1802/builds/838

Failed tests:

test_pyrepl

Failed subtests:

test_repl_eio - test.test_pyrepl.test_unix_console.TestUnixConsoleEIOHandling.test_repl_eio

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/subprocess.py", line 1138, in __del__
    _warn("subprocess %s is still running" % self.pid,
ResourceWarning: subprocess 3913832 is still running
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=10 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/unittest/case.py", line 668, in run
    with outcome.testPartExecutor(self):
ResourceWarning: unclosed file <_io.TextIOWrapper name=10 encoding='UTF-8'>
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=8 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/unittest/case.py", line 668, in run
    with outcome.testPartExecutor(self):
ResourceWarning: unclosed file <_io.TextIOWrapper name=8 encoding='UTF-8'>


Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/test/test_pyrepl/test_unix_console.py", line 390, in test_repl_eio
    _, err = proc.communicate(timeout=support.LONG_TIMEOUT)
             ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/subprocess.py", line 1220, in communicate
    stdout, stderr = self._communicate(input, endtime, timeout)
                     ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/subprocess.py", line 2153, in _communicate
    self._check_timeout(endtime, orig_timeout, stdout, stderr)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/subprocess.py", line 1267, in _check_timeout
    raise TimeoutExpired(
    ...<2 lines>...
            stderr=b''.join(stderr_seq) if stderr_seq else None)
subprocess.TimeoutExpired: Command '['/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/python', '-E', '-S', '/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/test/test_pyrepl/eio_test_script.py']' timed out after 300.0 seconds


Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/subprocess.py", line 1138, in __del__
    _warn("subprocess %s is still running" % self.pid,
ResourceWarning: subprocess 3900353 is still running
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=10 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/unittest/case.py", line 668, in run
    with outcome.testPartExecutor(self):
ResourceWarning: unclosed file <_io.TextIOWrapper name=10 encoding='UTF-8'>
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=8 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.14.bolen-ubuntu/build/Lib/unittest/case.py", line 668, in run
    with outcome.testPartExecutor(self):
ResourceWarning: unclosed file <_io.TextIOWrapper name=8 encoding='UTF-8'>

…145774) Avoid locking in the PyType_Lookup cache-miss path if the type's tp_version_tag is already valid.

pythongh-145685 : Improve scaling of type attribute lookups

9d1b301

Avoid locking in the PyType_Lookup cache-miss path if the type's tp_version_tag is already valid.

colesbury added topic-free-threading needs backport to 3.14 bugs and security fixes labels Mar 10, 2026

bedevere-app Bot mentioned this pull request Mar 10, 2026

Unexpected scaling bottleneck in _PyType_Lookup / _PyType_LookupStackRefAndVersion #145685

Closed

Clean up

68c122c

colesbury added the 🔨 test-with-refleak-buildbots Test PR w/ refleak buildbots; report in status section label Mar 10, 2026

bedevere-bot removed the 🔨 test-with-refleak-buildbots Test PR w/ refleak buildbots; report in status section label Mar 10, 2026

colesbury marked this pull request as ready for review March 10, 2026 19:25

colesbury requested a review from markshannon as a code owner March 10, 2026 19:25

bedevere-app Bot added the awaiting core review label Mar 10, 2026

colesbury requested a review from mpage March 10, 2026 19:25

mpage reviewed Mar 11, 2026

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mpage approved these changes Mar 11, 2026

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bedevere-app Bot added awaiting merge and removed awaiting core review labels Mar 11, 2026

colesbury merged commit cd52172 into python:main Mar 12, 2026
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bedevere-app Bot removed the awaiting merge label Mar 12, 2026

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Mar 12, 2026

colesbury deleted the gh-145685-lock-free-type-lookup branch April 20, 2026 15:20

ljfp pushed a commit to ljfp/cpython that referenced this pull request Apr 25, 2026

pythongh-145685 : Improve scaling of type attribute lookups (pythongh-…

645d773

…145774) Avoid locking in the PyType_Lookup cache-miss path if the type's tp_version_tag is already valid.

gh-145685: Improve scaling of type attribute lookups by colesbury · Pull Request #145774 · python/cpython · GitHub

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gh-145685: Improve scaling of type attribute lookups - #145774

gh-145685: Improve scaling of type attribute lookups#145774
colesbury merged 2 commits into
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colesbury:gh-145685-lock-free-type-lookup

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+                      END_TYPE_LOCK();
+                  }
+                  else {
+                      res = find_name_in_mro(type, name, out);

gh-145685: Improve scaling of type attribute lookups by colesbury · Pull Request #145774 · python/cpython · GitHub

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gh-145685: Improve scaling of type attribute lookups - #145774

gh-145685: Improve scaling of type attribute lookups#145774colesbury merged 2 commits intopython:mainpython/cpython:mainfrom colesbury:gh-145685-lock-free-type-lookupcolesbury/cpython:gh-145685-lock-free-type-lookupCopy head branch name to clipboard

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gh-145685: Improve scaling of type attribute lookups#145774
colesbury merged 2 commits into
python:mainfrom
colesbury:gh-145685-lock-free-type-lookup

colesbury commented Mar 10, 2026 •
edited by bedevere-app Bot
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