diff --git a/Doc/library/gc.rst b/Doc/library/gc.rst
index 1065ec308028412..8ce850ba777e7c7 100644
--- a/Doc/library/gc.rst
+++ b/Doc/library/gc.rst
@@ -40,18 +40,11 @@ The :mod:`gc` module provides the following functions:
 
 .. function:: collect(generation=2)
 
-   Perform a collection.  The optional argument *generation*
+   With no arguments, run a full collection.  The optional argument *generation*
    may be an integer specifying which generation to collect (from 0 to 2).  A
    :exc:`ValueError` is raised if the generation number is invalid. The sum of
    collected objects and uncollectable objects is returned.
 
-   Calling ``gc.collect(0)`` will perform a GC collection on the young generation.
-
-   Calling ``gc.collect(1)`` will perform a GC collection on the young generation
-   and an increment of the old generation.
-
-   Calling ``gc.collect(2)`` or ``gc.collect()`` performs a full collection
-
    The free lists maintained for a number of built-in types are cleared
    whenever a full collection or collection of the highest generation (2)
    is run.  Not all items in some free lists may be freed due to the
@@ -60,9 +53,6 @@ The :mod:`gc` module provides the following functions:
    The effect of calling ``gc.collect()`` while the interpreter is already
    performing a collection is undefined.
 
-   .. versionchanged:: 3.13
-      ``generation=1`` performs an increment of collection.
-
 
 .. function:: set_debug(flags)
 
@@ -78,20 +68,13 @@ The :mod:`gc` module provides the following functions:
 
 .. function:: get_objects(generation=None)
 
-
    Returns a list of all objects tracked by the collector, excluding the list
-   returned. If *generation* is not ``None``, return only the objects as follows:
-
-   * 0: All objects in the young generation
-   * 1: No objects, as there is no generation 1 (as of Python 3.13)
-   * 2: All objects in the old generation
+   returned. If *generation* is not ``None``, return only the objects tracked by
+   the collector that are in that generation.
 
    .. versionchanged:: 3.8
       New *generation* parameter.
 
-   .. versionchanged:: 3.13
-      Generation 1 is removed
-
    .. audit-event:: gc.get_objects generation gc.get_objects
 
 .. function:: get_stats()
@@ -118,27 +101,19 @@ The :mod:`gc` module provides the following functions:
    Set the garbage collection thresholds (the collection frequency). Setting
    *threshold0* to zero disables collection.
 
-   The GC classifies objects into two generations depending on whether they have
-   survived a collection. New objects are placed in the young generation. If an
-   object survives a collection it is moved into the old generation.
-
-   In order to decide when to run, the collector keeps track of the number of object
+   The GC classifies objects into three generations depending on how many
+   collection sweeps they have survived.  New objects are placed in the youngest
+   generation (generation ``0``).  If an object survives a collection it is moved
+   into the next older generation.  Since generation ``2`` is the oldest
+   generation, objects in that generation remain there after a collection.  In
+   order to decide when to run, the collector keeps track of the number object
    allocations and deallocations since the last collection.  When the number of
    allocations minus the number of deallocations exceeds *threshold0*, collection
-   starts. For each collection, all the objects in the young generation and some
-   fraction of the old generation is collected.
-
-   The fraction of the old generation that is collected is **inversely** proportional
-   to *threshold1*. The larger *threshold1* is, the slower objects in the old generation
-   are collected.
-   For the default value of 10, 1% of the old generation is scanned during each collection.
-
-   *threshold2* is ignored.
-
-   See `Garbage collector design <https://devguide.python.org/garbage_collector>`_ for more information.
-
-   .. versionchanged:: 3.13
-      *threshold2* is ignored
+   starts.  Initially only generation ``0`` is examined.  If generation ``0`` has
+   been examined more than *threshold1* times since generation ``1`` has been
+   examined, then generation ``1`` is examined as well.
+   With the third generation, things are a bit more complicated,
+   see `Collecting the oldest generation <https://devguide.python.org/garbage_collector/#collecting-the-oldest-generation>`_ for more information.
 
 
 .. function:: get_count()
diff --git a/Include/internal/pycore_gc.h b/Include/internal/pycore_gc.h
index b85957df5a6b9f4..d45814635248c04 100644
--- a/Include/internal/pycore_gc.h
+++ b/Include/internal/pycore_gc.h
@@ -143,23 +143,8 @@ static inline void _PyObject_GC_SET_SHARED_INLINE(PyObject *op) {
 #define _PyGC_PREV_MASK_FINALIZED  ((uintptr_t)1)
 /* 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.
- * 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_PREV_SHIFT           2
+/* The (N-2) most significant bits contain the real address. */
+#define _PyGC_PREV_SHIFT           (2)
 #define _PyGC_PREV_MASK            (((uintptr_t) -1) << _PyGC_PREV_SHIFT)
 
 /* set for debugging information */
@@ -185,13 +170,11 @@ 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 & _PyGC_PREV_MASK;
+    uintptr_t next = gc->_gc_next;
     return (PyGC_Head*)next;
 }
 static inline void _PyGCHead_SET_NEXT(PyGC_Head *gc, PyGC_Head *next) {
-    uintptr_t unext = (uintptr_t)next;
-    assert((unext & ~_PyGC_PREV_MASK) == 0);
-    gc->_gc_next = (gc->_gc_next & ~_PyGC_PREV_MASK) | unext;
+    gc->_gc_next = (uintptr_t)next;
 }
 
 // Lowest two bits of _gc_prev is used for _PyGC_PREV_MASK_* flags.
@@ -199,7 +182,6 @@ 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);
@@ -285,13 +267,6 @@ 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 */
@@ -313,8 +288,8 @@ struct _gc_runtime_state {
     int enabled;
     int debug;
     /* linked lists of container objects */
-    struct gc_generation young;
-    struct gc_generation old[2];
+    struct gc_generation generations[NUM_GENERATIONS];
+    PyGC_Head *generation0;
     /* a permanent generation which won't be collected */
     struct gc_generation permanent_generation;
     struct gc_generation_stats generation_stats[NUM_GENERATIONS];
@@ -325,12 +300,6 @@ 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;
-
-#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.
@@ -342,7 +311,6 @@ 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
@@ -355,7 +323,8 @@ 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_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. */
diff --git a/Include/internal/pycore_object.h b/Include/internal/pycore_object.h
index c7af720b1ce43d3..b92590926720468 100644
--- a/Include/internal/pycore_object.h
+++ b/Include/internal/pycore_object.h
@@ -462,12 +462,11 @@ static inline void _PyObject_GC_TRACK(
                           filename, lineno, __func__);
 
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    PyGC_Head *generation0 = &interp->gc.young.head;
+    PyGC_Head *generation0 = interp->gc.generation0;
     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;
+    _PyGCHead_SET_NEXT(gc, generation0);
     generation0->_gc_prev = (uintptr_t)gc;
 #endif
 }
diff --git a/Include/internal/pycore_runtime_init.h b/Include/internal/pycore_runtime_init.h
index e99febab2f3d57a..2715449a41437e3 100644
--- a/Include/internal/pycore_runtime_init.h
+++ b/Include/internal/pycore_runtime_init.h
@@ -126,12 +126,12 @@ extern PyTypeObject _PyExc_MemoryError;
         }, \
         .gc = { \
             .enabled = 1, \
-            .young = { .threshold = 2000, }, \
-            .old = { \
+            .generations = { \
+                /* .head is set in _PyGC_InitState(). */ \
+                { .threshold = 2000, }, \
+                { .threshold = 10, }, \
                 { .threshold = 10, }, \
-                { .threshold = 0, }, \
             }, \
-            .work_to_do = -5000, \
         }, \
         .qsbr = { \
             .wr_seq = QSBR_INITIAL, \
diff --git a/Lib/test/test_gc.py b/Lib/test/test_gc.py
index 2b3c0d3baddeafc..44227ee8952611e 100644
--- a/Lib/test/test_gc.py
+++ b/Lib/test/test_gc.py
@@ -388,11 +388,19 @@ def test_collect_generations(self):
         # each call to collect(N)
         x = []
         gc.collect(0)
-        # x is now in the old gen
+        # x is now in gen 1
         a, b, c = gc.get_count()
-        # We don't check a since its exact values depends on
+        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
         # 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:
@@ -828,10 +836,42 @@ def test_get_objects_generations(self):
         self.assertTrue(
                 any(l is element for element in gc.get_objects(generation=0))
         )
-        gc.collect()
+        self.assertFalse(
+                any(l is element for element in  gc.get_objects(generation=1))
+        )
+        self.assertFalse(
+                any(l is element for element in gc.get_objects(generation=2))
+        )
+        gc.collect(generation=0)
         self.assertFalse(
                 any(l is element for element in gc.get_objects(generation=0))
         )
+        self.assertTrue(
+                any(l is element for element in  gc.get_objects(generation=1))
+        )
+        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))
+        )
+        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))
+        )
         del l
         gc.collect()
 
@@ -1082,72 +1122,6 @@ def __del__(self):
         gc.collect()
         self.assertTrue(collected)
 
-
-class IncrementalGCTests(unittest.TestCase):
-
-    def setUp(self):
-        # Reenable GC as it is disabled module-wide
-        gc.enable()
-
-    def tearDown(self):
-        gc.disable()
-
-    @requires_gil_enabled("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:
-
-            #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(1000)
-        count = 1000
-
-        # There will be 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(20_000):
-            newhead = make_ll(20)
-            count += 20
-            newhead.surprise = head
-            olds.append(newhead)
-            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[:]
-        if not enabled:
-            gc.disable()
-
-
 class GCCallbackTests(unittest.TestCase):
     def setUp(self):
         # Save gc state and disable it.
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2024-09-30-21-02-10.gh-issue-124567.tv_B_C.rst b/Misc/NEWS.d/next/Core_and_Builtins/2024-09-30-21-02-10.gh-issue-124567.tv_B_C.rst
new file mode 100644
index 000000000000000..96091972a499ae9
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2024-09-30-21-02-10.gh-issue-124567.tv_B_C.rst
@@ -0,0 +1 @@
+Revert the incremental GC (in 3.13), since it's not clear the benefits outweigh the costs at this point.
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index f5fea5aa4dde085..7e7660a1191013e 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -159,12 +159,17 @@ gc_set_threshold_impl(PyObject *module, int threshold0, int group_right_1,
 {
     GCState *gcstate = get_gc_state();
 
-    gcstate->young.threshold = threshold0;
+    gcstate->generations[0].threshold = threshold0;
     if (group_right_1) {
-        gcstate->old[0].threshold = threshold1;
+        gcstate->generations[1].threshold = threshold1;
     }
     if (group_right_2) {
-        gcstate->old[1].threshold = threshold2;
+        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;
+        }
     }
     Py_RETURN_NONE;
 }
@@ -181,9 +186,9 @@ gc_get_threshold_impl(PyObject *module)
 {
     GCState *gcstate = get_gc_state();
     return Py_BuildValue("(iii)",
-                         gcstate->young.threshold,
-                         gcstate->old[0].threshold,
-                         0);
+                         gcstate->generations[0].threshold,
+                         gcstate->generations[1].threshold,
+                         gcstate->generations[2].threshold);
 }
 
 /*[clinic input]
@@ -203,14 +208,14 @@ 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->young.count, (int)gc->alloc_count);
+    _Py_atomic_add_int(&gcstate->generations[0].count, (int)gc->alloc_count);
     gc->alloc_count = 0;
 #endif
 
     return Py_BuildValue("(iii)",
-                         gcstate->young.count,
-                         gcstate->old[gcstate->visited_space].count,
-                         gcstate->old[gcstate->visited_space^1].count);
+                         gcstate->generations[0].count,
+                         gcstate->generations[1].count,
+                         gcstate->generations[2].count);
 }
 
 /*[clinic input]
diff --git a/Python/gc.c b/Python/gc.c
index 028657eb8999c19..45d2e0057b24b0e 100644
--- a/Python/gc.c
+++ b/Python/gc.c
@@ -45,7 +45,7 @@ typedef struct _gc_runtime_state GCState;
 // move_legacy_finalizers() removes this flag instead.
 // Between them, unreachable list is not normal list and we can not use
 // most gc_list_* functions for it.
-#define NEXT_MASK_UNREACHABLE  2
+#define NEXT_MASK_UNREACHABLE  (1)
 
 #define AS_GC(op) _Py_AS_GC(op)
 #define FROM_GC(gc) _Py_FROM_GC(gc)
@@ -95,48 +95,9 @@ gc_decref(PyGC_Head *g)
     g->_gc_prev -= 1 << _PyGC_PREV_SHIFT;
 }
 
-static inline int
-gc_old_space(PyGC_Head *g)
-{
-    return g->_gc_next & _PyGC_NEXT_MASK_OLD_SPACE_1;
-}
 
-static inline int
-flip_old_space(int space)
-{
-    assert(space == 0 || space == 1);
-    return space ^ _PyGC_NEXT_MASK_OLD_SPACE_1;
-}
+#define GEN_HEAD(gcstate, n) (&(gcstate)->generations[n].head)
 
-static inline void
-gc_flip_old_space(PyGC_Head *g)
-{
-    g->_gc_next ^= _PyGC_NEXT_MASK_OLD_SPACE_1;
-}
-
-static inline void
-gc_set_old_space(PyGC_Head *g, int space)
-{
-    assert(space == 0 || space == _PyGC_NEXT_MASK_OLD_SPACE_1);
-    g->_gc_next &= ~_PyGC_NEXT_MASK_OLD_SPACE_1;
-    g->_gc_next |= space;
-}
-
-static PyGC_Head *
-GEN_HEAD(GCState *gcstate, int n)
-{
-    assert((gcstate->visited_space & (~1)) == 0);
-    switch(n) {
-        case 0:
-            return &gcstate->young.head;
-        case 1:
-            return &gcstate->old[gcstate->visited_space].head;
-        case 2:
-            return &gcstate->old[gcstate->visited_space^1].head;
-        default:
-            Py_UNREACHABLE();
-    }
-}
 
 static GCState *
 get_gc_state(void)
@@ -155,12 +116,11 @@ _PyGC_InitState(GCState *gcstate)
         GEN.head._gc_prev = (uintptr_t)&GEN.head; \
     } while (0)
 
-    assert(gcstate->young.count == 0);
-    assert(gcstate->old[0].count == 0);
-    assert(gcstate->old[1].count == 0);
-    INIT_HEAD(gcstate->young);
-    INIT_HEAD(gcstate->old[0]);
-    INIT_HEAD(gcstate->old[1]);
+    for (int i = 0; i < NUM_GENERATIONS; i++) {
+        assert(gcstate->generations[i].count == 0);
+        INIT_HEAD(gcstate->generations[i]);
+    };
+    gcstate->generation0 = GEN_HEAD(gcstate, 0);
     INIT_HEAD(gcstate->permanent_generation);
 
 #undef INIT_HEAD
@@ -181,7 +141,6 @@ _PyGC_Init(PyInterpreterState *interp)
     if (gcstate->callbacks == NULL) {
         return _PyStatus_NO_MEMORY();
     }
-    gcstate->heap_size = 0;
 
     return _PyStatus_OK();
 }
@@ -259,7 +218,6 @@ gc_list_is_empty(PyGC_Head *list)
 static inline void
 gc_list_append(PyGC_Head *node, PyGC_Head *list)
 {
-    assert((list->_gc_prev & ~_PyGC_PREV_MASK) == 0);
     PyGC_Head *last = (PyGC_Head *)list->_gc_prev;
 
     // last <-> node
@@ -317,8 +275,6 @@ gc_list_merge(PyGC_Head *from, PyGC_Head *to)
         PyGC_Head *from_tail = GC_PREV(from);
         assert(from_head != from);
         assert(from_tail != from);
-        assert(gc_list_is_empty(to) ||
-            gc_old_space(to_tail) == gc_old_space(from_tail));
 
         _PyGCHead_SET_NEXT(to_tail, from_head);
         _PyGCHead_SET_PREV(from_head, to_tail);
@@ -387,8 +343,8 @@ enum flagstates {collecting_clear_unreachable_clear,
 static void
 validate_list(PyGC_Head *head, enum flagstates flags)
 {
-    assert((head->_gc_prev & ~_PyGC_PREV_MASK) == 0);
-    assert((head->_gc_next & ~_PyGC_PREV_MASK) == 0);
+    assert((head->_gc_prev & PREV_MASK_COLLECTING) == 0);
+    assert((head->_gc_next & NEXT_MASK_UNREACHABLE) == 0);
     uintptr_t prev_value = 0, next_value = 0;
     switch (flags) {
         case collecting_clear_unreachable_clear:
@@ -410,7 +366,7 @@ validate_list(PyGC_Head *head, enum flagstates flags)
     PyGC_Head *gc = GC_NEXT(head);
     while (gc != head) {
         PyGC_Head *trueprev = GC_PREV(gc);
-        PyGC_Head *truenext = GC_NEXT(gc);
+        PyGC_Head *truenext = (PyGC_Head *)(gc->_gc_next  & ~NEXT_MASK_UNREACHABLE);
         assert(truenext != NULL);
         assert(trueprev == prev);
         assert((gc->_gc_prev & PREV_MASK_COLLECTING) == prev_value);
@@ -420,54 +376,8 @@ validate_list(PyGC_Head *head, enum flagstates flags)
     }
     assert(prev == GC_PREV(head));
 }
-
-static void
-validate_old(GCState *gcstate)
-{
-    for (int space = 0; space < 2; space++) {
-        PyGC_Head *head = &gcstate->old[space].head;
-        PyGC_Head *gc = GC_NEXT(head);
-        while (gc != head) {
-            PyGC_Head *next = GC_NEXT(gc);
-            assert(gc_old_space(gc) == space);
-            gc = next;
-        }
-    }
-}
-
-static void
-validate_consistent_old_space(PyGC_Head *head)
-{
-    PyGC_Head *prev = head;
-    PyGC_Head *gc = GC_NEXT(head);
-    if (gc == head) {
-        return;
-    }
-    int old_space = gc_old_space(gc);
-    while (gc != head) {
-        PyGC_Head *truenext = GC_NEXT(gc);
-        assert(truenext != NULL);
-        assert(gc_old_space(gc) == old_space);
-        prev = gc;
-        gc = truenext;
-    }
-    assert(prev == GC_PREV(head));
-}
-
-static void
-gc_list_validate_space(PyGC_Head *head, int space) {
-    PyGC_Head *gc = GC_NEXT(head);
-    while (gc != head) {
-        assert(gc_old_space(gc) == space);
-        gc = GC_NEXT(gc);
-    }
-}
-
 #else
 #define validate_list(x, y) do{}while(0)
-#define validate_old(g) do{}while(0)
-#define validate_consistent_old_space(l) do{}while(0)
-#define gc_list_validate_space(l, s) do{}while(0)
 #endif
 
 /*** end of list stuff ***/
@@ -485,6 +395,10 @@ update_refs(PyGC_Head *containers)
     while (gc != containers) {
         next = GC_NEXT(gc);
         PyObject *op = FROM_GC(gc);
+        /* Move any object that might have become immortal to the
+         * permanent generation as the reference count is not accurately
+         * reflecting the actual number of live references to this object
+         */
         if (_Py_IsImmortal(op)) {
            gc_list_move(gc, &get_gc_state()->permanent_generation.head);
            gc = next;
@@ -605,13 +519,12 @@ visit_reachable(PyObject *op, void *arg)
         // Manually unlink gc from unreachable list because the list functions
         // don't work right in the presence of NEXT_MASK_UNREACHABLE flags.
         PyGC_Head *prev = GC_PREV(gc);
-        PyGC_Head *next = GC_NEXT(gc);
+        PyGC_Head *next = (PyGC_Head*)(gc->_gc_next & ~NEXT_MASK_UNREACHABLE);
         _PyObject_ASSERT(FROM_GC(prev),
                          prev->_gc_next & NEXT_MASK_UNREACHABLE);
         _PyObject_ASSERT(FROM_GC(next),
                          next->_gc_next & NEXT_MASK_UNREACHABLE);
-        prev->_gc_next = gc->_gc_next;  // copy flag bits
-        gc->_gc_next &= ~NEXT_MASK_UNREACHABLE;
+        prev->_gc_next = gc->_gc_next;  // copy NEXT_MASK_UNREACHABLE
         _PyGCHead_SET_PREV(next, prev);
 
         gc_list_append(gc, reachable);
@@ -663,9 +576,6 @@ move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
      * or to the right have been scanned yet.
      */
 
-    validate_consistent_old_space(young);
-    /* Record which old space we are in, and set NEXT_MASK_UNREACHABLE bit for convenience */
-    uintptr_t flags = NEXT_MASK_UNREACHABLE | (gc->_gc_next & _PyGC_NEXT_MASK_OLD_SPACE_1);
     while (gc != young) {
         if (gc_get_refs(gc)) {
             /* gc is definitely reachable from outside the
@@ -711,18 +621,17 @@ move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
             // But this may pollute the unreachable list head's 'next' pointer
             // too. That's semantically senseless but expedient here - the
             // damage is repaired when this function ends.
-            last->_gc_next = flags | (uintptr_t)gc;
+            last->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)gc);
             _PyGCHead_SET_PREV(gc, last);
-            gc->_gc_next = flags | (uintptr_t)unreachable;
+            gc->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)unreachable);
             unreachable->_gc_prev = (uintptr_t)gc;
         }
-        gc = _PyGCHead_NEXT(prev);
+        gc = (PyGC_Head*)prev->_gc_next;
     }
     // young->_gc_prev must be last element remained in the list.
     young->_gc_prev = (uintptr_t)prev;
-    young->_gc_next &= _PyGC_PREV_MASK;
     // don't let the pollution of the list head's next pointer leak
-    unreachable->_gc_next &= _PyGC_PREV_MASK;
+    unreachable->_gc_next &= ~NEXT_MASK_UNREACHABLE;
 }
 
 static void
@@ -781,8 +690,8 @@ move_legacy_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
         PyObject *op = FROM_GC(gc);
 
         _PyObject_ASSERT(op, gc->_gc_next & NEXT_MASK_UNREACHABLE);
-        next = GC_NEXT(gc);
         gc->_gc_next &= ~NEXT_MASK_UNREACHABLE;
+        next = (PyGC_Head*)gc->_gc_next;
 
         if (has_legacy_finalizer(op)) {
             gc_clear_collecting(gc);
@@ -805,8 +714,8 @@ clear_unreachable_mask(PyGC_Head *unreachable)
     PyGC_Head *gc, *next;
     for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) {
         _PyObject_ASSERT((PyObject*)FROM_GC(gc), gc->_gc_next & NEXT_MASK_UNREACHABLE);
-        next = GC_NEXT(gc);
         gc->_gc_next &= ~NEXT_MASK_UNREACHABLE;
+        next = (PyGC_Head*)gc->_gc_next;
     }
     validate_list(unreachable, collecting_set_unreachable_clear);
 }
@@ -976,7 +885,6 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
     /* Invoke the callbacks we decided to honor.  It's safe to invoke them
      * because they can't reference unreachable objects.
      */
-    int visited_space = get_gc_state()->visited_space;
     while (! gc_list_is_empty(&wrcb_to_call)) {
         PyObject *temp;
         PyObject *callback;
@@ -1011,7 +919,6 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
         Py_DECREF(op);
         if (wrcb_to_call._gc_next == (uintptr_t)gc) {
             /* object is still alive -- move it */
-            gc_set_old_space(gc, visited_space);
             gc_list_move(gc, old);
         }
         else {
@@ -1140,6 +1047,25 @@ delete_garbage(PyThreadState *tstate, GCState *gcstate,
 }
 
 
+// Show stats for objects in each generations
+static void
+show_stats_each_generations(GCState *gcstate)
+{
+    char buf[100];
+    size_t pos = 0;
+
+    for (int i = 0; i < NUM_GENERATIONS && pos < sizeof(buf); i++) {
+        pos += PyOS_snprintf(buf+pos, sizeof(buf)-pos,
+                             " %zd",
+                             gc_list_size(GEN_HEAD(gcstate, i)));
+    }
+
+    PySys_FormatStderr(
+        "gc: objects in each generation:%s\n"
+        "gc: objects in permanent generation: %zd\n",
+        buf, gc_list_size(&gcstate->permanent_generation.head));
+}
+
 /* Deduce which objects among "base" are unreachable from outside the list
    and move them to 'unreachable'. The process consist in the following steps:
 
@@ -1213,6 +1139,7 @@ deduce_unreachable(PyGC_Head *base, PyGC_Head *unreachable) {
      * the reachable objects instead.  But this is a one-time cost, probably not
      * worth complicating the code to speed just a little.
      */
+    gc_list_init(unreachable);
     move_unreachable(base, unreachable);  // gc_prev is pointer again
     validate_list(base, collecting_clear_unreachable_clear);
     validate_list(unreachable, collecting_set_unreachable_set);
@@ -1251,295 +1178,220 @@ handle_resurrected_objects(PyGC_Head *unreachable, PyGC_Head* still_unreachable,
 }
 
 
-#define UNTRACK_TUPLES 1
-#define UNTRACK_DICTS 2
-
-static void
-gc_collect_region(PyThreadState *tstate,
-                  PyGC_Head *from,
-                  PyGC_Head *to,
-                  int untrack,
-                  struct gc_collection_stats *stats);
-
-static inline Py_ssize_t
-gc_list_set_space(PyGC_Head *list, int space)
-{
-    Py_ssize_t size = 0;
-    PyGC_Head *gc;
-    for (gc = GC_NEXT(list); gc != list; gc = GC_NEXT(gc)) {
-        gc_set_old_space(gc, space);
-        size++;
-    }
-    return size;
-}
-
-/* Making progress in the incremental collector
- * In order to eventually collect all cycles
- * the incremental collector must progress through the old
- * space faster than objects are added to the old space.
- *
- * Each young or incremental collection adds a number of
- * objects, S (for survivors) to the old space, and
- * incremental collectors scan I objects from the old space.
- * I > S must be true. We also want I > S * N to be where
- * N > 1. Higher values of N mean that the old space is
- * scanned more rapidly.
- * The default incremental threshold of 10 translates to
- * N == 1.4 (1 + 4/threshold)
+/* Invoke progress callbacks to notify clients that garbage collection
+ * is starting or stopping
  */
-
-/* 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)
+invoke_gc_callback(PyThreadState *tstate, const char *phase,
+                   int generation, Py_ssize_t collected,
+                   Py_ssize_t uncollectable)
 {
-    gcstate->generation_stats[gen].collected += stats->collected;
-    gcstate->generation_stats[gen].uncollectable += stats->uncollectable;
-    gcstate->generation_stats[gen].collections += 1;
-}
+    assert(!_PyErr_Occurred(tstate));
 
-static void
-gc_collect_young(PyThreadState *tstate,
-                 struct gc_collection_stats *stats)
-{
+    /* we may get called very early */
     GCState *gcstate = &tstate->interp->gc;
-    PyGC_Head *young = &gcstate->young.head;
-    PyGC_Head *visited = &gcstate->old[gcstate->visited_space].head;
-    GC_STAT_ADD(0, collections, 1);
-#ifdef Py_STATS
-    {
-        Py_ssize_t count = 0;
-        PyGC_Head *gc;
-        for (gc = GC_NEXT(young); gc != young; gc = GC_NEXT(gc)) {
-            count++;
-        }
+    if (gcstate->callbacks == NULL) {
+        return;
     }
-#endif
 
-    PyGC_Head survivors;
-    gc_list_init(&survivors);
-    gc_collect_region(tstate, young, &survivors, UNTRACK_TUPLES, stats);
-    Py_ssize_t survivor_count = 0;
-    if (gcstate->visited_space) {
-        /* objects in visited space have bit set, so we set it here */
-        survivor_count = gc_list_set_space(&survivors, 1);
-    }
-    else {
-        PyGC_Head *gc;
-        for (gc = GC_NEXT(&survivors); gc != &survivors; gc = GC_NEXT(gc)) {
-#ifdef GC_DEBUG
-            assert(gc_old_space(gc) == 0);
-#endif
-            survivor_count++;
+    /* The local variable cannot be rebound, check it for sanity */
+    assert(PyList_CheckExact(gcstate->callbacks));
+    PyObject *info = NULL;
+    if (PyList_GET_SIZE(gcstate->callbacks) != 0) {
+        info = Py_BuildValue("{sisnsn}",
+            "generation", generation,
+            "collected", collected,
+            "uncollectable", uncollectable);
+        if (info == NULL) {
+            PyErr_FormatUnraisable("Exception ignored on invoking gc callbacks");
+            return;
         }
     }
-    (void)survivor_count;  // Silence compiler warning
-    gc_list_merge(&survivors, visited);
-    validate_old(gcstate);
-    gcstate->young.count = 0;
-    gcstate->old[gcstate->visited_space].count++;
-    Py_ssize_t scale_factor = gcstate->old[0].threshold;
-    if (scale_factor < 1) {
-        scale_factor = 1;
-    }
-    gcstate->work_to_do += gcstate->heap_size / SCAN_RATE_DIVISOR / scale_factor;
-    add_stats(gcstate, 0, stats);
-}
 
-#ifndef NDEBUG
-static inline int
-IS_IN_VISITED(PyGC_Head *gc, int visited_space)
-{
-    assert(visited_space == 0 || flip_old_space(visited_space) == 0);
-    return gc_old_space(gc) == visited_space;
-}
-#endif
-
-struct container_and_flag {
-    PyGC_Head *container;
-    int visited_space;
-    uintptr_t size;
-};
-
-/* A traversal callback for adding to container) */
-static int
-visit_add_to_container(PyObject *op, void *arg)
-{
-    OBJECT_STAT_INC(object_visits);
-    struct container_and_flag *cf = (struct container_and_flag *)arg;
-    int visited = cf->visited_space;
-    assert(visited == get_gc_state()->visited_space);
-    if (!_Py_IsImmortal(op) && _PyObject_IS_GC(op)) {
-        PyGC_Head *gc = AS_GC(op);
-        if (_PyObject_GC_IS_TRACKED(op) &&
-            gc_old_space(gc) != visited) {
-            gc_flip_old_space(gc);
-            gc_list_move(gc, cf->container);
-            cf->size++;
-        }
+    PyObject *phase_obj = PyUnicode_FromString(phase);
+    if (phase_obj == NULL) {
+        Py_XDECREF(info);
+        PyErr_FormatUnraisable("Exception ignored on invoking gc callbacks");
+        return;
     }
-    return 0;
-}
 
-static uintptr_t
-expand_region_transitively_reachable(PyGC_Head *container, PyGC_Head *gc, GCState *gcstate)
-{
-    validate_list(container, collecting_clear_unreachable_clear);
-    struct container_and_flag arg = {
-        .container = container,
-        .visited_space = gcstate->visited_space,
-        .size = 0
-    };
-    assert(GC_NEXT(gc) == container);
-    while (gc != container) {
-        /* Survivors will be moved to visited space, so they should
-         * have been marked as visited */
-        assert(IS_IN_VISITED(gc, gcstate->visited_space));
-        PyObject *op = FROM_GC(gc);
-        if (_Py_IsImmortal(op)) {
-            PyGC_Head *next = GC_NEXT(gc);
-            gc_list_move(gc, &get_gc_state()->permanent_generation.head);
-            gc = next;
-            continue;
+    PyObject *stack[] = {phase_obj, info};
+    for (Py_ssize_t i=0; i<PyList_GET_SIZE(gcstate->callbacks); i++) {
+        PyObject *r, *cb = PyList_GET_ITEM(gcstate->callbacks, i);
+        Py_INCREF(cb); /* make sure cb doesn't go away */
+        r = PyObject_Vectorcall(cb, stack, 2, NULL);
+        if (r == NULL) {
+            PyErr_WriteUnraisable(cb);
         }
-        traverseproc traverse = Py_TYPE(op)->tp_traverse;
-        (void) traverse(op,
-                        visit_add_to_container,
-                        &arg);
-        gc = GC_NEXT(gc);
+        else {
+            Py_DECREF(r);
+        }
+        Py_DECREF(cb);
     }
-    return arg.size;
+    Py_DECREF(phase_obj);
+    Py_XDECREF(info);
+    assert(!_PyErr_Occurred(tstate));
 }
 
-/* Do bookkeeping for a completed GC cycle */
-static void
-completed_cycle(GCState *gcstate)
-{
-#ifdef Py_DEBUG
-    PyGC_Head *not_visited = &gcstate->old[gcstate->visited_space^1].head;
-    assert(gc_list_is_empty(not_visited));
-#endif
-    gcstate->visited_space = flip_old_space(gcstate->visited_space);
-    /* Make sure all young objects have old space bit set correctly */
-    PyGC_Head *young = &gcstate->young.head;
-    PyGC_Head *gc = GC_NEXT(young);
-    while (gc != young) {
-        PyGC_Head *next = GC_NEXT(gc);
-        gc_set_old_space(gc, gcstate->visited_space);
-        gc = next;
-    }
-    gcstate->work_to_do = 0;
-}
 
-static void
-gc_collect_increment(PyThreadState *tstate, struct gc_collection_stats *stats)
-{
-    GC_STAT_ADD(1, collections, 1);
-    GCState *gcstate = &tstate->interp->gc;
-    PyGC_Head *not_visited = &gcstate->old[gcstate->visited_space^1].head;
-    PyGC_Head *visited = &gcstate->old[gcstate->visited_space].head;
-    PyGC_Head increment;
-    gc_list_init(&increment);
-    Py_ssize_t scale_factor = gcstate->old[0].threshold;
-    if (scale_factor < 1) {
-        scale_factor = 1;
-    }
-    gc_list_merge(&gcstate->young.head, &increment);
-    gcstate->young.count = 0;
-    gc_list_validate_space(&increment, gcstate->visited_space);
-    Py_ssize_t increment_size = 0;
-    while (increment_size < gcstate->work_to_do) {
-        if (gc_list_is_empty(not_visited)) {
-            break;
+/* Find the oldest generation (highest numbered) where the count
+ * exceeds the threshold.  Objects in the that generation and
+ * generations younger than it will be collected. */
+static int
+gc_select_generation(GCState *gcstate)
+{
+    for (int i = NUM_GENERATIONS-1; i >= 0; i--) {
+        if (gcstate->generations[i].count > gcstate->generations[i].threshold) {
+            /* Avoid quadratic performance degradation in number
+               of tracked objects (see also issue #4074):
+
+               To limit the cost of garbage collection, there are two strategies;
+                 - make each collection faster, e.g. by scanning fewer objects
+                 - do less collections
+               This heuristic is about the latter strategy.
+
+               In addition to the various configurable thresholds, we only trigger a
+               full collection if the ratio
+
+                long_lived_pending / long_lived_total
+
+               is above a given value (hardwired to 25%).
+
+               The reason is that, while "non-full" collections (i.e., collections of
+               the young and middle generations) will always examine roughly the same
+               number of objects -- determined by the aforementioned thresholds --,
+               the cost of a full collection is proportional to the total number of
+               long-lived objects, which is virtually unbounded.
+
+               Indeed, it has been remarked that doing a full collection every
+               <constant number> of object creations entails a dramatic performance
+               degradation in workloads which consist in creating and storing lots of
+               long-lived objects (e.g. building a large list of GC-tracked objects would
+               show quadratic performance, instead of linear as expected: see issue #4074).
+
+               Using the above ratio, instead, yields amortized linear performance in
+               the total number of objects (the effect of which can be summarized
+               thusly: "each full garbage collection is more and more costly as the
+               number of objects grows, but we do fewer and fewer of them").
+
+               This heuristic was suggested by Martin von Löwis on python-dev in
+               June 2008. His original analysis and proposal can be found at:
+               http://mail.python.org/pipermail/python-dev/2008-June/080579.html
+            */
+            if (i == NUM_GENERATIONS - 1
+                && gcstate->long_lived_pending < gcstate->long_lived_total / 4)
+            {
+                continue;
+            }
+            return i;
         }
-        PyGC_Head *gc = _PyGCHead_NEXT(not_visited);
-        gc_list_move(gc, &increment);
-        increment_size++;
-        gc_set_old_space(gc, gcstate->visited_space);
-        increment_size += expand_region_transitively_reachable(&increment, gc, gcstate);
-    }
-    gc_list_validate_space(&increment, gcstate->visited_space);
-    PyGC_Head survivors;
-    gc_list_init(&survivors);
-    gc_collect_region(tstate, &increment, &survivors, UNTRACK_TUPLES, stats);
-    gc_list_validate_space(&survivors, gcstate->visited_space);
-    gc_list_merge(&survivors, visited);
-    assert(gc_list_is_empty(&increment));
-    gcstate->work_to_do += gcstate->heap_size / SCAN_RATE_DIVISOR / scale_factor;
-    gcstate->work_to_do -= increment_size;
-
-    validate_old(gcstate);
-    add_stats(gcstate, 1, stats);
-    if (gc_list_is_empty(not_visited)) {
-        completed_cycle(gcstate);
     }
+    return -1;
 }
 
 
-static void
-gc_collect_full(PyThreadState *tstate,
-                struct gc_collection_stats *stats)
-{
-    GC_STAT_ADD(2, collections, 1);
-    GCState *gcstate = &tstate->interp->gc;
-    validate_old(gcstate);
-    PyGC_Head *young = &gcstate->young.head;
-    PyGC_Head *pending = &gcstate->old[gcstate->visited_space^1].head;
-    PyGC_Head *visited = &gcstate->old[gcstate->visited_space].head;
-    /* merge all generations into visited */
-    gc_list_validate_space(young, gcstate->visited_space);
-    gc_list_set_space(pending, gcstate->visited_space);
-    gc_list_merge(young, pending);
-    gcstate->young.count = 0;
-    gc_list_merge(pending, visited);
-
-    gc_collect_region(tstate, visited, visited,
-                      UNTRACK_TUPLES | UNTRACK_DICTS,
-                      stats);
-    gcstate->young.count = 0;
-    gcstate->old[0].count = 0;
-    gcstate->old[1].count = 0;
-
-    gcstate->work_to_do = - gcstate->young.threshold * 2;
-    _PyGC_ClearAllFreeLists(tstate->interp);
-    validate_old(gcstate);
-    add_stats(gcstate, 2, stats);
-}
-
-/* This is the main function. Read this to understand how the
+/* This is the main function.  Read this to understand how the
  * collection process works. */
-static void
-gc_collect_region(PyThreadState *tstate,
-                  PyGC_Head *from,
-                  PyGC_Head *to,
-                  int untrack,
-                  struct gc_collection_stats *stats)
+static Py_ssize_t
+gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
 {
+    int i;
+    Py_ssize_t m = 0; /* # objects collected */
+    Py_ssize_t n = 0; /* # unreachable objects that couldn't be collected */
+    PyGC_Head *young; /* the generation we are examining */
+    PyGC_Head *old; /* next older generation */
     PyGC_Head unreachable; /* non-problematic unreachable trash */
     PyGC_Head finalizers;  /* objects with, & reachable from, __del__ */
-    PyGC_Head *gc; /* initialize to prevent a compiler warning */
+    PyGC_Head *gc;
+    PyTime_t t1 = 0;   /* initialize to prevent a compiler warning */
     GCState *gcstate = &tstate->interp->gc;
 
+    // gc_collect_main() must not be called before _PyGC_Init
+    // or after _PyGC_Fini()
     assert(gcstate->garbage != NULL);
     assert(!_PyErr_Occurred(tstate));
 
-    gc_list_init(&unreachable);
-    deduce_unreachable(from, &unreachable);
-    validate_consistent_old_space(from);
-    if (untrack & UNTRACK_TUPLES) {
-        untrack_tuples(from);
+    int expected = 0;
+    if (!_Py_atomic_compare_exchange_int(&gcstate->collecting, &expected, 1)) {
+        // Don't start a garbage collection if one is already in progress.
+        return 0;
     }
-    if (untrack & UNTRACK_DICTS) {
-        untrack_dicts(from);
+
+    if (generation == GENERATION_AUTO) {
+        // Select the oldest generation that needs collecting. We will collect
+        // objects from that generation and all generations younger than it.
+        generation = gc_select_generation(gcstate);
+        if (generation < 0) {
+            // No generation needs to be collected.
+            _Py_atomic_store_int(&gcstate->collecting, 0);
+            return 0;
+        }
     }
-    validate_consistent_old_space(to);
-    if (from != to) {
-        gc_list_merge(from, to);
+
+    assert(generation >= 0 && generation < NUM_GENERATIONS);
+
+#ifdef Py_STATS
+    if (_Py_stats) {
+        _Py_stats->object_stats.object_visits = 0;
+    }
+#endif
+    GC_STAT_ADD(generation, collections, 1);
+
+    if (reason != _Py_GC_REASON_SHUTDOWN) {
+        invoke_gc_callback(tstate, "start", generation, 0, 0);
+    }
+
+    if (gcstate->debug & _PyGC_DEBUG_STATS) {
+        PySys_WriteStderr("gc: collecting generation %d...\n", generation);
+        show_stats_each_generations(gcstate);
+        // ignore error: don't interrupt the GC if reading the clock fails
+        (void)PyTime_PerfCounterRaw(&t1);
+    }
+
+    if (PyDTrace_GC_START_ENABLED()) {
+        PyDTrace_GC_START(generation);
+    }
+
+    /* update collection and allocation counters */
+    if (generation+1 < NUM_GENERATIONS) {
+        gcstate->generations[generation+1].count += 1;
+    }
+    for (i = 0; i <= generation; i++) {
+        gcstate->generations[i].count = 0;
+    }
+
+    /* merge younger generations with one we are currently collecting */
+    for (i = 0; i < generation; i++) {
+        gc_list_merge(GEN_HEAD(gcstate, i), GEN_HEAD(gcstate, generation));
+    }
+
+    /* handy references */
+    young = GEN_HEAD(gcstate, generation);
+    if (generation < NUM_GENERATIONS-1) {
+        old = GEN_HEAD(gcstate, generation+1);
     }
-    validate_consistent_old_space(to);
+    else {
+        old = young;
+    }
+    validate_list(old, collecting_clear_unreachable_clear);
+
+    deduce_unreachable(young, &unreachable);
+
+    untrack_tuples(young);
     /* Move reachable objects to next generation. */
+    if (young != old) {
+        if (generation == NUM_GENERATIONS - 2) {
+            gcstate->long_lived_pending += gc_list_size(young);
+        }
+        gc_list_merge(young, old);
+    }
+    else {
+        /* We only un-track dicts in full collections, to avoid quadratic
+           dict build-up. See issue #14775. */
+        untrack_dicts(young);
+        gcstate->long_lived_pending = 0;
+        gcstate->long_lived_total = gc_list_size(young);
+    }
 
     /* All objects in unreachable are trash, but objects reachable from
      * legacy finalizers (e.g. tp_del) can't safely be deleted.
@@ -1553,8 +1405,10 @@ gc_collect_region(PyThreadState *tstate,
      * and we move those into the finalizers list too.
      */
     move_legacy_finalizer_reachable(&finalizers);
+
     validate_list(&finalizers, collecting_clear_unreachable_clear);
     validate_list(&unreachable, collecting_set_unreachable_clear);
+
     /* Print debugging information. */
     if (gcstate->debug & _PyGC_DEBUG_COLLECTABLE) {
         for (gc = GC_NEXT(&unreachable); gc != &unreachable; gc = GC_NEXT(gc)) {
@@ -1563,101 +1417,91 @@ gc_collect_region(PyThreadState *tstate,
     }
 
     /* Clear weakrefs and invoke callbacks as necessary. */
-    stats->collected += handle_weakrefs(&unreachable, to);
-    gc_list_validate_space(to, gcstate->visited_space);
-    validate_list(to, collecting_clear_unreachable_clear);
+    m += handle_weakrefs(&unreachable, old);
+
+    validate_list(old, collecting_clear_unreachable_clear);
     validate_list(&unreachable, collecting_set_unreachable_clear);
 
     /* Call tp_finalize on objects which have one. */
     finalize_garbage(tstate, &unreachable);
+
     /* Handle any objects that may have resurrected after the call
      * to 'finalize_garbage' and continue the collection with the
      * objects that are still unreachable */
     PyGC_Head final_unreachable;
-    gc_list_init(&final_unreachable);
-    handle_resurrected_objects(&unreachable, &final_unreachable, to);
+    handle_resurrected_objects(&unreachable, &final_unreachable, old);
 
     /* Call tp_clear on objects in the final_unreachable set.  This will cause
     * the reference cycles to be broken.  It may also cause some objects
     * in finalizers to be freed.
     */
-    stats->collected += gc_list_size(&final_unreachable);
-    delete_garbage(tstate, gcstate, &final_unreachable, to);
+    m += gc_list_size(&final_unreachable);
+    delete_garbage(tstate, gcstate, &final_unreachable, old);
 
     /* Collect statistics on uncollectable objects found and print
      * debugging information. */
-    Py_ssize_t n = 0;
     for (gc = GC_NEXT(&finalizers); gc != &finalizers; gc = GC_NEXT(gc)) {
         n++;
-        if (gcstate->debug & _PyGC_DEBUG_COLLECTABLE)
+        if (gcstate->debug & _PyGC_DEBUG_UNCOLLECTABLE)
             debug_cycle("uncollectable", FROM_GC(gc));
     }
-    stats->uncollectable = n;
+    if (gcstate->debug & _PyGC_DEBUG_STATS) {
+        PyTime_t t2;
+        (void)PyTime_PerfCounterRaw(&t2);
+        double d = PyTime_AsSecondsDouble(t2 - t1);
+        PySys_WriteStderr(
+            "gc: done, %zd unreachable, %zd uncollectable, %.4fs elapsed\n",
+            n+m, n, d);
+    }
+
     /* Append instances in the uncollectable set to a Python
      * reachable list of garbage.  The programmer has to deal with
      * this if they insist on creating this type of structure.
      */
-    handle_legacy_finalizers(tstate, gcstate, &finalizers, to);
-    gc_list_validate_space(to, gcstate->visited_space);
-    validate_list(to, collecting_clear_unreachable_clear);
-}
+    handle_legacy_finalizers(tstate, gcstate, &finalizers, old);
+    validate_list(old, collecting_clear_unreachable_clear);
 
-/* Invoke progress callbacks to notify clients that garbage collection
- * is starting or stopping
- */
-static void
-do_gc_callback(GCState *gcstate, const char *phase,
-                   int generation, struct gc_collection_stats *stats)
-{
-    assert(!PyErr_Occurred());
+    /* Clear free list only during the collection of the highest
+     * generation */
+    if (generation == NUM_GENERATIONS-1) {
+        _PyGC_ClearAllFreeLists(tstate->interp);
+    }
 
-    /* The local variable cannot be rebound, check it for sanity */
-    assert(PyList_CheckExact(gcstate->callbacks));
-    PyObject *info = NULL;
-    if (PyList_GET_SIZE(gcstate->callbacks) != 0) {
-        info = Py_BuildValue("{sisnsn}",
-            "generation", generation,
-            "collected", stats->collected,
-            "uncollectable", stats->uncollectable);
-        if (info == NULL) {
-            PyErr_FormatUnraisable("Exception ignored on invoking gc callbacks");
-            return;
+    if (_PyErr_Occurred(tstate)) {
+        if (reason == _Py_GC_REASON_SHUTDOWN) {
+            _PyErr_Clear(tstate);
+        }
+        else {
+            PyErr_FormatUnraisable("Exception ignored in garbage collection");
         }
     }
 
-    PyObject *phase_obj = PyUnicode_FromString(phase);
-    if (phase_obj == NULL) {
-        Py_XDECREF(info);
-        PyErr_FormatUnraisable("Exception ignored on invoking gc callbacks");
-        return;
+    /* Update stats */
+    struct gc_generation_stats *stats = &gcstate->generation_stats[generation];
+    stats->collections++;
+    stats->collected += m;
+    stats->uncollectable += n;
+
+    GC_STAT_ADD(generation, objects_collected, m);
+#ifdef Py_STATS
+    if (_Py_stats) {
+        GC_STAT_ADD(generation, object_visits,
+            _Py_stats->object_stats.object_visits);
+        _Py_stats->object_stats.object_visits = 0;
     }
+#endif
 
-    PyObject *stack[] = {phase_obj, info};
-    for (Py_ssize_t i=0; i<PyList_GET_SIZE(gcstate->callbacks); i++) {
-        PyObject *r, *cb = PyList_GET_ITEM(gcstate->callbacks, i);
-        Py_INCREF(cb); /* make sure cb doesn't go away */
-        r = PyObject_Vectorcall(cb, stack, 2, NULL);
-        if (r == NULL) {
-            PyErr_WriteUnraisable(cb);
-        }
-        else {
-            Py_DECREF(r);
-        }
-        Py_DECREF(cb);
+    if (PyDTrace_GC_DONE_ENABLED()) {
+        PyDTrace_GC_DONE(n + m);
     }
-    Py_DECREF(phase_obj);
-    Py_XDECREF(info);
-    assert(!PyErr_Occurred());
-}
 
-static void
-invoke_gc_callback(GCState *gcstate, const char *phase,
-                   int generation, struct gc_collection_stats *stats)
-{
-    if (gcstate->callbacks == NULL) {
-        return;
+    if (reason != _Py_GC_REASON_SHUTDOWN) {
+        invoke_gc_callback(tstate, "stop", generation, m, n);
     }
-    do_gc_callback(gcstate, phase, generation, stats);
+
+    assert(!_PyErr_Occurred(tstate));
+    _Py_atomic_store_int(&gcstate->collecting, 0);
+    return n + m;
 }
 
 static int
@@ -1719,25 +1563,20 @@ _PyGC_GetObjects(PyInterpreterState *interp, int generation)
     GCState *gcstate = &interp->gc;
 
     PyObject *result = PyList_New(0);
-    /* Generation:
-     * -1: Return all objects
-     * 0: All young objects
-     * 1: No objects
-     * 2: All old objects
-     */
-    if (result == NULL || generation == 1) {
-        return result;
+    if (result == NULL) {
+        return NULL;
     }
-    if (generation <= 0) {
-        if (append_objects(result, &gcstate->young.head)) {
-            goto error;
+
+    if (generation == -1) {
+        /* If generation is -1, get all objects from all generations */
+        for (int i = 0; i < NUM_GENERATIONS; i++) {
+            if (append_objects(result, GEN_HEAD(gcstate, i))) {
+                goto error;
+            }
         }
     }
-    if (generation != 0) {
-        if (append_objects(result, &gcstate->old[0].head)) {
-            goto error;
-        }
-        if (append_objects(result, &gcstate->old[1].head)) {
+    else {
+        if (append_objects(result, GEN_HEAD(gcstate, generation))) {
             goto error;
         }
     }
@@ -1752,20 +1591,10 @@ void
 _PyGC_Freeze(PyInterpreterState *interp)
 {
     GCState *gcstate = &interp->gc;
-    /* The permanent_generation has its old space bit set to zero */
-    if (gcstate->visited_space) {
-        gc_list_set_space(&gcstate->young.head, 0);
-    }
-    gc_list_merge(&gcstate->young.head, &gcstate->permanent_generation.head);
-    gcstate->young.count = 0;
-    PyGC_Head*old0 = &gcstate->old[0].head;
-    PyGC_Head*old1 = &gcstate->old[1].head;
-    gc_list_merge(old0, &gcstate->permanent_generation.head);
-    gcstate->old[0].count = 0;
-    gc_list_set_space(old1, 0);
-    gc_list_merge(old1, &gcstate->permanent_generation.head);
-    gcstate->old[1].count = 0;
-    validate_old(gcstate);
+    for (int i = 0; i < NUM_GENERATIONS; ++i) {
+        gc_list_merge(GEN_HEAD(gcstate, i), &gcstate->permanent_generation.head);
+        gcstate->generations[i].count = 0;
+    }
 }
 
 void
@@ -1773,8 +1602,7 @@ _PyGC_Unfreeze(PyInterpreterState *interp)
 {
     GCState *gcstate = &interp->gc;
     gc_list_merge(&gcstate->permanent_generation.head,
-                  &gcstate->old[0].head);
-    validate_old(gcstate);
+                  GEN_HEAD(gcstate, NUM_GENERATIONS-1));
 }
 
 Py_ssize_t
@@ -1810,85 +1638,29 @@ PyGC_IsEnabled(void)
     return gcstate->enabled;
 }
 
-// Show stats for objects in each generations
-static void
-show_stats_each_generations(GCState *gcstate)
-{
-    char buf[100];
-    size_t pos = 0;
-
-    for (int i = 0; i < NUM_GENERATIONS && pos < sizeof(buf); i++) {
-        pos += PyOS_snprintf(buf+pos, sizeof(buf)-pos,
-                             " %zd",
-                             gc_list_size(GEN_HEAD(gcstate, i)));
-    }
-    PySys_FormatStderr(
-        "gc: objects in each generation:%s\n"
-        "gc: objects in permanent generation: %zd\n",
-        buf, gc_list_size(&gcstate->permanent_generation.head));
-}
-
+/* Public API to invoke gc.collect() from C */
 Py_ssize_t
-_PyGC_Collect(PyThreadState *tstate, int generation, _PyGC_Reason reason)
+PyGC_Collect(void)
 {
+    PyThreadState *tstate = _PyThreadState_GET();
     GCState *gcstate = &tstate->interp->gc;
 
-    int expected = 0;
-    if (!_Py_atomic_compare_exchange_int(&gcstate->collecting, &expected, 1)) {
-        // Don't start a garbage collection if one is already in progress.
+    if (!gcstate->enabled) {
         return 0;
     }
 
-    struct gc_collection_stats stats = { 0 };
-    if (reason != _Py_GC_REASON_SHUTDOWN) {
-        invoke_gc_callback(gcstate, "start", generation, &stats);
-    }
-    if (gcstate->debug & _PyGC_DEBUG_STATS) {
-        PySys_WriteStderr("gc: collecting generation %d...\n", generation);
-        show_stats_each_generations(gcstate);
-    }
-    if (PyDTrace_GC_START_ENABLED()) {
-        PyDTrace_GC_START(generation);
-    }
+    Py_ssize_t n;
     PyObject *exc = _PyErr_GetRaisedException(tstate);
-    switch(generation) {
-        case 0:
-            gc_collect_young(tstate, &stats);
-            break;
-        case 1:
-            gc_collect_increment(tstate, &stats);
-            break;
-        case 2:
-            gc_collect_full(tstate, &stats);
-            break;
-        default:
-            Py_UNREACHABLE();
-    }
-    if (PyDTrace_GC_DONE_ENABLED()) {
-        PyDTrace_GC_DONE(stats.uncollectable + stats.collected);
-    }
-    if (reason != _Py_GC_REASON_SHUTDOWN) {
-        invoke_gc_callback(gcstate, "stop", generation, &stats);
-    }
+    n = gc_collect_main(tstate, NUM_GENERATIONS - 1, _Py_GC_REASON_MANUAL);
     _PyErr_SetRaisedException(tstate, exc);
-    GC_STAT_ADD(generation, objects_collected, stats.collected);
-#ifdef Py_STATS
-    if (_Py_stats) {
-        GC_STAT_ADD(generation, object_visits,
-            _Py_stats->object_stats.object_visits);
-        _Py_stats->object_stats.object_visits = 0;
-    }
-#endif
-    validate_old(gcstate);
-    _Py_atomic_store_int(&gcstate->collecting, 0);
-    return stats.uncollectable + stats.collected;
+
+    return n;
 }
 
-/* Public API to invoke gc.collect() from C */
 Py_ssize_t
-PyGC_Collect(void)
+_PyGC_Collect(PyThreadState *tstate, int generation, _PyGC_Reason reason)
 {
-    return _PyGC_Collect(_PyThreadState_GET(), 2, _Py_GC_REASON_MANUAL);
+    return gc_collect_main(tstate, generation, reason);
 }
 
 void
@@ -1900,7 +1672,7 @@ _PyGC_CollectNoFail(PyThreadState *tstate)
        during interpreter shutdown (and then never finish it).
        See http://bugs.python.org/issue8713#msg195178 for an example.
        */
-    _PyGC_Collect(_PyThreadState_GET(), 2, _Py_GC_REASON_SHUTDOWN);
+    gc_collect_main(tstate, NUM_GENERATIONS - 1, _Py_GC_REASON_SHUTDOWN);
 }
 
 void
@@ -1974,9 +1746,9 @@ _PyGC_Fini(PyInterpreterState *interp)
      * This bug was originally fixed when reported as gh-90228.  The bug was
      * re-introduced in gh-94673.
      */
-    finalize_unlink_gc_head(&gcstate->young.head);
-    finalize_unlink_gc_head(&gcstate->old[0].head);
-    finalize_unlink_gc_head(&gcstate->old[1].head);
+    for (int i = 0; i < NUM_GENERATIONS; i++) {
+        finalize_unlink_gc_head(&gcstate->generations[i].head);
+    }
     finalize_unlink_gc_head(&gcstate->permanent_generation.head);
 }
 
@@ -2062,11 +1834,10 @@ _PyObject_GC_Link(PyObject *op)
     GCState *gcstate = &tstate->interp->gc;
     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->generations[0].count++; /* number of allocated GC objects */
+    if (gcstate->generations[0].count > gcstate->generations[0].threshold &&
         gcstate->enabled &&
-        gcstate->young.threshold &&
+        gcstate->generations[0].threshold &&
         !_Py_atomic_load_int_relaxed(&gcstate->collecting) &&
         !_PyErr_Occurred(tstate))
     {
@@ -2077,9 +1848,11 @@ _PyObject_GC_Link(PyObject *op)
 void
 _Py_RunGC(PyThreadState *tstate)
 {
-    if (tstate->interp->gc.enabled) {
-        _PyGC_Collect(tstate, 1, _Py_GC_REASON_HEAP);
+    GCState *gcstate = get_gc_state();
+    if (!gcstate->enabled) {
+        return;
     }
+    gc_collect_main(tstate, GENERATION_AUTO, _Py_GC_REASON_HEAP);
 }
 
 static PyObject *
@@ -2190,10 +1963,9 @@ PyObject_GC_Del(void *op)
 #endif
     }
     GCState *gcstate = get_gc_state();
-    if (gcstate->young.count > 0) {
-        gcstate->young.count--;
+    if (gcstate->generations[0].count > 0) {
+        gcstate->generations[0].count--;
     }
-    gcstate->heap_size--;
     PyObject_Free(((char *)op)-presize);
 }
 
@@ -2215,36 +1987,26 @@ PyObject_GC_IsFinalized(PyObject *obj)
     return 0;
 }
 
-static int
-visit_generation(gcvisitobjects_t callback, void *arg, struct gc_generation *gen)
-{
-    PyGC_Head *gc_list, *gc;
-    gc_list = &gen->head;
-    for (gc = GC_NEXT(gc_list); gc != gc_list; gc = GC_NEXT(gc)) {
-        PyObject *op = FROM_GC(gc);
-        Py_INCREF(op);
-        int res = callback(op, arg);
-        Py_DECREF(op);
-        if (!res) {
-            return -1;
-        }
-    }
-    return 0;
-}
-
 void
 PyUnstable_GC_VisitObjects(gcvisitobjects_t callback, void *arg)
 {
+    size_t i;
     GCState *gcstate = get_gc_state();
     int origenstate = gcstate->enabled;
     gcstate->enabled = 0;
-    if (visit_generation(callback, arg, &gcstate->young)) {
-        goto done;
-    }
-    if (visit_generation(callback, arg, &gcstate->old[0])) {
-        goto done;
+    for (i = 0; i < NUM_GENERATIONS; i++) {
+        PyGC_Head *gc_list, *gc;
+        gc_list = GEN_HEAD(gcstate, i);
+        for (gc = GC_NEXT(gc_list); gc != gc_list; gc = GC_NEXT(gc)) {
+            PyObject *op = FROM_GC(gc);
+            Py_INCREF(op);
+            int res = callback(op, arg);
+            Py_DECREF(op);
+            if (!res) {
+                goto done;
+            }
+        }
     }
-    visit_generation(callback, arg, &gcstate->old[1]);
 done:
     gcstate->enabled = origenstate;
 }
diff --git a/Python/gc_free_threading.c b/Python/gc_free_threading.c
index 1969ed608ea524c..f6c766bc2c2428c 100644
--- a/Python/gc_free_threading.c
+++ b/Python/gc_free_threading.c
@@ -836,7 +836,7 @@ void
 _PyGC_InitState(GCState *gcstate)
 {
     // TODO: move to pycore_runtime_init.h once the incremental GC lands.
-    gcstate->young.threshold = 2000;
+    gcstate->generations[0].threshold = 2000;
 }
 
 
@@ -1155,8 +1155,8 @@ cleanup_worklist(struct worklist *worklist)
 static bool
 gc_should_collect(GCState *gcstate)
 {
-    int count = _Py_atomic_load_int_relaxed(&gcstate->young.count);
-    int threshold = gcstate->young.threshold;
+    int count = _Py_atomic_load_int_relaxed(&gcstate->generations[0].count);
+    int threshold = gcstate->generations[0].threshold;
     if (count <= threshold || threshold == 0 || !gcstate->enabled) {
         return false;
     }
@@ -1164,7 +1164,7 @@ gc_should_collect(GCState *gcstate)
     // objects. A few tests rely on immediate scheduling of the GC so we ignore
     // the scaled threshold if generations[1].threshold is set to zero.
     return (count > gcstate->long_lived_total / 4 ||
-            gcstate->old[0].threshold == 0);
+            gcstate->generations[1].threshold == 0);
 }
 
 static void
@@ -1178,7 +1178,7 @@ record_allocation(PyThreadState *tstate)
     if (gc->alloc_count >= LOCAL_ALLOC_COUNT_THRESHOLD) {
         // TODO: Use Py_ssize_t for the generation count.
         GCState *gcstate = &tstate->interp->gc;
-        _Py_atomic_add_int(&gcstate->young.count, (int)gc->alloc_count);
+        _Py_atomic_add_int(&gcstate->generations[0].count, (int)gc->alloc_count);
         gc->alloc_count = 0;
 
         if (gc_should_collect(gcstate) &&
@@ -1197,7 +1197,7 @@ record_deallocation(PyThreadState *tstate)
     gc->alloc_count--;
     if (gc->alloc_count <= -LOCAL_ALLOC_COUNT_THRESHOLD) {
         GCState *gcstate = &tstate->interp->gc;
-        _Py_atomic_add_int(&gcstate->young.count, (int)gc->alloc_count);
+        _Py_atomic_add_int(&gcstate->generations[0].count, (int)gc->alloc_count);
         gc->alloc_count = 0;
     }
 }
@@ -1209,12 +1209,10 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
 
     // update collection and allocation counters
     if (generation+1 < NUM_GENERATIONS) {
-        state->gcstate->old[generation].count += 1;
+        state->gcstate->generations[generation+1].count += 1;
     }
-
-    state->gcstate->young.count = 0;
-    for (int i = 1; i <= generation; ++i) {
-        state->gcstate->old[i-1].count = 0;
+    for (int i = 0; i <= generation; i++) {
+        state->gcstate->generations[i].count = 0;
     }
 
     HEAD_LOCK(&_PyRuntime);