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gh-126703: Add PyCFunction freelist #128692

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Apr 5, 2025
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gh-126703: Add PyCFunction freelist #128692

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20cc6b9
add pycfunction freelist
eendebakpt Jan 9, 2025
e86ccad
📜🤖 Added by blurb_it.
blurb-it[bot] Jan 9, 2025
a76224b
Merge branch 'main' into pycfunctionobject_freelist
eendebakpt Jan 10, 2025
6c9d056
Merge branch 'main' into pycfunctionobject_freelist
eendebakpt Jan 15, 2025
7a165db
Merge branch 'main' into pycfunctionobject_freelist
eendebakpt Jan 27, 2025
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4 changes: 4 additions & 0 deletions Include/internal/pycore_freelist_state.h
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,8 @@ extern "C" {
# define Py_futureiters_MAXFREELIST 255
# define Py_object_stack_chunks_MAXFREELIST 4
# define Py_unicode_writers_MAXFREELIST 1
# define Py_pycfunctionobject_MAXFREELIST 16
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How are these lengths being chosen? 16 seems fine for PyCFunction, but I would think that methods are created much more frequently.

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Choosing the number is a bit of an art (and a good number will depend what kind of code is executed).

How well the freelist works depends on the dynamics of the allocation and deallocation. For example the size of Py_unicode_writers_MAXFREELIST is only 1. I suspect this is because the typical use case is: create a writer, write some data, release it again. And during the writing of the data no new unicode writers are constructed.

With the diff below you can see whether a PyCMethodObject is obtained from the freelist (if the size > 0), or whether a new one is allocated (when size = 0).

diff --git a/Include/internal/pycore_freelist.h b/Include/internal/pycore_freelist.h
index 84a5ab30f3e..90d6616cbfd 100644
--- a/Include/internal/pycore_freelist.h
+++ b/Include/internal/pycore_freelist.h
@@ -28,6 +28,10 @@ _Py_freelists_GET(void)
 #endif
 }

+#define _Py_FREELIST_FREE_PRINT(NAME, op, freefunc) \
+    _PyFreeList_Free_Print(&_Py_freelists_GET()->NAME, _PyObject_CAST(op), \
+                     Py_ ## NAME ## _MAXFREELIST, freefunc, #NAME)
+
 // Pushes `op` to the freelist, calls `freefunc` if the freelist is full
 #define _Py_FREELIST_FREE(NAME, op, freefunc) \
     _PyFreeList_Free(&_Py_freelists_GET()->NAME, _PyObject_CAST(op), \
@@ -69,6 +73,16 @@ _PyFreeList_Free(struct _Py_freelist *fl, void *obj, Py_ssize_t maxsize,
     }
 }

+static inline void
+_PyFreeList_Free_Print(struct _Py_freelist *fl, void *obj, Py_ssize_t maxsize,
+                 freefunc dofree, const char *name)
+{
+    if (!_PyFreeList_Push(fl, obj, maxsize)) {
+        printf("    %s: no space to store object\n", name);
+        dofree(obj);
+    }
+}
+
 static inline void *
 _PyFreeList_PopNoStats(struct _Py_freelist *fl)
 {
diff --git a/Objects/methodobject.c b/Objects/methodobject.c
index fc77055b0a2..58d928d78bf 100644
--- a/Objects/methodobject.c
+++ b/Objects/methodobject.c
@@ -86,6 +86,7 @@ PyCMethod_New(PyMethodDef *ml, PyObject *self, PyObject *module, PyTypeObject *c
                             "flag but no class");
             return NULL;
         }
+        printf("PyCMethodObject: try allocation (freelist size %d)\n", _Py_FREELIST_SIZE(pycmethodobject));
         PyCMethodObject *om = _Py_FREELIST_POP(PyCMethodObject, pycmethodobject);
         if (om == NULL) {
             om = PyObject_GC_New(PyCMethodObject, &PyCMethod_Type);
@@ -102,6 +103,7 @@ PyCMethod_New(PyMethodDef *ml, PyObject *self, PyObject *module, PyTypeObject *c
                             "but no METH_METHOD flag");
             return NULL;
         }
+        printf("PyCFunctionObject: try allocation (freelist size %d)\n", _Py_FREELIST_SIZE(pycfunctionobject));
         op = _Py_FREELIST_POP(PyCFunctionObject, pycfunctionobject);
         if (op == NULL) {
             op = PyObject_GC_New(PyCFunctionObject, &PyCFunction_Type);
@@ -180,11 +182,11 @@ meth_dealloc(PyObject *self)
     Py_XDECREF(m->m_module);
     if (m->m_ml->ml_flags & METH_METHOD) {
         assert(Py_IS_TYPE(self, &PyCMethod_Type));
-        _Py_FREELIST_FREE(pycmethodobject, m, PyObject_GC_Del);
+        _Py_FREELIST_FREE_PRINT(pycmethodobject, m, PyObject_GC_Del);
     }
     else {
         assert(Py_IS_TYPE(self, &PyCFunction_Type));
-        _Py_FREELIST_FREE(pycfunctionobject, m, PyObject_GC_Del);
+        _Py_FREELIST_FREE_PRINT(pycfunctionobject, m, PyObject_GC_Del);
     }
     Py_TRASHCAN_END;
 }

When I use this on python -m test test_operator:

  • there are many allocations at the start of the program (because nothing has been deallocated, so there is nothing on the freelist)
  • there is a dynamic section where often objects are allocated and deallocated, the freelist size is changing a lot, but it is not reaching the maximum size of 16
  • at the end the python interpreter is closing down, so many objects are deallocated

Based on this there would be no need to increase the freelist size. On the other hand, it is almost free to increase the size (the only memory is the objects on the freelist).

I am fine with changing the size (anything between 4 and 400 would be fine with me), but I am not sure it matters or how to make a more informed decision.

Full output is at:

https://gist.github.com/eendebakpt/7a867587450dca4689bb46271fb01ec2

# define Py_pycmethodobject_MAXFREELIST 16
# define Py_pymethodobjects_MAXFREELIST 20

// A generic freelist of either PyObjects or other data structures.
Expand Down Expand Up @@ -49,6 +51,8 @@ struct _Py_freelists {
struct _Py_freelist futureiters;
struct _Py_freelist object_stack_chunks;
struct _Py_freelist unicode_writers;
struct _Py_freelist pycfunctionobject;
struct _Py_freelist pycmethodobject;
struct _Py_freelist pymethodobjects;
};

Expand Down
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-01-09-22-12-03.gh-issue-126703.6rAxZ7.rst
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@@ -0,0 +1 @@
Improve performance of builtin methods by using a freelist.
24 changes: 19 additions & 5 deletions Objects/methodobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@
#include "Python.h"
#include "pycore_call.h" // _Py_CheckFunctionResult()
#include "pycore_ceval.h" // _Py_EnterRecursiveCallTstate()
#include "pycore_freelist.h"
#include "pycore_object.h"
#include "pycore_pyerrors.h"
#include "pycore_pystate.h" // _PyThreadState_GET()
Expand Down Expand Up @@ -85,9 +86,12 @@ PyCMethod_New(PyMethodDef *ml, PyObject *self, PyObject *module, PyTypeObject *c
"flag but no class");
return NULL;
}
PyCMethodObject *om = PyObject_GC_New(PyCMethodObject, &PyCMethod_Type);
PyCMethodObject *om = _Py_FREELIST_POP(PyCMethodObject, pycmethodobject);
if (om == NULL) {
return NULL;
om = PyObject_GC_New(PyCMethodObject, &PyCMethod_Type);
if (om == NULL) {
return NULL;
}
}
om->mm_class = (PyTypeObject*)Py_NewRef(cls);
op = (PyCFunctionObject *)om;
Expand All @@ -98,9 +102,12 @@ PyCMethod_New(PyMethodDef *ml, PyObject *self, PyObject *module, PyTypeObject *c
"but no METH_METHOD flag");
return NULL;
}
op = PyObject_GC_New(PyCFunctionObject, &PyCFunction_Type);
op = _Py_FREELIST_POP(PyCFunctionObject, pycfunctionobject);
if (op == NULL) {
return NULL;
op = PyObject_GC_New(PyCFunctionObject, &PyCFunction_Type);
if (op == NULL) {
return NULL;
}
}
}

Expand Down Expand Up @@ -171,7 +178,14 @@ meth_dealloc(PyObject *self)
Py_XDECREF(PyCFunction_GET_CLASS(m));
Py_XDECREF(m->m_self);
Py_XDECREF(m->m_module);
PyObject_GC_Del(m);
if (m->m_ml->ml_flags & METH_METHOD) {
assert(Py_IS_TYPE(self, &PyCMethod_Type));
_Py_FREELIST_FREE(pycmethodobject, m, PyObject_GC_Del);
}
else {
assert(Py_IS_TYPE(self, &PyCFunction_Type));
_Py_FREELIST_FREE(pycfunctionobject, m, PyObject_GC_Del);
}
Py_TRASHCAN_END;
}

Expand Down
2 changes: 2 additions & 0 deletions Objects/object.c
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Expand Up @@ -937,6 +937,8 @@ _PyObject_ClearFreeLists(struct _Py_freelists *freelists, int is_finalization)
}
clear_freelist(&freelists->unicode_writers, is_finalization, PyMem_Free);
clear_freelist(&freelists->ints, is_finalization, free_object);
clear_freelist(&freelists->pycfunctionobject, is_finalization, PyObject_GC_Del);
clear_freelist(&freelists->pycmethodobject, is_finalization, PyObject_GC_Del);
clear_freelist(&freelists->pymethodobjects, is_finalization, free_object);
}

Expand Down
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