GH-117759: Document incremental GC (GH-123266)

* Update what's new

* Update gc module docs and fix inconsistency in gc.get_objects

Doc/library/gc.rst

@@ -40,11 +40,18 @@ The :mod:`gc` module provides the following functions:
 
 .. function:: collect(generation=2)
 
-   With no arguments, run a full collection.  The optional argument *generation*
+   Perform a 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
+   :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
@@ -53,6 +60,9 @@ 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)
 
@@ -68,13 +78,20 @@ 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 tracked by
-   the collector that are in that generation.
+   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
 
    .. 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()
@@ -101,19 +118,27 @@ 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 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
+   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
    allocations and deallocations since the last collection.  When the number of
    allocations minus the number of deallocations exceeds *threshold0*, collection
-   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.
+   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
 
 
 .. function:: get_count()

Doc/whatsnew/3.13.rst

@@ -495,6 +495,19 @@ 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.
+
+There are now only two generations: young and old.
+When :func:`gc.collect` is not called directly, the
+GC is invoked a little less frequently. When invoked, it
+collects the young generation and an increment of the
+old generation, instead of collecting one or more generations.
+
+The behavior of :func:`!gc.collect` changes slightly:
+
+* ``gc.collect(1)``: Performs an increment of GC,
+  rather than collecting generation 1.
+* Other calls to :func:`!gc.collect` are unchanged.
+
 (Contributed by Mark Shannon in :gh:`108362`.)
 
 

Python/gc.c

@@ -1712,20 +1712,25 @@ _PyGC_GetObjects(PyInterpreterState *interp, int generation)
     GCState *gcstate = &interp->gc;
 
     PyObject *result = PyList_New(0);
-    if (result == NULL) {
-        return NULL;
+    /* Generation:
+     * -1: Return all objects
+     * 0: All young objects
+     * 1: No objects
+     * 2: All old objects
+     */
+    if (result == NULL || generation == 1) {
+        return result;
     }
-
-    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->young.head)) {
+            goto error;
         }
     }
-    else {
-        if (append_objects(result, GEN_HEAD(gcstate, generation))) {
+    if (generation != 0) {
+        if (append_objects(result, &gcstate->old[0].head)) {
+            goto error;
+        }
+        if (append_objects(result, &gcstate->old[1].head)) {
             goto error;
         }
     }