8000 GH-117759: Document incremental GC (GH-123266) · python/cpython@f49a916 · GitHub
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GH-117759: Document incremental GC (GH-123266)
* Update what's new * Update gc module docs and fix inconsistency in gc.get_objects
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‎Doc/library/gc.rst

Lines changed: 40 additions & 15 deletions
Original file line numberDiff line numberDiff line change
@@ -40,11 +40,18 @@ The :mod:`gc` module provides the following functions:
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.. function:: collect(generation=2)
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43-
With no arguments, run a full collection. The optional argument *generation*
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Perform a collection. The optional argument *generation*
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may be an integer specifying which generation to collect (from 0 to 2). A
45-
:exc:`ValueError` is raised if the generation number is invalid. The sum of
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:exc:`ValueError` is raised if the generation number is invalid. The sum of
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collected objects and uncol 8000 lectable objects is returned.
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48+
Calling ``gc.collect(0)`` will perform a GC collection on the young generation.
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Calling ``gc.collect(1)`` will perform a GC collection on the young generation
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and an increment of the old generation.
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53+
Calling ``gc.collect(2)`` or ``gc.collect()`` performs a full collection
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4855
The free lists maintained for a number of built-in types are cleared
4956
whenever a full collection or collection of the highest generation (2)
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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:
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The effect of calling ``gc.collect()`` while the interpreter is already
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performing a collection is undefined.
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.. versionchanged:: 3.13
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``generation=1`` performs an increment of collection.
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.. function:: set_debug(flags)
5868

@@ -68,13 +78,20 @@ The :mod:`gc` module provides the following functions:
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.. function:: get_objects(generation=None)
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81+
7182
Returns a list of all objects tracked by the collector, excluding the list
72-
returned. If *generation* is not ``None``, return only the objects tracked by
73-
the collector that are in that generation.
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returned. If *generation* is not ``None``, return only the objects as follows:
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* 0: All objects in the young generation
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* 1: No objects, as there is no generation 1 (as of Python 3.13)
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* 2: All objects in the old generation
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.. versionchanged:: 3.8
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New *generation* parameter.
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.. versionchanged:: 3.13
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Generation 1 is removed
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.. audit-event:: gc.get_objects generation gc.get_objects
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.. function:: get_stats()
@@ -101,19 +118,27 @@ The :mod:`gc` module provides the following functions:
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Set the garbage collection thresholds (the collection frequency). Setting
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*threshold0* to zero disables collection.
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The GC classifies objects into three generations depending on how many
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collection sweeps they have survived. New objects are placed in the youngest
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generation (generation ``0``). If an object survives a collection it is moved
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into the next older generation. Since generation ``2`` is the oldest
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generation, objects in that generation remain there after a collection. In
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order to decide when to run, the collector keeps track of the number object
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The GC classifies objects into two generations depending on whether they have
122+
survived a collection. New objects are placed in the young generation. If an
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object survives a collection it is moved into the old generation.
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In order to decide when to run, the collector keeps track of the number of object
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allocations and deallocations since the last collection. When the number of
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allocations minus the number of deallocations exceeds *threshold0*, collection
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starts. Initially only generation ``0`` is examined. If generation ``0`` has
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been examined more than *threshold1* times since generation ``1`` has been
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examined, then generation ``1`` is examined as well.
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With the third generation, things are a bit more complicated,
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see `Collecting the oldest generation <https://devguide.python.org/garbage_collector/#collecting-the-oldest-generation>`_ for more information.
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starts. For each collection, all the objects in the young generation and some
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fraction of the old generation is collected.
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The fraction of the old generation that is collected is **inversely** proportional
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to *threshold1*. The larger *threshold1* is, the slower objects in the old generation
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are collected.
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For the default value of 10, 1% of the old generation is scanned during each collection.
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*threshold2* is ignored.
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See `Garbage collector design <https://devguide.python.org/garbage_collector>`_ for more information.
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.. versionchanged:: 3.13
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*threshold2* is ignored
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.. function:: get_count()

‎Doc/whatsnew/3.13.rst

Lines changed: 13 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -495,6 +495,19 @@ Incremental garbage collection
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The cycle garbage collector is now incremental.
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This means that maximum pause times are reduced
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by an order of magnitude or more for larger heaps.
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There are now only two generations: young and old.
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When :func:`gc.collect` is not called directly, the
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GC is invoked a little less frequently. When invoked, it
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collects the young generation and an increment of the
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old generation, instead of collecting one or more generations.
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The behavior of :func:`!gc.collect` changes slightly:
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* ``gc.collect(1)``: Performs an increment of GC,
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rather than collecting generation 1.
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* Other calls to :func:`!gc.collect` are unchanged.
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(Contributed by Mark Shannon in :gh:`108362`.)
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‎Python/gc.c

Lines changed: 16 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -1712,20 +1712,25 @@ _PyGC_GetObjects(PyInterpreterState *interp, int generation)
17121712
GCState *gcstate = &interp->gc;
17131713

17141714
PyObject *result = PyList_New(0);
1715-
if (result == NULL) {
1716-
return NULL;
1715+
/* Generation:
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* -1: Return all objects
1717+
* 0: All young objects
1718+
* 1: No objects
1719+
* 2: All old objects
1720+
*/
1721+
if (result == NULL || generation == 1) {
1722+
return result;
17171723
}
1718-
1719-
if (generation == -1) {
1720-
/* If generation is -1, get all objects from all generations */
1721-
for (int i = 0; i < NUM_GENERATIONS; i++) {
1722-
if (append_objects(result, GEN_HEAD(gcstate, i))) {
1723-
goto error;
1724-
}
1724+
if (generation <= 0) {
1725+
if (append_objects(result, &gcstate->young.head)) {
1726+
goto error;
17251727
}
17261728
}
1727-
else {
1728-
if (append_objects(result, GEN_HEAD(gcstate, generation))) {
1729+
if (generation != 0) {
1730+
if (append_objects(result, &gcstate->old[0].head)) {
1731+
goto error;
1732+
}
1733+
if (append_objects(result, &gcstate->old[1].head)) {
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goto error;
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}
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}

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