GH-117108: Change the size of the GC increment to about 1% of the total heap size.

Include/internal/pycore_gc.h

@@ -282,6 +282,7 @@ struct _gc_runtime_state {
  /* a list of callbacks to be invoked when collection is performed */
  PyObject *callbacks;
 
+ Py_ssize_t heap_size;
  Py_ssize_t work_to_do;
  /* Which of the old spaces is the visited space */
  int visited_space;

Lib/test/test_gc.py

@@ -1058,7 +1058,18 @@ class Z:
  callback.assert_not_called()
  gc.enable()
 
+
+class IncremetalGCTests(unittest.TestCase):
+
+ def setUp(self):
+ gc.enable()
+
+ def tearDown(self):
+ gc.disable()
+
  @unittest.skipIf(Py_GIL_DISABLED, "Free threading does not support incremental GC")
+ # Use small increments to emulate longer running process in a shorter time
+ @gc_threshold(200, 10)
  def test_incremental_gc_handles_fast_cycle_creation(self):
 
  class LinkedList:
@@ -1080,28 +1091,31 @@ def make_ll(depth):
  head = LinkedList(head, head.prev)
  return head
 
- head = make_ll(10000)
- count = 10000
+ head = make_ll(1000)
+ count = 1000
 
- # 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.
+ # There will some objects we aren't counting,
+ # e.g. the gc stats dicts. This test checks
+ # that the counts don't grow, so we try to
+ # correct for the uncounted objects
+ # This is just an estimate.
+ CORRECTION = 20
 
  enabled = gc.isenabled()
  gc.enable()
  olds = []
- for i in range(1000):
- newhead = make_ll(200)
- count += 200
+ for i in range(20_000):
+ newhead = make_ll(20)
+ count += 20
  newhead.surprise = head
  olds.append(newhead)
- if len(olds) == 50:
+ if len(olds) == 20:
  stats = gc.get_stats()
  young = stats[0]
  incremental = stats[1]
  old = stats[2]
  collected = young['collected'] + incremental['collected'] + old['collected']
+ count += CORRECTION
  live = count - collected
  self.assertLess(live, 25000)
  del olds[:]

Misc/NEWS.d/next/Core and Builtins/2024-03-21-12-10-11.gh-issue-117108._6jIrB.rst

@@ -0,0 +1,3 @@
+The cycle GC now chooses the size of increments based on the total heap
+size, instead of the rate of object creation. This ensures that it can keep
+up with growing heaps.

Python/gc.c

@@ -182,6 +182,7 @@ _PyGC_Init(PyInterpreterState *interp)
  if (gcstate->callbacks == NULL) {
  return _PyStatus_NO_MEMORY();
  }
+ gcstate->heap_size = 0;
 
  return _PyStatus_OK();
 }
@@ -1258,9 +1259,9 @@ gc_list_set_space(PyGC_Head *list, uintptr_t space)
  * N == 1.4 (1 + 4/threshold)
  */
 
-/* Multiply by 4 so that the default incremental threshold of 10
- * scans objects at 20% the rate of object creation */
-#define SCAN_RATE_MULTIPLIER 2
+/* Divide by 10, so that the default incremental threshold of 10
+ * scans objects at 1% of the heap size */
+#define SCAN_RATE_DIVISOR 10
 
 static void
 add_stats(GCState *gcstate, int gen, struct gc_collection_stats *stats)
@@ -1313,7 +1314,7 @@ gc_collect_young(PyThreadState *tstate,
  if (scale_factor < 1) {
  scale_factor = 1;
  }
- gcstate->work_to_do += survivor_count + survivor_count * SCAN_RATE_MULTIPLIER / scale_factor;
+ gcstate->work_to_do += gcstate->heap_size / SCAN_RATE_DIVISOR / scale_factor;
  add_stats(gcstate, 0, stats);
 }
 
@@ -1384,12 +1385,9 @@ expand_region_transitively_reachable(PyGC_Head *container, PyGC_Head *gc, GCStat
 static void
 completed_cycle(GCState *gcstate)
 {
- PyGC_Head *not_visited = &gcstate->old[gcstate->visited_space^1].head;
- assert(gc_list_is_empty(not_visited));
+ assert(gc_list_is_empty(&gcstate->old[gcstate->visited_space^1].head));
  gcstate->visited_space = flip_old_space(gcstate->visited_space);
- if (gcstate->work_to_do > 0) {
- gcstate->work_to_do = 0;
- }
+ gcstate->work_to_do = 0;
 }
 
 static void
@@ -1404,13 +1402,13 @@ gc_collect_increment(PyThreadState *tstate, struct gc_collection_stats *stats)
  if (scale_factor < 1) {
  scale_factor = 1;
  }
- Py_ssize_t increment_size = 0;
  gc_list_merge(&gcstate->young.head, &increment);
  gcstate->young.count = 0;
  if (gcstate->visited_space) {
  /* objects in visited space have bit set, so we set it here */
  gc_list_set_space(&increment, 1);
  }
+ Py_ssize_t increment_size = 0;
  while (increment_size < gcstate->work_to_do) {
  if (gc_list_is_empty(not_visited)) {
  break;
@@ -1425,14 +1423,11 @@ gc_collect_increment(PyThreadState *tstate, struct gc_collection_stats *stats)
  PyGC_Head survivors;
  gc_list_init(&survivors);
  gc_collect_region(tstate, &increment, &survivors, UNTRACK_TUPLES, stats);
- Py_ssize_t survivor_count = gc_list_size(&survivors);
  gc_list_merge(&survivors, visited);
  assert(gc_list_is_empty(&increment));
- gcstate->work_to_do += survivor_count + survivor_count * SCAN_RATE_MULTIPLIER / scale_factor;
+ gcstate->work_to_do += gcstate->heap_size / SCAN_RATE_DIVISOR / scale_factor;
  gcstate->work_to_do -= increment_size;
- if (gcstate->work_to_do < 0) {
- gcstate->work_to_do = 0;
- }
+
  validate_old(gcstate);
  add_stats(gcstate, 1, stats);
  if (gc_list_is_empty(not_visited)) {
@@ -1974,6 +1969,7 @@ _PyObject_GC_Link(PyObject *op)
  gc->_gc_next = 0;
  gc->_gc_prev = 0;
  gcstate->young.count++; /* number of allocated GC objects */
+ gcstate->heap_size++;
  if (gcstate->young.count > gcstate->young.threshold &&
  gcstate->enabled &&
  gcstate->young.threshold &&
@@ -2095,6 +2091,7 @@ PyObject_GC_Del(void *op)
  if (gcstate->young.count > 0) {
  gcstate->young.count--;
  }
+ gcstate->heap_size--;
  PyObject_Free(((char *)op)-presize);
 }