GH-108362: Incremental Cycle GC (GH-116206)

Author: markshannon

Doc/whatsnew/3.13.rst

@@ -111,6 +111,14 @@ Improved Error Messages
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
     TypeError: split() got an unexpected keyword argument 'max_split'. Did you mean 'maxsplit'?
 
+Incremental Garbage Collection
+------------------------------
+
+* The cycle garbage collector is now incremental.
+  This means that maximum pause times are reduced
+  by an order of magnitude or more for larger heaps.
+
+
 Other Language Changes
 ======================
 
@@ -350,6 +358,28 @@ fractions
   sign handling, minimum width and grouping. (Contributed by Mark Dickinson
   in :gh:`111320`.)
 
+gc
+--
+
+* The cyclic garbage collector is now incremental, which changes the meanings
+  of the results of :meth:`gc.get_threshold` and :meth:`gc.get_threshold` as
+  well as :meth:`gc.get_count` and :meth:`gc.get_stats`.
+* :meth:`gc.get_threshold` returns a three-tuple for backwards compatibility,
+  the first value is the threshold for young collections, as before, the second
+  value determines the rate at which the old collection is scanned; the
+  default is 10 and higher values mean that the old collection is scanned more slowly.
+  The third value is meangless and is always zero.
+* :meth:`gc.set_threshold` ignores any items after the second.
+* :meth:`gc.get_count` and :meth:`gc.get_stats`.
+  These functions return the same format of results as before.
+  The only difference is that instead of the results refering to
+  the young, aging and old generations, the results refer to the
+  young generation and the aging and collecting spaces of the old generation.
+
+In summary, code that attempted to manipulate the behavior of the cycle GC may
+not work exactly as intended, but it is very unlikely to harmful.
+All other code will work just fine.
+
 glob
 ----
 

Include/internal/pycore_gc.h

@@ -109,11 +109,14 @@ static inline void _PyObject_GC_SET_SHARED_INLINE(PyObject *op) {
 
 /* Bit flags for _gc_prev */
 /* Bit 0 is set when tp_finalize is called */
-#define _PyGC_PREV_MASK_FINALIZED  (1)
+#define _PyGC_PREV_MASK_FINALIZED  1
 /* Bit 1 is set when the object is in generation which is GCed currently. */
-#define _PyGC_PREV_MASK_COLLECTING (2)
-/* The (N-2) most significant bits contain the real address. */
-#define _PyGC_PREV_SHIFT           (2)
+#define _PyGC_PREV_MASK_COLLECTING 2
+
+/* Bit 0 is set if the object belongs to old space 1 */
+#define _PyGC_NEXT_MASK_OLD_SPACE_1    1
+
+#define _PyGC_PREV_SHIFT           2
 #define _PyGC_PREV_MASK            (((uintptr_t) -1) << _PyGC_PREV_SHIFT)
 
 /* set for debugging information */
@@ -139,18 +142,21 @@ typedef enum {
 // Lowest bit of _gc_next is used for flags only in GC.
 // But it is always 0 for normal code.
 static inline PyGC_Head* _PyGCHead_NEXT(PyGC_Head *gc) {
-    uintptr_t next = gc->_gc_next;
+    uintptr_t next = gc->_gc_next & _PyGC_PREV_MASK;
     return (PyGC_Head*)next;
 }
 static inline void _PyGCHead_SET_NEXT(PyGC_Head *gc, PyGC_Head *next) {
-    gc->_gc_next = (uintptr_t)next;
+    uintptr_t unext = (uintptr_t)next;
+    assert((unext & ~_PyGC_PREV_MASK) == 0);
+    gc->_gc_next = (gc->_gc_next & ~_PyGC_PREV_MASK) | unext;
 }
 
 // Lowest two bits of _gc_prev is used for _PyGC_PREV_MASK_* flags.
 static inline PyGC_Head* _PyGCHead_PREV(PyGC_Head *gc) {
     uintptr_t prev = (gc->_gc_prev & _PyGC_PREV_MASK);
     return (PyGC_Head*)prev;
 }
+
 static inline void _PyGCHead_SET_PREV(PyGC_Head *gc, PyGC_Head *prev) {
     uintptr_t uprev = (uintptr_t)prev;
     assert((uprev & ~_PyGC_PREV_MASK) == 0);
@@ -236,6 +242,13 @@ struct gc_generation {
                   generations */
 };
 
+struct gc_collection_stats {
+    /* number of collected objects */
+    Py_ssize_t collected;
+    /* total number of uncollectable objects (put into gc.garbage) */
+    Py_ssize_t uncollectable;
+};
+
 /* Running stats per generation */
 struct gc_generation_stats {
     /* total number of collections */
@@ -257,8 +270,8 @@ struct _gc_runtime_state {
     int enabled;
     int debug;
     /* linked lists of container objects */
-    struct gc_generation generations[NUM_GENERATIONS];
-    PyGC_Head *generation0;
+    struct gc_generation young;
+    struct gc_generation old[2];
     /* a permanent generation which won't be collected */
     struct gc_generation permanent_generation;
     struct gc_generation_stats generation_stats[NUM_GENERATIONS];
@@ -268,6 +281,12 @@ struct _gc_runtime_state {
     PyObject *garbage;
     /* a list of callbacks to be invoked when collection is performed */
     PyObject *callbacks;
+
+    Py_ssize_t work_to_do;
+    /* Which of the old spaces is the visited space */
+    int visited_space;
+
+#ifdef Py_GIL_DISABLED
     /* This is the number of objects that survived the last full
        collection. It approximates the number of long lived objects
        tracked by the GC.
@@ -279,6 +298,7 @@ struct _gc_runtime_state {
        collections, and are awaiting to undergo a full collection for
        the first time. */
     Py_ssize_t long_lived_pending;
+#endif
 };
 
 #ifdef Py_GIL_DISABLED
@@ -291,9 +311,8 @@ struct _gc_thread_state {
 
 extern void _PyGC_InitState(struct _gc_runtime_state *);
 
-extern Py_ssize_t _PyGC_Collect(PyThreadState *tstate, int generation,
-                                _PyGC_Reason reason);
-extern Py_ssize_t _PyGC_CollectNoFail(PyThreadState *tstate);
+extern Py_ssize_t _PyGC_Collect(PyThreadState *tstate, int generation, _PyGC_Reason reason);
+extern void _PyGC_CollectNoFail(PyThreadState *tstate);
 
 /* Freeze objects tracked by the GC and ignore them in future collections. */
 extern void _PyGC_Freeze(PyInterpreterState *interp);

Include/internal/pycore_object.h

@@ -125,19 +125,8 @@ static inline void _Py_RefcntAdd(PyObject* op, Py_ssize_t n)
 }
 #define _Py_RefcntAdd(op, n) _Py_RefcntAdd(_PyObject_CAST(op), n)
 
-static inline void _Py_SetImmortal(PyObject *op)
-{
-    if (op) {
-#ifdef Py_GIL_DISABLED
-        op->ob_tid = _Py_UNOWNED_TID;
-        op->ob_ref_local = _Py_IMMORTAL_REFCNT_LOCAL;
-        op->ob_ref_shared = 0;
-#else
-        op->ob_refcnt = _Py_IMMORTAL_REFCNT;
-#endif
-    }
-}
-#define _Py_SetImmortal(op) _Py_SetImmortal(_PyObject_CAST(op))
+extern void _Py_SetImmortal(PyObject *op);
+extern void _Py_SetImmortalUntracked(PyObject *op);
 
 // Makes an immortal object mortal again with the specified refcnt. Should only
 // be used during runtime finalization.
@@ -325,11 +314,12 @@ static inline void _PyObject_GC_TRACK(
                           filename, lineno, __func__);
 
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    PyGC_Head *generation0 = interp->gc.generation0;
+    PyGC_Head *generation0 = &interp->gc.young.head;
     PyGC_Head *last = (PyGC_Head*)(generation0->_gc_prev);
     _PyGCHead_SET_NEXT(last, gc);
     _PyGCHead_SET_PREV(gc, last);
     _PyGCHead_SET_NEXT(gc, generation0);
+    assert((gc->_gc_next & _PyGC_NEXT_MASK_OLD_SPACE_1) == 0);
     generation0->_gc_prev = (uintptr_t)gc;
 #endif
 }

Include/internal/pycore_runtime_init.h

@@ -168,12 +168,12 @@ extern PyTypeObject _PyExc_MemoryError;
         }, \
         .gc = { \
             .enabled = 1, \
-            .generations = { \
-                /* .head is set in _PyGC_InitState(). */ \
-                { .threshold = 700, }, \
-                { .threshold = 10, }, \
+            .young = { .threshold = 2000, }, \
+            .old = { \
                 { .threshold = 10, }, \
+                { .threshold = 0, }, \
             }, \
+            .work_to_do = -5000, \
         }, \
         .qsbr = { \
             .wr_seq = QSBR_INITIAL, \

Lib/test/test_gc.py

@@ -384,19 +384,11 @@ def test_collect_generations(self):
         # each call to collect(N)
         x = []
         gc.collect(0)
-        # x is now in gen 1
+        # x is now in the old gen
         a, b, c = gc.get_count()
-        gc.collect(1)
-        # x is now in gen 2
-        d, e, f = gc.get_count()
-        gc.collect(2)
-        # x is now in gen 3
-        g, h, i = gc.get_count()
-        # We don't check a, d, g since their exact values depends on
+        # We don't check a since its exact values depends on
         # internal implementation details of the interpreter.
         self.assertEqual((b, c), (1, 0))
-        self.assertEqual((e, f), (0, 1))
-        self.assertEqual((h, i), (0, 0))
 
     def test_trashcan(self):
         class Ouch:
@@ -847,16 +839,6 @@ def test_get_objects_generations(self):
         self.assertFalse(
                 any(l is element for element in gc.get_objects(generation=2))
         )
-        gc.collect(generation=1)
-        self.assertFalse(
-                any(l is element for element in gc.get_objects(generation=0))
-        )
-        self.assertFalse(
-                any(l is element for element in  gc.get_objects(generation=1))
-        )
-        self.assertTrue(
-                any(l is element for element in gc.get_objects(generation=2))
-        )
         gc.collect(generation=2)
         self.assertFalse(
                 any(l is element for element in gc.get_objects(generation=0))
@@ -1076,6 +1058,56 @@ class Z:
         callback.assert_not_called()
         gc.enable()
 
+    @unittest.skipIf(Py_GIL_DISABLED, "Free threading does not support incremental GC")
+    def test_incremental_gc_handles_fast_cycle_creation(self):
+
+        class LinkedList:
+
+            #Use slots to reduce number of implicit objects
+            __slots__ = "next", "prev", "surprise"
+
+            def __init__(self, next=None, prev=None):
+                self.next = next
+                if next is not None:
+                    next.prev = self
+                self.prev = prev
+                if prev is not None:
+                    prev.next = self
+
+        def make_ll(depth):
+            head = LinkedList()
+            for i in range(depth):
+                head = LinkedList(head, head.prev)
+            return head
+
+        head = make_ll(10000)
+        count = 10000
+
+        # We expect the counts to go negative eventually
+        # as there will some objects we aren't counting,
+        # e.g. the gc stats dicts. The test merely checks
+        # that the counts don't grow.
+
+        enabled = gc.isenabled()
+        gc.enable()
+        olds = []
+        for i in range(1000):
+            newhead = make_ll(200)
+            count += 200
+            newhead.surprise = head
+            olds.append(newhead)
+            if len(olds) == 50:
+                stats = gc.get_stats()
+                young = stats[0]
+                incremental = stats[1]
+                old = stats[2]
+                collected = young['collected'] + incremental['collected'] + old['collected']
+                live = count - collected
+                self.assertLess(live, 25000)
+                del olds[:]
+        if not enabled:
+            gc.disable()
+
 
 class GCCallbackTests(unittest.TestCase):
     def setUp(self):

Misc/NEWS.d/next/Core and Builtins/2024-01-07-04-22-51.gh-issue-108362.oB9Gcf.rst

@@ -0,0 +1,12 @@
+Implement an incremental cyclic garbage collector. By collecting the old
+generation in increments, there is no need for a full heap scan. This can
+hugely reduce maximum pause time for programs with large heaps.
+
+Reduce the number of generations from three to two. The old generation is
+split into two spaces, "visited" and "pending".
+
+Collection happens in two steps::
+* An increment is formed from the young generation and a small part of the pending space.
+* This increment is scanned and the survivors moved to the end of the visited space.
+
+When the collecting space becomes empty, the two spaces are swapped.

Modules/gcmodule.c

@@ -158,17 +158,12 @@ gc_set_threshold_impl(PyObject *module, int threshold0, int group_right_1,
 {
     GCState *gcstate = get_gc_state();
 
-    gcstate->generations[0].threshold = threshold0;
+    gcstate->young.threshold = threshold0;
     if (group_right_1) {
-        gcstate->generations[1].threshold = threshold1;
+        gcstate->old[0].threshold = threshold1;
     }
     if (group_right_2) {
-        gcstate->generations[2].threshold = threshold2;
-
-        /* generations higher than 2 get the same threshold */
-        for (int i = 3; i < NUM_GENERATIONS; i++) {
-            gcstate->generations[i].threshold = gcstate->generations[2].threshold;
-        }
+        gcstate->old[1].threshold = threshold2;
     }
     Py_RETURN_NONE;
 }
@@ -185,9 +180,9 @@ gc_get_threshold_impl(PyObject *module)
 {
     GCState *gcstate = get_gc_state();
     return Py_BuildValue("(iii)",
-                         gcstate->generations[0].threshold,
-                         gcstate->generations[1].threshold,
-                         gcstate->generations[2].threshold);
+                         gcstate->young.threshold,
+                         gcstate->old[0].threshold,
+                         0);
 }
 
 /*[clinic input]
@@ -207,14 +202,14 @@ gc_get_count_impl(PyObject *module)
     struct _gc_thread_state *gc = &tstate->gc;
 
     // Flush the local allocation count to the global count
-    _Py_atomic_add_int(&gcstate->generations[0].count, (int)gc->alloc_count);
+    _Py_atomic_add_int(&gcstate->young.count, (int)gc->alloc_count);
     gc->alloc_count = 0;
 #endif
 
     return Py_BuildValue("(iii)",
-                         gcstate->generations[0].count,
-                         gcstate->generations[1].count,
-                         gcstate->generations[2].count);
+                         gcstate->young.count,
+                         gcstate->old[gcstate->visited_space].count,
+                         gcstate->old[gcstate->visited_space^1].count);
 }
 
 /*[clinic input]

Objects/object.c

@@ -2401,6 +2401,27 @@ _Py_NewReferenceNoTotal(PyObject *op)
     new_reference(op);
 }
 
+void
+_Py_SetImmortalUntracked(PyObject *op)
+{
+#ifdef Py_GIL_DISABLED
+    op->ob_tid = _Py_UNOWNED_TID;
+    op->ob_ref_local = _Py_IMMORTAL_REFCNT_LOCAL;
+    op->ob_ref_shared = 0;
+#else
+    op->ob_refcnt = _Py_IMMORTAL_REFCNT;
+#endif
+}
+
+void
+_Py_SetImmortal(PyObject *op)
+{
+    if (PyObject_IS_GC(op) && _PyObject_GC_IS_TRACKED(op)) {
+        _PyObject_GC_UNTRACK(op);
+    }
+    _Py_SetImmortalUntracked(op);
+}
+
 void
 _Py_ResurrectReference(PyObject *op)
 {