GH-126491: GC: Mark objects reachable from roots before doing cycle collection (GH-126502)

* Mark almost all reachable objects before doing collection phase

* Add stats for objects marked

* Visit new frames before each increment

* Remove lazy dict tracking

* Update docs

* Clearer calculation of work to do.

Author: markshannon

Include/cpython/pystats.h

@@ -99,6 +99,8 @@ typedef struct _gc_stats {
     uint64_t collections;
     uint64_t object_visits;
     uint64_t objects_collected;
+    uint64_t objects_transitively_reachable;
+    uint64_t objects_not_transitively_reachable;
 } GCStats;
 
 typedef struct _uop_stats {

Include/internal/pycore_dict.h

@@ -43,8 +43,6 @@ extern int _PyDict_Next(
 
 extern int _PyDict_HasOnlyStringKeys(PyObject *mp);
 
-extern void _PyDict_MaybeUntrack(PyObject *mp);
-
 // Export for '_ctypes' shared extension
 PyAPI_FUNC(Py_ssize_t) _PyDict_SizeOf(PyDictObject *);
 

Include/internal/pycore_frame.h

@@ -75,6 +75,7 @@ typedef struct _PyInterpreterFrame {
     _PyStackRef *stackpointer;
     uint16_t return_offset;  /* Only relevant during a function call */
     char owner;
+    char visited;
     /* Locals and stack */
     _PyStackRef localsplus[1];
 } _PyInterpreterFrame;
@@ -207,6 +208,7 @@ _PyFrame_Initialize(
 #endif
     frame->return_offset = 0;
     frame->owner = FRAME_OWNED_BY_THREAD;
+    frame->visited = 0;
 
     for (int i = null_locals_from; i < code->co_nlocalsplus; i++) {
         frame->localsplus[i] = PyStackRef_NULL;
@@ -389,6 +391,7 @@ _PyFrame_PushTrampolineUnchecked(PyThreadState *tstate, PyCodeObject *code, int
     frame->instr_ptr = _PyCode_CODE(code);
 #endif
     frame->owner = FRAME_OWNED_BY_THREAD;
+    frame->visited = 0;
     frame->return_offset = 0;
 
 #ifdef Py_GIL_DISABLED

Include/internal/pycore_gc.h

@@ -10,11 +10,11 @@ extern "C" {
 
 /* GC information is stored BEFORE the object structure. */
 typedef struct {
-    // Pointer to next object in the list.
+    // Tagged pointer to next object in the list.
     // 0 means the object is not tracked
     uintptr_t _gc_next;
 
-    // Pointer to previous object in the list.
+    // Tagged pointer to previous object in the list.
     // Lowest two bits are used for flags documented later.
     uintptr_t _gc_prev;
 } PyGC_Head;
@@ -302,6 +302,11 @@ struct gc_generation_stats {
     Py_ssize_t uncollectable;
 };
 
+enum _GCPhase {
+    GC_PHASE_MARK = 0,
+    GC_PHASE_COLLECT = 1
+};
+
 struct _gc_runtime_state {
     /* List of objects that still need to be cleaned up, singly linked
      * via their gc headers' gc_prev pointers.  */
@@ -325,10 +330,12 @@ struct _gc_runtime_state {
     /* a list of callbacks to be invoked when collection is performed */
     PyObject *callbacks;
 
+    Py_ssize_t prior_heap_size;
     Py_ssize_t heap_size;
     Py_ssize_t work_to_do;
     /* Which of the old spaces is the visited space */
     int visited_space;
+    int phase;
 
 #ifdef Py_GIL_DISABLED
     /* This is the number of objects that survived the last full

Include/internal/pycore_object.h

@@ -466,8 +466,8 @@ static inline void _PyObject_GC_TRACK(
     PyGC_Head *last = (PyGC_Head*)(generation0->_gc_prev);
     _PyGCHead_SET_NEXT(last, gc);
     _PyGCHead_SET_PREV(gc, last);
-    /* Young objects will be moved into the visited space during GC, so set the bit here */
-    gc->_gc_next = ((uintptr_t)generation0) | (uintptr_t)interp->gc.visited_space;
+    uintptr_t not_visited = 1 ^ interp->gc.visited_space;
+    gc->_gc_next = ((uintptr_t)generation0) | not_visited;
     generation0->_gc_prev = (uintptr_t)gc;
 #endif
 }

Include/internal/pycore_runtime_init.h

@@ -134,6 +134,7 @@ extern PyTypeObject _PyExc_MemoryError;
                 { .threshold = 0, }, \
             }, \
             .work_to_do = -5000, \
+            .phase = GC_PHASE_MARK, \
         }, \
         .qsbr = { \
             .wr_seq = QSBR_INITIAL, \

InternalDocs/garbage_collector.md

@@ -351,6 +351,7 @@ follows these steps in order:
    the reference counts fall to 0, triggering the destruction of all unreachable
    objects.
 
+
 Optimization: incremental collection
 ====================================
 
@@ -484,6 +485,46 @@ specifically in a generation by calling `gc.collect(generation=NUM)`.
 ```
 
 
+Optimization: visiting reachable objects
+========================================
+
+An object cannot be garbage if it can be reached.
+
+To avoid having to identify reference cycles across the whole heap, we can
+reduce the amount of work done considerably by first moving most reachable objects
+to the `visited` space. Empirically, most reachable objects can be reached from a
+small set of global objects and local variables.
+This step does much less work per object, so reduces the time spent
+performing garbage collection by at least half.
+
+> [!NOTE]
+> Objects that are not determined to be reachable by this pass are not necessarily
+> unreachable. We still need to perform the main algorithm to determine which objects
+> are actually unreachable.
+
+We use the same technique of forming a transitive closure as the incremental
+collector does to find reachable objects, seeding the list with some global
+objects and the currently executing frames.
+
+This phase moves objects to the `visited` space, as follows:
+
+1. All objects directly referred to by any builtin class, the `sys` module, the `builtins`
+module and all objects directly referred to from stack frames are added to a working
+set of reachable objects.
+2. Until this working set is empty:
+   1. Pop an object from the set and move it to the `visited` space
+   2. For each object directly reachable from that object:
+      * If it is not already in `visited` space and it is a GC object,
+        add it to the working set
+
+
+Before each increment of collection is performed, the stacks are scanned
+to check for any new stack frames that have been created since the last
+increment. All objects directly referred to from those stack frames are
+added to the working set.
+Then the above algorithm is repeated, starting from step 2.
+
+
 Optimization: reusing fields to save memory
 ===========================================
 
@@ -532,8 +573,8 @@ of `PyGC_Head` discussed in the `Memory layout and object structure`_ section:
   currently in.  Instead, when that's needed, ad hoc tricks (like the
   `NEXT_MASK_UNREACHABLE` flag) are employed.
 
-Optimization: delay tracking containers
-=======================================
+Optimization: delayed untracking of containers
+==============================================
 
 Certain types of containers cannot participate in a reference cycle, and so do
 not need to be tracked by the garbage collector. Untracking these objects
@@ -548,8 +589,8 @@ a container:
 As a general rule, instances of atomic types aren't tracked and instances of
 non-atomic types (containers, user-defined objects...) are.  However, some
 type-specific optimizations can be present in order to suppress the garbage
-collector footprint of simple instances. Some examples of native types that
-benefit from delayed tracking:
+collector footprint of simple instances. Historically, both dictionaries and
+tuples were untracked during garbage collection. Now it is only tuples:
 
 - Tuples containing only immutable objects (integers, strings etc,
   and recursively, tuples of immutable objects) do not need to be tracked. The
@@ -558,14 +599,8 @@ benefit from delayed tracking:
   tuples at creation time. Instead, all tuples except the empty tuple are tracked
   when created. During garbage collection it is determined whether any surviving
   tuples can be untracked. A tuple can be untracked if all of its contents are
-  already not tracked. Tuples are examined for untracking in all garbage collection
-  cycles. It may take more than one cycle to untrack a tuple.
-
-- Dictionaries containing only immutable objects also do not need to be tracked.
-  Dictionaries are untracked when created. If a tracked item is inserted into a
-  dictionary (either as a key or value), the dictionary becomes tracked. During a
-  full garbage collection (all generations), the collector will untrack any dictionaries
-  whose contents are not tracked.
+  already not tracked. Tuples are examined for untracking when moved from the
+  young to the old generation.
 
 The garbage collector module provides the Python function `is_tracked(obj)`, which returns
 the current tracking status of the object. Subsequent garbage collections may change the
@@ -578,11 +613,9 @@ tracking status of the object.
       False
       >>> gc.is_tracked([])
       True
-      >>> gc.is_tracked({})
+      >>> gc.is_tracked(("a", 1))
       False
       >>> gc.is_tracked({"a": 1})
-      False
-      >>> gc.is_tracked({"a": []})
       True
 ```
 

Lib/test/test_dict.py

@@ -880,115 +880,6 @@ class C(object):
             gc.collect()
             self.assertIs(ref(), None, "Cycle was not collected")
 
-    def _not_tracked(self, t):
-        # Nested containers can take several collections to untrack
-        gc.collect()
-        gc.collect()
-        self.assertFalse(gc.is_tracked(t), t)
-
-    def _tracked(self, t):
-        self.assertTrue(gc.is_tracked(t), t)
-        gc.collect()
-        gc.collect()
-        self.assertTrue(gc.is_tracked(t), t)
-
-    def test_string_keys_can_track_values(self):
-        # Test that this doesn't leak.
-        for i in range(10):
-            d = {}
-            for j in range(10):
-                d[str(j)] = j
-            d["foo"] = d
-
-    @support.cpython_only
-    def test_track_literals(self):
-        # Test GC-optimization of dict literals
-        x, y, z, w = 1.5, "a", (1, None), []
-
-        self._not_tracked({})
-        self._not_tracked({x:(), y:x, z:1})
-        self._not_tracked({1: "a", "b": 2})
-        self._not_tracked({1: 2, (None, True, False, ()): int})
-        self._not_tracked({1: object()})
-
-        # Dicts with mutable elements are always tracked, even if those
-        # elements are not tracked right now.
-        self._tracked({1: []})
-        self._tracked({1: ([],)})
-        self._tracked({1: {}})
-        self._tracked({1: set()})
-
-    @support.cpython_only
-    def test_track_dynamic(self):
-        # Test GC-optimization of dynamically-created dicts
-        class MyObject(object):
-            pass
-        x, y, z, w, o = 1.5, "a", (1, object()), [], MyObject()
-
-        d = dict()
-        self._not_tracked(d)
-        d[1] = "a"
-        self._not_tracked(d)
-        d[y] = 2
-        self._not_tracked(d)
-        d[z] = 3
-        self._not_tracked(d)
-        self._not_tracked(d.copy())
-        d[4] = w
-        self._tracked(d)
-        self._tracked(d.copy())
-        d[4] = None
-        self._not_tracked(d)
-        self._not_tracked(d.copy())
-
-        # dd isn't tracked right now, but it may mutate and therefore d
-        # which contains it must be tracked.
-        d = dict()
-        dd = dict()
-        d[1] = dd
-        self._not_tracked(dd)
-        self._tracked(d)
-        dd[1] = d
-        self._tracked(dd)
-
-        d = dict.fromkeys([x, y, z])
-        self._not_tracked(d)
-        dd = dict()
-        dd.update(d)
-        self._not_tracked(dd)
-        d = dict.fromkeys([x, y, z, o])
-        self._tracked(d)
-        dd = dict()
-        dd.update(d)
-        self._tracked(dd)
-
-        d = dict(x=x, y=y, z=z)
-        self._not_tracked(d)
-        d = dict(x=x, y=y, z=z, w=w)
-        self._tracked(d)
-        d = dict()
-        d.update(x=x, y=y, z=z)
-        self._not_tracked(d)
-        d.update(w=w)
-        self._tracked(d)
-
-        d = dict([(x, y), (z, 1)])
-        self._not_tracked(d)
-        d = dict([(x, y), (z, w)])
-        self._tracked(d)
-        d = dict()
-        d.update([(x, y), (z, 1)])
-        self._not_tracked(d)
-        d.update([(x, y), (z, w)])
-        self._tracked(d)
-
-    @support.cpython_only
-    def test_track_subtypes(self):
-        # Dict subtypes are always tracked
-        class MyDict(dict):
-            pass
-        self._tracked(MyDict())
-
     def make_shared_key_dict(self, n):
         class C:
             pass