bpo-45522: Allow to disable freelists on build time (GH-29056)

Freelists for object structs can now be disabled. A new ``configure`` option ``--without-freelists`` can be used to disable all freelists except empty tuple singleton. Internal Py*_MAXFREELIST macros can now be defined as 0 without causing compiler warnings and segfaults. Signed-off-by: Christian Heimes <christian@python.org>
python · Oct 21, 2021 · 9942f42 · 9942f42
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commit 9942f42
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diff --git a/Doc/whatsnew/3.11.rst b/Doc/whatsnew/3.11.rst
@@ -481,6 +481,11 @@ Build Changes
   ``isinf()``, ``isnan()``, ``round()``.
   (Contributed by Victor Stinner in :issue:`45440`.)
 
+* Freelists for object structs can now be disabled. A new :program:`configure`
+  option :option:`!--without-freelists` can be used to disable all freelists
+  except empty tuple singleton.
+  (Contributed by Christian Heimes in :issue:`45522`)
+
 C API Changes
 =============
 

diff --git a/Include/internal/pycore_interp.h b/Include/internal/pycore_interp.h
@@ -85,12 +85,31 @@ struct _Py_unicode_state {
     struct _Py_unicode_ids ids;
 };
 
+#ifndef WITH_FREELISTS
+// without freelists
+#  define PyFloat_MAXFREELIST 0
+// for tuples only store empty tuple singleton
+#  define PyTuple_MAXSAVESIZE 1
+#  define PyTuple_MAXFREELIST 1
+#  define PyList_MAXFREELIST 0
+#  define PyDict_MAXFREELIST 0
+#  define PyFrame_MAXFREELIST 0
+#  define _PyAsyncGen_MAXFREELIST 0
+#  define PyContext_MAXFREELIST 0
+#endif
+
+#ifndef PyFloat_MAXFREELIST
+#  define PyFloat_MAXFREELIST   100
+#endif
+
 struct _Py_float_state {
+#if PyFloat_MAXFREELIST > 0
     /* Special free list
        free_list is a singly-linked list of available PyFloatObjects,
        linked via abuse of their ob_type members. */
     int numfree;
     PyFloatObject *free_list;
+#endif
 };
 
 /* Speed optimization to avoid frequent malloc/free of small tuples */
@@ -119,33 +138,44 @@ struct _Py_tuple_state {
 #endif
 
 struct _Py_list_state {
+#if PyList_MAXFREELIST > 0
     PyListObject *free_list[PyList_MAXFREELIST];
     int numfree;
+#endif
 };
 
 #ifndef PyDict_MAXFREELIST
 #  define PyDict_MAXFREELIST 80
 #endif
 
 struct _Py_dict_state {
+#if PyDict_MAXFREELIST > 0
     /* Dictionary reuse scheme to save calls to malloc and free */
     PyDictObject *free_list[PyDict_MAXFREELIST];
     int numfree;
     PyDictKeysObject *keys_free_list[PyDict_MAXFREELIST];
     int keys_numfree;
+#endif
 };
 
+#ifndef PyFrame_MAXFREELIST
+#  define PyFrame_MAXFREELIST 200
+#endif
+
 struct _Py_frame_state {
+#if PyFrame_MAXFREELIST > 0
     PyFrameObject *free_list;
     /* number of frames currently in free_list */
     int numfree;
+#endif
 };
 
 #ifndef _PyAsyncGen_MAXFREELIST
 #  define _PyAsyncGen_MAXFREELIST 80
 #endif
 
 struct _Py_async_gen_state {
+#if _PyAsyncGen_MAXFREELIST > 0
     /* Freelists boost performance 6-10%; they also reduce memory
        fragmentation, as _PyAsyncGenWrappedValue and PyAsyncGenASend
        are short-living objects that are instantiated for every
@@ -155,12 +185,19 @@ struct _Py_async_gen_state {
 
     struct PyAsyncGenASend* asend_freelist[_PyAsyncGen_MAXFREELIST];
     int asend_numfree;
+#endif
 };
 
+#ifndef PyContext_MAXFREELIST
+#  define PyContext_MAXFREELIST 255
+#endif
+
 struct _Py_context_state {
+#if PyContext_MAXFREELIST > 0
     // List of free PyContext objects
     PyContext *freelist;
     int numfree;
+#endif
 };
 
 struct _Py_exc_state {

diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
@@ -825,7 +825,18 @@ def test_debugmallocstats(self):
         from test.support.script_helper import assert_python_ok
         args = ['-c', 'import sys; sys._debugmallocstats()']
         ret, out, err = assert_python_ok(*args)
-        self.assertIn(b"free PyDictObjects", err)
+
+        # Output of sys._debugmallocstats() depends on configure flags.
+        # The sysconfig vars are not available on Windows.
+        if sys.platform != "win32":
+            with_freelists = sysconfig.get_config_var("WITH_FREELISTS")
+            with_pymalloc = sysconfig.get_config_var("WITH_PYMALLOC")
+            if with_freelists:
+                self.assertIn(b"free PyDictObjects", err)
+            if with_pymalloc:
+                self.assertIn(b'Small block threshold', err)
+            if not with_freelists and not with_pymalloc:
+                self.assertFalse(err)
 
         # The function has no parameter
         self.assertRaises(TypeError, sys._debugmallocstats, True)

diff --git a/Misc/NEWS.d/next/C API/2021-10-19-13-07-46.bpo-45522.kGAwmZ.rst b/Misc/NEWS.d/next/C API/2021-10-19-13-07-46.bpo-45522.kGAwmZ.rst
@@ -0,0 +1,2 @@
+The internal freelists for frame, float, list, dict, async generators, and
+context objects can now be disabled.
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
@@ -240,17 +240,20 @@ uint64_t _pydict_global_version = 0;
 #include "clinic/dictobject.c.h"
 
 
+#if PyDict_MAXFREELIST > 0
 static struct _Py_dict_state *
 get_dict_state(void)
 {
     PyInterpreterState *interp = _PyInterpreterState_GET();
     return &interp->dict_state;
 }
+#endif
 
 
 void
 _PyDict_ClearFreeList(PyInterpreterState *interp)
 {
+#if PyDict_MAXFREELIST > 0
     struct _Py_dict_state *state = &interp->dict_state;
     while (state->numfree) {
         PyDictObject *op = state->free_list[--state->numfree];
@@ -260,14 +263,15 @@ _PyDict_ClearFreeList(PyInterpreterState *interp)
     while (state->keys_numfree) {
         PyObject_Free(state->keys_free_list[--state->keys_numfree]);
     }
+#endif
 }
 
 
 void
 _PyDict_Fini(PyInterpreterState *interp)
 {
     _PyDict_ClearFreeList(interp);
-#ifdef Py_DEBUG
+#if defined(Py_DEBUG) && PyDict_MAXFREELIST > 0
     struct _Py_dict_state *state = &interp->dict_state;
     state->numfree = -1;
     state->keys_numfree = -1;
@@ -279,9 +283,11 @@ _PyDict_Fini(PyInterpreterState *interp)
 void
 _PyDict_DebugMallocStats(FILE *out)
 {
+#if PyDict_MAXFREELIST > 0
     struct _Py_dict_state *state = get_dict_state();
     _PyDebugAllocatorStats(out, "free PyDictObject",
                            state->numfree, sizeof(PyDictObject));
+#endif
 }
 
 #define DK_MASK(dk) (DK_SIZE(dk)-1)
@@ -570,6 +576,7 @@ new_keys_object(uint8_t log2_size)
         es = sizeof(Py_ssize_t);
     }
 
+#if PyDict_MAXFREELIST > 0
     struct _Py_dict_state *state = get_dict_state();
 #ifdef Py_DEBUG
     // new_keys_object() must not be called after _PyDict_Fini()
@@ -579,6 +586,7 @@ new_keys_object(uint8_t log2_size)
         dk = state->keys_free_list[--state->keys_numfree];
     }
     else
+#endif
     {
         dk = PyObject_Malloc(sizeof(PyDictKeysObject)
                              + (es<<log2_size)
@@ -611,6 +619,7 @@ free_keys_object(PyDictKeysObject *keys)
         Py_XDECREF(entries[i].me_key);
         Py_XDECREF(entries[i].me_value);
     }
+#if PyDict_MAXFREELIST > 0
     struct _Py_dict_state *state = get_dict_state();
 #ifdef Py_DEBUG
     // free_keys_object() must not be called after _PyDict_Fini()
@@ -620,6 +629,7 @@ free_keys_object(PyDictKeysObject *keys)
         state->keys_free_list[state->keys_numfree++] = keys;
         return;
     }
+#endif
     PyObject_Free(keys);
 }
 
@@ -638,6 +648,7 @@ new_dict(PyDictKeysObject *keys, PyDictValues *values, Py_ssize_t used, int free
 {
     PyDictObject *mp;
     assert(keys != NULL);
+#if PyDict_MAXFREELIST > 0
     struct _Py_dict_state *state = get_dict_state();
 #ifdef Py_DEBUG
     // new_dict() must not be called after _PyDict_Fini()
@@ -649,7 +660,9 @@ new_dict(PyDictKeysObject *keys, PyDictValues *values, Py_ssize_t used, int free
         assert (Py_IS_TYPE(mp, &PyDict_Type));
         _Py_NewReference((PyObject *)mp);
     }
-    else {
+    else
+#endif
+    {
         mp = PyObject_GC_New(PyDictObject, &PyDict_Type);
         if (mp == NULL) {
             dictkeys_decref(keys);
@@ -1259,6 +1272,7 @@ dictresize(PyDictObject *mp, uint8_t log2_newsize)
 #ifdef Py_REF_DEBUG
         _Py_RefTotal--;
 #endif
+#if PyDict_MAXFREELIST > 0
         struct _Py_dict_state *state = get_dict_state();
 #ifdef Py_DEBUG
         // dictresize() must not be called after _PyDict_Fini()
@@ -1269,7 +1283,9 @@ dictresize(PyDictObject *mp, uint8_t log2_newsize)
         {
             state->keys_free_list[state->keys_numfree++] = oldkeys;
         }
-        else {
+        else
+#endif
+        {
             PyObject_Free(oldkeys);
         }
     }
@@ -1987,6 +2003,7 @@ dict_dealloc(PyDictObject *mp)
         assert(keys->dk_refcnt == 1);
         dictkeys_decref(keys);
     }
+#if PyDict_MAXFREELIST > 0
     struct _Py_dict_state *state = get_dict_state();
 #ifdef Py_DEBUG
     // new_dict() must not be called after _PyDict_Fini()
@@ -1995,7 +2012,9 @@ dict_dealloc(PyDictObject *mp)
     if (state->numfree < PyDict_MAXFREELIST && Py_IS_TYPE(mp, &PyDict_Type)) {
         state->free_list[state->numfree++] = mp;
     }
-    else {
+    else
+#endif
+    {
         Py_TYPE(mp)->tp_free((PyObject *)mp);
     }
     Py_TRASHCAN_END

diff --git a/Objects/floatobject.c b/Objects/floatobject.c
@@ -28,12 +28,14 @@ class float "PyObject *" "&PyFloat_Type"
 #endif
 
 
+#if PyFloat_MAXFREELIST > 0
 static struct _Py_float_state *
 get_float_state(void)
 {
     PyInterpreterState *interp = _PyInterpreterState_GET();
     return &interp->float_state;
 }
+#endif
 
 
 double
@@ -126,8 +128,10 @@ PyFloat_GetInfo(void)
 PyObject *
 PyFloat_FromDouble(double fval)
 {
+    PyFloatObject *op;
+#if PyFloat_MAXFREELIST > 0
     struct _Py_float_state *state = get_float_state();
-    PyFloatObject *op = state->free_list;
+    op = state->free_list;
     if (op != NULL) {
 #ifdef Py_DEBUG
         // PyFloat_FromDouble() must not be called after _PyFloat_Fini()
@@ -136,7 +140,9 @@ PyFloat_FromDouble(double fval)
         state->free_list = (PyFloatObject *) Py_TYPE(op);
         state->numfree--;
     }
-    else {
+    else
+#endif
+    {
         op = PyObject_Malloc(sizeof(PyFloatObject));
         if (!op) {
             return PyErr_NoMemory();
@@ -233,6 +239,7 @@ PyFloat_FromString(PyObject *v)
 static void
 float_dealloc(PyFloatObject *op)
 {
+#if PyFloat_MAXFREELIST > 0
     if (PyFloat_CheckExact(op)) {
         struct _Py_float_state *state = get_float_state();
 #ifdef Py_DEBUG
@@ -247,7 +254,9 @@ float_dealloc(PyFloatObject *op)
         Py_SET_TYPE(op, (PyTypeObject *)state->free_list);
         state->free_list = op;
     }
-    else {
+    else
+#endif
+    {
         Py_TYPE(op)->tp_free((PyObject *)op);
     }
 }
@@ -2036,6 +2045,7 @@ _PyFloat_InitTypes(void)
 void
 _PyFloat_ClearFreeList(PyInterpreterState *interp)
 {
+#if PyFloat_MAXFREELIST > 0
     struct _Py_float_state *state = &interp->float_state;
     PyFloatObject *f = state->free_list;
     while (f != NULL) {
@@ -2045,13 +2055,14 @@ _PyFloat_ClearFreeList(PyInterpreterState *interp)
     }
     state->free_list = NULL;
     state->numfree = 0;
+#endif
 }
 
 void
 _PyFloat_Fini(PyInterpreterState *interp)
 {
     _PyFloat_ClearFreeList(interp);
-#ifdef Py_DEBUG
+#if defined(Py_DEBUG) && PyFloat_MAXFREELIST > 0
     struct _Py_float_state *state = &interp->float_state;
     state->numfree = -1;
 #endif
@@ -2061,10 +2072,12 @@ _PyFloat_Fini(PyInterpreterState *interp)
 void
 _PyFloat_DebugMallocStats(FILE *out)
 {
+#if PyFloat_MAXFREELIST > 0
     struct _Py_float_state *state = get_float_state();
     _PyDebugAllocatorStats(out,
                            "free PyFloatObject",
                            state->numfree, sizeof(PyFloatObject));
+#endif
 }
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		The internal freelists for frame, float, list, dict, async generators, and
		context objects can now be disabled.