Region GC

Include/internal/pycore_cown.h

@@ -22,14 +22,14 @@ typedef uint64_t _PyCown_ipid_t;
 typedef uint64_t _PyCown_thread_id_t;
 
 //PyAPI_FUNC(PyObject*) _PyCown_New();
-// PyAPI_FUNC(int) _PyCown_SetValue(_PyCownObject* self, PyObject* value);
+PyAPI_FUNC(PyObject*) _PyCown_GetValue(_PyCownObject* self);
+PyAPI_FUNC(int) _PyCown_SetValue(_PyCownObject* self, PyObject* value);
 PyAPI_FUNC(_PyCown_ipid_t) _PyCown_ThisInterpreterId(void);
 PyAPI_FUNC(_PyCown_thread_id_t) _PyCown_ThisThreadId(void);
 PyAPI_FUNC(int) _PyCown_RegionOpen(_PyCownObject *self, _PyRegionObject* region, _PyCown_ipid_t ip);
-PyAPI_FUNC(int) _PyCown_AcquireGC(_PyCownObject *self, Py_region_t *region);
+PyAPI_FUNC(void) _PyCown_SetCollecting(_PyCownObject *self, int value);
 PyAPI_FUNC(int) _PyCown_SwitchFromGcToIp(_PyCownObject *self);
-PyAPI_FUNC(int) _PyCown_SwitchFromIpToGc(_PyCownObject *self, Py_region_t *contained_region);
-PyAPI_FUNC(int) _PyCown_ReleaseGC(_PyCownObject *self);
+PyAPI_FUNC(int) _PyCown_SwitchFromIpToGc(_PyCownObject *self);
 
 
 #ifdef __cplusplus

Include/internal/pycore_gc.h

@@ -118,20 +118,18 @@ static inline void _PyObject_GC_SET_SHARED(PyObject *op) {
 /* Bit 1 is set when the object is in generation which is GCed currently. */
 #define _PyGC_PREV_MASK_COLLECTING ((uintptr_t)2)
 
-/* Bit 0 in _gc_next is the old space bit.
+/* Bit flags for _gc_next */
+/* Bit 0 is the old space bit.
+ * It describes the generation space the object is in.
  * It is set as follows:
  * Young: gcstate->visited_space
  * old[0]: 0
  * old[1]: 1
  * permanent: 0
- *
- * During a collection all objects handled should have the bit set to
- * gcstate->visited_space, as objects are moved from the young gen
- * and the increment into old[gcstate->visited_space].
- * When object are moved from the pending space, old[gcstate->visited_space^1]
- * into the increment, the old space bit is flipped.
 */
-#define _PyGC_NEXT_MASK_OLD_SPACE_1    1
+#define _PyGC_NEXT_MASK_OLD_SPACE_1 ((uintptr_t)1)
+/* Bit 1 is set when the object is in the unreachable list. */
+#define _PyGC_NEXT_MASK_UNREACHABLE ((uintptr_t)2)
 
 #define _PyGC_PREV_SHIFT           2
 #define _PyGC_PREV_MASK            (((uintptr_t) -1) << _PyGC_PREV_SHIFT)
@@ -340,6 +338,9 @@ extern void _PyGC_InitState(struct _gc_runtime_state *);
 
 extern Py_ssize_t _PyGC_Collect(PyThreadState *tstate, int generation, _PyGC_Reason reason);
 extern void _PyGC_CollectNoFail(PyThreadState *tstate);
+extern Py_ssize_t _PyGC_CollectRegion(PyThreadState *tstate, PyObject *region, _PyGC_Reason reason);
+extern void _PyGC_IncreaseRegionBudget(PyThreadState *tstate);
+extern bool _PyGC_CanRunRegionGC(PyThreadState *tstate);
 
 /* Freeze objects tracked by the GC and ignore them in future collections. */
 extern void _PyGC_Freeze(PyInterpreterState *interp);

Include/internal/pycore_global_strings.h

@@ -712,6 +712,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(readonly)
         STRUCT_FOR_ID(real)
         STRUCT_FOR_ID(reducer_override)
+        STRUCT_FOR_ID(region)
         STRUCT_FOR_ID(registry)
         STRUCT_FOR_ID(rel_tol)
         STRUCT_FOR_ID(release)

Include/internal/pycore_interp_structs.h

@@ -227,6 +227,9 @@ struct _gc_runtime_state {
     int visited_space;
     int phase;
 
+    /* How many objects in regions can be collected within this cycle */
+    Py_ssize_t region_budget;
+
 #ifdef Py_GIL_DISABLED
     /* This is the number of objects that survived the last full
        collection. It approximates the number of long lived objects

Include/internal/pycore_region.h

@@ -98,6 +98,9 @@ typedef struct _Py_region_data {
      */
     PyGC_Head gc_list;
 
+    /* List of unreachable objects in the region, saved to be deleted later. */
+    PyGC_Head unreachable;
+
 #ifdef Py_OWNERSHIP_INVARIANT
     _Py_ownership_invariant_region_data invariant_data;
 #endif
@@ -133,6 +136,7 @@ static inline Py_region_t __PyRegion_Get(PyObject *obj, int follow_pending) {
 
 PyAPI_FUNC(int) _PyRegion_New(_PyRegionObject *bridge);
 PyAPI_FUNC(int) _PyRegion_Dissolve(Py_region_t region);
+PyAPI_FUNC(void) _PyRegion_IncRc(Py_region_t region);
 PyAPI_FUNC(void) _PyRegion_DecRc(Py_region_t region);
 
 PyAPI_FUNC(Py_ssize_t) _PyRegion_GetLrc(Py_region_t region);

Include/internal/pycore_regionobject.h

@@ -22,6 +22,8 @@ struct _PyRegionObject {
     /** The name of the region or NULL */
     PyObject *name;
     PyObject *dict;
+    /* A link in a list of regions to be garbage collected. */
+    struct _PyRegionObject *next;
 };
 #define _PyRegionObject_CAST(op) _Py_CAST(_PyRegionObject*, op)
 
@@ -30,4 +32,4 @@ PyAPI_DATA(PyTypeObject) _PyRegion_Type;
 #ifdef __cplusplus
 }
 #endif
-#endif /* !Py_INTERNAL_REGIONOBJECT_H */
+#endif /* !Py_INTERNAL_REGIONOBJECT_H */

InternalDocs/garbage_collector.md

@@ -199,22 +199,22 @@ unreachable:
 
 ```pycon
 >>> import gc
->>> 
+>>>
 >>> class Link:
 ...    def __init__(self, next_link=None):
 ...        self.next_link = next_link
-...  
+...
 >>> link_3 = Link()
 >>> link_2 = Link(link_3)
 >>> link_1 = Link(link_2)
 >>> link_3.next_link = link_1
 >>> A = link_1
 >>> del link_1, link_2, link_3
->>> 
+>>>
 >>> link_4 = Link()
 >>> link_4.next_link = link_4
 >>> del link_4
->>> 
+>>>
 >>> # Collect the unreachable Link object (and its .__dict__ dict).
 >>> gc.collect()
 2
@@ -278,7 +278,7 @@ state in the previous image and after examining the objects referred to by `link
 the GC knows that `link_3` is reachable after all, so it is moved back to the
 original list and its `gc_ref` field is set to 1 so that if the GC visits it again,
 it will know that it's reachable. To avoid visiting an object twice, the GC marks all
-objects that have already been visited once (by unsetting the `PREV_MASK_COLLECTING`
+objects that have already been visited once (by unsetting the `_PyGC_PREV_MASK_COLLECTING`
 flag) so that if an object that has already been processed is referenced by some other
 object, the GC does not process it twice.
 
@@ -465,11 +465,11 @@ specifically in a generation by calling `gc.collect(generation=NUM)`.
 >>> # Create a reference cycle.
 >>> x = MyObj()
 >>> x.self = x
->>> 
+>>>
 >>> # Initially the object is in the young generation.
 >>> gc.get_objects(generation=0)
 [..., <__main__.MyObj object at 0x7fbcc12a3400>, ...]
->>> 
+>>>
 >>> # After a collection of the youngest generation the object
 >>> # moves to the old generation.
 >>> gc.collect(generation=0)
@@ -725,21 +725,27 @@ of `PyGC_Head` discussed in the `Memory layout and object structure`_ section:
 
 - The `_gc_prev` field is normally used as the "previous" pointer to maintain the
   doubly linked list but its lowest two bits are used to keep the flags
-  `PREV_MASK_COLLECTING` and `_PyGC_PREV_MASK_FINALIZED`. Between collections,
+  `_PyGC_PREV_MASK_COLLECTING` and `_PyGC_PREV_MASK_FINALIZED`. Between collections,
   the only flag that can be present is `_PyGC_PREV_MASK_FINALIZED` that indicates
   if an object has been already finalized. During collections `_gc_prev` is
   temporarily used for storing a copy of the reference count (`gc_ref`), in
   addition to two flags, and the GC linked list becomes a singly linked list until
   `_gc_prev` is restored.
 
-- The `_gc_next` field is used as the "next" pointer to maintain the doubly linked
-  list but during collection its lowest bit is used to keep the
-  `NEXT_MASK_UNREACHABLE` flag that indicates if an object is tentatively
+- The `_gc_next` field is normally used as the "next" pointer to maintain the
+  doubly linked list but its lowest two bits are used to keep the flags
+  `_PyGC_NEXT_MASK_OLD_SPACE_1` and `_PyGC_NEXT_MASK_UNREACHABLE`.
+  During collection, the `_PyGC_NEXT_MASK_UNREACHABLE` flag indicates if an object is tentatively
   unreachable during the cycle detection algorithm.  This is a drawback to using only
   doubly linked lists to implement partitions:  while most needed operations are
   constant-time, there is no efficient way to determine which partition an object is
   currently in.  Instead, when that's needed, ad hoc tricks (like the
-  `NEXT_MASK_UNREACHABLE` flag) are employed.
+  `_PyGC_NEXT_MASK_UNREACHABLE` flag) are employed.
+  The `_PyGC_NEXT_MASK_OLD_SPACE_1` flag
+  indicates whether the object belongs to the pending space or the
+  visited space. The objects in the pending space are yet to be processed
+  during future incremental collections. Which space is which is determined
+  by gcstate->visited_space.
 
 Optimization: delayed untracking containers
 ===========================================