8000 gh-134761: Use deferred reference counting for `threading` concurrency primitives by ZeroIntensity · Pull Request #134762 · python/cpython · GitHub
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gh-134761: Use deferred reference counting for threading concurrency primitives #134762

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gh-134761: Use deferred reference counting for threading concurrency primitives #134762

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e02137e
Use deferred reference counting for threading primitives.
ZeroIntensity May 26, 2025
6093acb
Document and test sys._defer_refcount()
ZeroIntensity May 26, 2025
478d482
Add blurb.
ZeroIntensity May 26, 2025
f0fc73c
Remove sys._defer_refcount() documentation.
ZeroIntensity May 27, 2025
d407ad9
Remove stray newline change.
ZeroIntensity May 27, 2025
c750d92
Move call to _PyObject_SetDeferredRefcount()
ZeroIntensity May 27, 2025
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19 changes: 19 additions & 0 deletions Lib/test/test_sys.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1343,6 +1343,25 @@ def test_pystats(self):
def test_disable_gil_abi(self):
self.assertEqual('t' in sys.abiflags, support.Py_GIL_DISABLED)

@test.support.cpython_only
@unittest.skipUnless(hasattr(sys, '_defer_refcount'), "requires _defer_refcount()")
def test_defer_refcount(self):
_testinternalcapi = import_helper.import_module('_testinternalcapi')

class Test:
pass

ref = Test()
if support.Py_GIL_DISABLED:
self.assertTrue(sys._defer_refcount(ref))
self.assertTrue(_testinternalcapi.has_deferred_refcount(ref))
self.assertFalse(sys._defer_refcount(ref))
self.assertFalse(sys._defer_refcount(42))
else:
self.assertFalse(sys._defer_refcount(ref))
self.assertFalse(_testinternalcapi.has_deferred_refcount(ref))
self.assertFalse(sys._defer_refcount(42))


@test.support.cpython_only
class UnraisableHookTest(unittest.TestCase):
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9 changes: 9 additions & 0 deletions Lib/threading.py
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8000
Original file line number Diff line number Diff line change
Expand Up @@ -73,6 +73,11 @@
_profile_hook = None
_trace_hook = None

def _defer_refcount(op):
"""Improve multithreaded scaling on the free-threade build."""
if hasattr(_sys, "_defer_refcount"):
_sys._defer_refcount(op)

def setprofile(func):
"""Set a profile function for all threads started from the threading module.

Expand Down Expand Up @@ -298,6 +303,7 @@ def __init__(self, lock=None):
if hasattr(lock, '_is_owned'):
self._is_owned = lock._is_owned
self._waiters = _deque()
_defer_refcount(self)

def _at_fork_reinit(self):
self._lock._at_fork_reinit()
Expand Down Expand Up @@ -466,6 +472,7 @@ def __init__(self, value=1):
raise ValueError("semaphore initial value must be >= 0")
self._cond = Condition(Lock())
self._value = value
_defer_refcount(self)

def __repr__(self):
cls = self.__class__
Expand Down Expand Up @@ -595,6 +602,7 @@ class Event:
def __init__(self):
self._cond = Condition(Lock())
self._flag = False
_defer_refcount(self)

def __repr__(self):
cls = self.__class__
Expand Down Expand Up @@ -700,6 +708,7 @@ def __init__(self, parties, action=None, timeout=None):
self._parties = parties
self._state = 0 # 0 filling, 1 draining, -1 resetting, -2 broken
self._count = 0
_defer_refcount(self)

def __repr__(self):
cls = self.__class__
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2025-05-26-19-49-16.gh-issue-134761.LeMefI.rst
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@@ -0,0 +1,2 @@
Improve performance when using :mod:`threading` primitives across multiple
threads.
2 changes: 2 additions & 0 deletions Modules/_threadmodule.c
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Expand Up @@ -950,6 +950,7 @@ lock_new_impl(PyTypeObject *type)
if (self == NULL) {
return NULL;
}
_PyObject_SetDeferredRefcount((PyObject *)self);
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Can you move the call after initializing self->lock? Same remark for rlock_new_impl() below.

self->lock = (PyMutex){0};
return (PyObject *)self;
}
Expand Down Expand Up @@ -1221,6 +1222,7 @@ rlock_new_impl(PyTypeObject *type)
if (self == NULL) {
return NULL;
}
_PyObject_SetDeferredRefcount((PyObject *)self);
self->lock = (_PyRecursiveMutex){0};
return (PyObject *) self;
}
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32 changes: 31 additions & 1 deletion Python/clinic/sysmodule.c.h
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19 changes: 19 additions & 0 deletions Python/sysmodule.c
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Expand Up @@ -15,6 +15,7 @@ Data members:
*/

#include "Python.h"
#include "object.h"
#include "pycore_audit.h" // _Py_AuditHookEntry
#include "pycore_call.h" // _PyObject_CallNoArgs()
#include "pycore_ceval.h" // _PyEval_SetAsyncGenFinalizer()
Expand Down Expand Up @@ -2653,6 +2654,23 @@ sys__is_gil_enabled_impl(PyObject *module)
#endif
}

/*[clinic input]
sys._defer_refcount -> bool
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Please document it in Doc/library/sys.rst. If it "should not be used", add a clear explanation why it should not be used there. If it's not documented, the lack of documentation doesn't prevent users from using it.

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See Ken and Donghee's comments.

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Well, I disageee with them. IMO we should document sys functions.

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I would only be supportive of documenting this if we were allowed to change it in a minor version with no deprecation period. My understanding is that PyUnstable in the C API allows that, but exposing to sys._x means we are stuck with at least 2 deprecation cycle and recommended 5 deprecation cycles. Users should not rely on this function in the first place except in very specific scenarios.

One way to "bypass" this is make the function a no-op in future versions of Python once we solve this issue altogether. But I don't know what users will rely on by then so I'm a bit worried.

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Hmm, I thought we were allowed to change sys._x things in minor versions without deprecation. If not, that's a problem.

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I think we're getting a bit hung up on this point. We can add or remove the documentation for _defer_refcount later, it's not too important. Does everything else look fine here?

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Well I am preparing a better proposal for this approach. Give me hours.

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@ZeroIntensity ZeroIntensity May 28, 2025 •
edited
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Ok, cool. Feel free to cc me on it.

Something we also need to consider is whether we want to address this for 3.14. Should this general idea be considered a bugfix or a feature?

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See: #134819

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I think that this is improvement rather than bug fix.


op: object
/

Defer reference counting for the object, allowing for better scaling across multiple threads.

This function should be used for specialized purposes only.
[clinic start generated code]*/

static int
sys__defer_refcount_impl(PyObject *module, PyObject *op)
/*[clinic end generated code: output=3b965122056085f5 input=a081971a76c49e64]*/
{
return PyUnstable_Object_EnableDeferredRefcount(op);
}

#ifndef MS_WINDOWS
static PerfMapState perf_map_state;
Expand Down Expand Up @@ -2834,6 +2852,7 @@ static PyMethodDef sys_methods[] = {
SYS__GET_CPU_COUNT_CONFIG_METHODDEF
SYS__IS_GIL_ENABLED_METHODDEF
SYS__DUMP_TRACELETS_METHODDEF
SYS__DEFER_REFCOUNT_METHODDEF
{NULL, NULL} // sentinel
};

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