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Reference Cycle Garbage Collection

==================================

Neil Schemenauer <nas@arctrix.com>

Based on a post on the python-dev list. Ideas from Guido van Rossum,

Eric Tiedemann, and various others.

http://www.arctrix.com/nas/python/gc/

The following mailing list threads provide a historical perspective on

the design of this module. Note that a fair amount of refinement has

occurred since those discussions.

http://mail.python.org/pipermail/python-dev/2000-March/002385.html

http://mail.python.org/pipermail/python-dev/2000-March/002434.html

http://mail.python.org/pipermail/python-dev/2000-March/002497.html

For a highlevel view of the collection process, read the collect

function.

#include "Python.h"

#include "frameobject.h" /* for PyFrame_ClearFreeList */

/* Get an object's GC head */

#define AS_GC(o) ((PyGC_Head *)(o)-1)

/* Get the object given the GC head */

#define FROM_GC(g) ((PyObject *)(((PyGC_Head *)g)+1))

/*** Global GC state ***/

struct gc_generation {

PyGC_Head head;

int threshold; /* collection threshold */

int count; /* count of allocations or collections of younger

generations */

};

#define NUM_GENERATIONS 3

#define GEN_HEAD(n) (&generations[n].head)

/* linked lists of container objects */

static struct gc_generation generations[NUM_GENERATIONS] = {

/* PyGC_Head, threshold, count */

{{{GEN_HEAD(0), GEN_HEAD(0), 0}}, 700, 0},

{{{GEN_HEAD(1), GEN_HEAD(1), 0}}, 10, 0},

{{{GEN_HEAD(2), GEN_HEAD(2), 0}}, 10, 0},

};

PyGC_Head *_PyGC_generation0 = GEN_HEAD(0);

static int enabled = 1; /* automatic collection enabled? */

/* true if we are currently running the collector */

static int collecting = 0;

/* list of uncollectable objects */

static PyObject *garbage = NULL;

/* Python string to use if unhandled exception occurs */

static PyObject *gc_str = NULL;

/* a list of callbacks to be invoked when collection is performed */

static PyObject *callbacks = NULL;

/* This is the number of objects that survived the last full collection. It

approximates the number of long lived objects tracked by the GC.

(by "full collection", we mean a collection of the oldest generation).

static Py_ssize_t long_lived_total = 0;

/* This is the number of objects that survived all "non-full" collections,

and are awaiting to undergo a full collection for the first time.

static Py_ssize_t long_lived_pending = 0;

NOTE: about the counting of long-lived objects.

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

NOTE: about untracking of mutable objects.

Certain types of container cannot participate in a reference cycle, and

so do not need to be tracked by the garbage collector. Untracking these

objects reduces the cost of garbage collections. However, determining

which objects may be untracked is not free, and the costs must be

weighed against the benefits for garbage collection.

There are two possible strategies for when to untrack a container:

i) When the container is created.

ii) When the container is examined by the garbage collector.

Tuples containing only immutable objects (integers, strings etc, and

recursively, tuples of immutable objects) do not need to be tracked.

The interpreter creates a large number of tuples, many of which will

not survive until garbage collection. It is therefore not worthwhile

to untrack eligible 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.

The module provides the python function is_tracked(obj), which returns

the CURRENT tracking status of the object. Subsequent garbage

collections may change the tracking status of the object.

Untracking of certain containers was introduced in issue #4688, and

the algorithm was refined in response to issue #14775.

/* set for debugging information */

#define DEBUG_STATS (1<<0) /* print collection statistics */

#define DEBUG_COLLECTABLE (1<<1) /* print collectable objects */

#define DEBUG_UNCOLLECTABLE (1<<2) /* print uncollectable objects */

#define DEBUG_SAVEALL (1<<5) /* save all garbage in gc.garbage */

#define DEBUG_LEAK DEBUG_COLLECTABLE | \

DEBUG_UNCOLLECTABLE | \

DEBUG_SAVEALL

static int debug;

static PyObject *tmod = NULL;

/* Running stats per generation */

struct gc_generation_stats {

/* total number of collections */

Py_ssize_t collections;

/* total number of collected objects */

Py_ssize_t collected;

/* total number of uncollectable objects (put into gc.garbage) */

Py_ssize_t uncollectable;

};

static struct gc_generation_stats generation_stats[NUM_GENERATIONS];

/*--------------------------------------------------------------------------

gc_refs values.

Between collections, every gc'ed object has one of two gc_refs values:

GC_UNTRACKED

The initial state; objects returned by PyObject_GC_Malloc are in this

state. The object doesn't live in any generation list, and its

tp_traverse slot must not be called.

GC_REACHABLE

The object lives in some generation list, and its tp_traverse is safe to

call. An object transitions to GC_REACHABLE when PyObject_GC_Track

is called.

During a collection, gc_refs can temporarily take on other states:

>= 0

At the start of a collection, update_refs() copies the true refcount

to gc_refs, for each object in the generation being collected.

subtract_refs() then adjusts gc_refs so that it equals the number of

times an object is referenced directly from outside the generation

being collected.

gc_refs remains >= 0 throughout these steps.

GC_TENTATIVELY_UNREACHABLE

move_unreachable() then moves objects not reachable (whether directly or

indirectly) from outside the generation into an "unreachable" set.

Objects that are found to be reachable have gc_refs set to GC_REACHABLE

again. Objects that are found to be unreachable have gc_refs set to

GC_TENTATIVELY_UNREACHABLE. It's "tentatively" because the pass doing

this can't be sure until it ends, and GC_TENTATIVELY_UNREACHABLE may

transition back to GC_REACHABLE.

Only objects with GC_TENTATIVELY_UNREACHABLE still set are candidates

for collection. If it's decided not to collect such an object (e.g.,

it has a __del__ method), its gc_refs is restored to GC_REACHABLE again.

----------------------------------------------------------------------------

#define GC_UNTRACKED _PyGC_REFS_UNTRACKED

#define GC_REACHABLE _PyGC_REFS_REACHABLE

#define GC_TENTATIVELY_UNREACHABLE _PyGC_REFS_TENTATIVELY_UNREACHABLE

#define IS_TRACKED(o) (_PyGC_REFS(o) != GC_UNTRACKED)

#define IS_REACHABLE(o) (_PyGC_REFS(o) == GC_REACHABLE)

#define IS_TENTATIVELY_UNREACHABLE(o) ( \

_PyGC_REFS(o) == GC_TENTATIVELY_UNREACHABLE)

/*** list functions ***/

static void

gc_list_init(PyGC_Head *list)

{

list->gc.gc_prev = list;

list->gc.gc_next = list;

}

static int

gc_list_is_empty(PyGC_Head *list)

{

return (list->gc.gc_next == list);

}

#if 0

/* This became unused after gc_list_move() was introduced. */

/* Append `node` to `list`. */

static void

gc_list_append(PyGC_Head *node, PyGC_Head *list)

{

node->gc.gc_next = list;

node->gc.gc_prev = list->gc.gc_prev;

node->gc.gc_prev->gc.gc_next = node;

list->gc.gc_prev = node;

}

#endif

/* Remove `node` from the gc list it's currently in. */

static void

gc_list_remove(PyGC_Head *node)

{

node->gc.gc_prev->gc.gc_next = node->gc.gc_next;

node->gc.gc_next->gc.gc_prev = node->gc.gc_prev;

node->gc.gc_next = NULL; /* object is not currently tracked */

}

/* Move `node` from the gc list it's currently in (which is not explicitly

* named here) to the end of `list`. This is semantically the same as

* gc_list_remove(node) followed by gc_list_append(node, list).

static void

gc_list_move(PyGC_Head *node, PyGC_Head *list)

{

PyGC_Head *new_prev;

PyGC_Head *current_prev = node->gc.gc_prev;

PyGC_Head *current_next = node->gc.gc_next;

/* Unlink from current list. */

current_prev->gc.gc_next = current_next;

current_next->gc.gc_prev = current_prev;

/* Relink at end of new list. */

new_prev = node->gc.gc_prev = list->gc.gc_prev;

new_prev->gc.gc_next = list->gc.gc_prev = node;

node->gc.gc_next = list;

}

/* append list `from` onto list `to`; `from` becomes an empty list */

static void

gc_list_merge(PyGC_Head *from, PyGC_Head *to)

{

PyGC_Head *tail;

assert(from != to);

if (!gc_list_is_empty(from)) {

tail = to->gc.gc_prev;

tail->gc.gc_next = from->gc.gc_next;

tail->gc.gc_next->gc.gc_prev = tail;

to->gc.gc_prev = from->gc.gc_prev;

to->gc.gc_prev->gc.gc_next = to;

}

gc_list_init(from);

}

static Py_ssize_t

gc_list_size(PyGC_Head *list)

{

PyGC_Head *gc;

Py_ssize_t n = 0;

for (gc = list->gc.gc_next; gc != list; gc = gc->gc.gc_next) {

n++;

}

return n;

}

/* Append objects in a GC list to a Python list.

* Return 0 if all OK, < 0 if error (out of memory for list).

static int

append_objects(PyObject *py_list, PyGC_Head *gc_list)

{

PyGC_Head *gc;

for (gc = gc_list->gc.gc_next; gc != gc_list; gc = gc->gc.gc_next) {

PyObject *op = FROM_GC(gc);

if (op != py_list) {

if (PyList_Append(py_list, op)) {

return -1; /* exception */

}

return 0;

}

/*** end of list stuff ***/

/* Set all gc_refs = ob_refcnt. After this, gc_refs is > 0 for all objects

* in containers, and is GC_REACHABLE for all tracked gc objects not in

* containers.

static void

update_refs(PyGC_Head *containers)

{

PyGC_Head *gc = containers->gc.gc_next;

for (; gc != containers; gc = gc->gc.gc_next) {

assert(_PyGCHead_REFS(gc) == GC_REACHABLE);

_PyGCHead_SET_REFS(gc, Py_REFCNT(FROM_GC(gc)));

/* Python's cyclic gc should never see an incoming refcount

* of 0: if something decref'ed to 0, it should have been

* deallocated immediately at that time.

* Possible cause (if the assert triggers): a tp_dealloc

* routine left a gc-aware object tracked during its teardown

* phase, and did something-- or allowed something to happen --

* that called back into Python. gc can trigger then, and may

* see the still-tracked dying object. Before this assert

* was added, such mistakes went on to allow gc to try to

* delete the object again. In a debug build, that caused

* a mysterious segfault, when _Py_ForgetReference tried

* to remove the object from the doubly-linked list of all

* objects a second time. In a release build, an actual

* double deallocation occurred, which leads to corruption

* of the allocator's internal bookkeeping pointers. That's

* so serious that maybe this should be a release-build

* check instead of an assert?

assert(_PyGCHead_REFS(gc) != 0);

}

/* A traversal callback for subtract_refs. */

static int

visit_decref(PyObject *op, void *data)

{

assert(op != NULL);

if (PyObject_IS_GC(op)) {

PyGC_Head *gc = AS_GC(op);

/* We're only interested in gc_refs for objects in the

* generation being collected, which can be recognized

* because only they have positive gc_refs.

assert(_PyGCHead_REFS(gc) != 0); /* else refcount was too small */

if (_PyGCHead_REFS(gc) > 0)

_PyGCHead_DECREF(gc);

}

return 0;

}

/* Subtract internal references from gc_refs. After this, gc_refs is >= 0

* for all objects in containers, and is GC_REACHABLE for all tracked gc

* objects not in containers. The ones with gc_refs > 0 are directly

* reachable from outside containers, and so can't be collected.

static void

subtract_refs(PyGC_Head *containers)

{

traverseproc traverse;

PyGC_Head *gc = containers->gc.gc_next;

for (; gc != containers; gc=gc->gc.gc_next) {

traverse = Py_TYPE(FROM_GC(gc))->tp_traverse;

(void) traverse(FROM_GC(gc),

(visitproc)visit_decref,

NULL);

}

/* A traversal callback for move_unreachable. */

static int

visit_reachable(PyObject *op, PyGC_Head *reachable)

{

if (PyObject_IS_GC(op)) {

PyGC_Head *gc = AS_GC(op);

const Py_ssize_t gc_refs = _PyGCHead_REFS(gc);

if (gc_refs == 0) {

/* This is in move_unreachable's 'young' list, but

* the traversal hasn't yet gotten to it. All

* we need to do is tell move_unreachable that it's

* reachable.

_PyGCHead_SET_REFS(gc, 1);

}

else if (gc_refs == GC_TENTATIVELY_UNREACHABLE) {

/* This had gc_refs = 0 when move_unreachable got

* to it, but turns out it's reachable after all.

* Move it back to move_unreachable's 'young' list,

* and move_unreachable will eventually get to it

* again.

gc_list_move(gc, reachable);

_PyGCHead_SET_REFS(gc, 1);

}

/* Else there's nothing to do.

* If gc_refs > 0, it must be in move_unreachable's 'young'

* list, and move_unreachable will eventually get to it.

* If gc_refs == GC_REACHABLE, it's either in some other

* generation so we don't care about it, or move_unreachable

* already dealt with it.

* If gc_refs == GC_UNTRACKED, it must be ignored.

else {

assert(gc_refs > 0

|| gc_refs == GC_REACHABLE

|| gc_refs == GC_UNTRACKED);

}

return 0;

}

/* Move the unreachable objects from young to unreachable. After this,

* all objects in young have gc_refs = GC_REACHABLE, and all objects in

* unreachable have gc_refs = GC_TENTATIVELY_UNREACHABLE. All tracked

* gc objects not in young or unreachable still have gc_refs = GC_REACHABLE.

* All objects in young after this are directly or indirectly reachable

* from outside the original young; and all objects in unreachable are

* not.

static void

move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)

{

PyGC_Head *gc = young->gc.gc_next;

/* Invariants: all objects "to the left" of us in young have gc_refs

* = GC_REACHABLE, and are indeed reachable (directly or indirectly)

* from outside the young list as it was at entry. All other objects

* from the original young "to the left" of us are in unreachable now,

* and have gc_refs = GC_TENTATIVELY_UNREACHABLE. All objects to the

* left of us in 'young' now have been scanned, and no objects here

* or to the right have been scanned yet.

while (gc != young) {

PyGC_Head *next;

if (_PyGCHead_REFS(gc)) {

/* gc is definitely reachable from outside the

* original 'young'. Mark it as such, and traverse

* its pointers to find any other objects that may

* be directly reachable from it. Note that the

* call to tp_traverse may append objects to young,

* so we have to wait until it returns to determine

* the next object to visit.

PyObject *op = FROM_GC(gc);

traverseproc traverse = Py_TYPE(op)->tp_traverse;

assert(_PyGCHead_REFS(gc) > 0);

_PyGCHead_SET_REFS(gc, GC_REACHABLE);

(void) traverse(op,

(visitproc)visit_reachable,

(void *)young);

next = gc->gc.gc_next;

if (PyTuple_CheckExact(op)) {

_PyTuple_MaybeUntrack(op);

}

else {

/* This *may* be unreachable. To make progress,

* assume it is. gc isn't directly reachable from

* any object we've already traversed, but may be

* reachable from an object we haven't gotten to yet.

* visit_reachable will eventually move gc back into

* young if that's so, and we'll see it again.

next = gc->gc.gc_next;

gc_list_move(gc, unreachable);

_PyGCHead_SET_REFS(gc, GC_TENTATIVELY_UNREACHABLE);

}

gc = next;

}

/* Try to untrack all currently tracked dictionaries */

static void

untrack_dicts(PyGC_Head *head)

{

PyGC_Head *next, *gc = head->gc.gc_next;

while (gc != head) {

PyObject *op = FROM_GC(gc);

next = gc->gc.gc_next;

if (PyDict_CheckExact(op))

_PyDict_MaybeUntrack(op);

gc = next;

}

/* Return true if object has a pre-PEP 442 finalization method. */

static int

has_legacy_finalizer(PyObject *op)

{

return op->ob_type->tp_del != NULL;

}

/* Move the objects in unreachable with tp_del slots into `finalizers`.

* Objects moved into `finalizers` have gc_refs set to GC_REACHABLE; the

* objects remaining in unreachable are left at GC_TENTATIVELY_UNREACHABLE.

static void

move_legacy_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)

{

PyGC_Head *gc;

PyGC_Head *next;

/* March over unreachable. Move objects with finalizers into

* `finalizers`.

for (gc = unreachable->gc.gc_next; gc != unreachable; gc = next) {

PyObject *op = FROM_GC(gc);

assert(IS_TENTATIVELY_UNREACHABLE(op));

next = gc->gc.gc_next;

if (has_legacy_finalizer(op)) {

gc_list_move(gc, finalizers);

_PyGCHead_SET_REFS(gc, GC_REACHABLE);

}

/* A traversal callback for move_legacy_finalizer_reachable. */

static int

visit_move(PyObject *op, PyGC_Head *tolist)

{

if (PyObject_IS_GC(op)) {

if (IS_TENTATIVELY_UNREACHABLE(op)) {

PyGC_Head *gc = AS_GC(op);

gc_list_move(gc, tolist);

_PyGCHead_SET_REFS(gc, GC_REACHABLE);

}

return 0;

}

/* Move objects that are reachable from finalizers, from the unreachable set

* into finalizers set.

static void

move_legacy_finalizer_reachable(PyGC_Head *finalizers)

{

traverseproc traverse;

PyGC_Head *gc = finalizers->gc.gc_next;

for (; gc != finalizers; gc = gc->gc.gc_next) {

/* Note that the finalizers list may grow during this. */

traverse = Py_TYPE(FROM_GC(gc))->tp_traverse;

(void) traverse(FROM_GC(gc),

(visitproc)visit_move,

(void *)finalizers);

}

/* Clear all weakrefs to unreachable objects, and if such a weakref has a

* callback, invoke it if necessary. Note that it's possible for such

* weakrefs to be outside the unreachable set -- indeed, those are precisely

* the weakrefs whose callbacks must be invoked. See gc_weakref.txt for

* overview & some details. Some weakrefs with callbacks may be reclaimed

* directly by this routine; the number reclaimed is the return value. Other

* weakrefs with callbacks may be moved into the `old` generation. Objects

* moved into `old` have gc_refs set to GC_REACHABLE; the objects remaining in

* unreachable are left at GC_TENTATIVELY_UNREACHABLE. When this returns,

* no object in `unreachable` is weakly referenced anymore.

static int

handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)

{

PyGC_Head *gc;

PyObject *op; /* generally FROM_GC(gc) */

PyWeakReference *wr; /* generally a cast of op */

PyGC_Head wrcb_to_call; /* weakrefs with callbacks to call */

PyGC_Head *next;

int num_freed = 0;

gc_list_init(&wrcb_to_call);

/* Clear all weakrefs to the objects in unreachable. If such a weakref

* also has a callback, move it into `wrcb_to_call` if the callback

* needs to be invoked. Note that we cannot invoke any callbacks until

* all weakrefs to unreachable objects are cleared, lest the callback

* resurrect an unreachable object via a still-active weakref. We

* make another pass over wrcb_to_call, invoking callbacks, after this

* pass completes.

for (gc = unreachable->gc.gc_next; gc != unreachable; gc = next) {

PyWeakReference **wrlist;

op = FROM_GC(gc);

assert(IS_TENTATIVELY_UNREACHABLE(op));

next = gc->gc.gc_next;

if (! PyType_SUPPORTS_WEAKREFS(Py_TYPE(op)))

continue;

/* It supports weakrefs. Does it have any? */

wrlist = (PyWeakReference **)

PyObject_GET_WEAKREFS_LISTPTR(op);

/* `op` may have some weakrefs. March over the list, clear

* all the weakrefs, and move the weakrefs with callbacks

* that must be called into wrcb_to_call.

for (wr = *wrlist; wr != NULL; wr = *wrlist) {

PyGC_Head *wrasgc; /* AS_GC(wr) */

/* _PyWeakref_ClearRef clears the weakref but leaves

* the callback pointer intact. Obscure: it also

* changes *wrlist.

assert(wr->wr_object == op);

_PyWeakref_ClearRef(wr);

assert(wr->wr_object == Py_None);

if (wr->wr_callback == NULL)

continue; /* no callback */

/* Headache time. `op` is going away, and is weakly referenced by

* `wr`, which has a callback. Should the callback be invoked? If wr

* is also trash, no:

* 1. There's no need to call it. The object and the weakref are

* both going away, so it's legitimate to pretend the weakref is

* going away first. The user has to ensure a weakref outlives its

* referent if they want a guarantee that the wr callback will get

* invoked.

* 2. It may be catastrophic to call it. If the callback is also in

* cyclic trash (CT), then although the CT is unreachable from

* outside the current generation, CT may be reachable from the

* callback. Then the callback could resurrect insane objects.

* Since the callback is never needed and may be unsafe in this case,

* wr is simply left in the unreachable set. Note that because we

* already called _PyWeakref_ClearRef(wr), its callback will never

* trigger.

* OTOH, if wr isn't part of CT, we should invoke the callback: the

* weakref outlived the trash. Note that since wr isn't CT in this

* case, its callback can't be CT either -- wr acted as an external

* root to this generation, and therefore its callback did too. So

* nothing in CT is reachable from the callback either, so it's hard

* to imagine how calling it later could create a problem for us. wr

* is moved to wrcb_to_call in this case.

if (IS_TENTATIVELY_UNREACHABLE(wr))

continue;

assert(IS_REACHABLE(wr));

/* Create a new reference so that wr can't go away

* before we can process it again.

Py_INCREF(wr);

/* Move wr to wrcb_to_call, for the next pass. */

wrasgc = AS_GC(wr);

assert(wrasgc != next); /* wrasgc is reachable, but

next isn't, so they can't

be the same */

gc_list_move(wrasgc, &wrcb_to_call);

}

/* Invoke the callbacks we decided to honor. It's safe to invoke them

* because they can't reference unreachable objects.

while (! gc_list_is_empty(&wrcb_to_call)) {

PyObject *temp;

PyObject *callback;

gc = wrcb_to_call.gc.gc_next;

op = FROM_GC(gc);

assert(IS_REACHABLE(op));

assert(PyWeakref_Check(op));

wr = (PyWeakReference *)op;

callback = wr->wr_callback;

assert(callback != NULL);

/* copy-paste of weakrefobject.c's handle_callback() */

temp = PyObject_CallFunctionObjArgs(callback, wr, NULL);

if (temp == NULL)

PyErr_WriteUnraisable(callback);

else

Py_DECREF(temp);

/* Give up the reference we created in the first pass. When

* op's refcount hits 0 (which it may or may not do right now),

* op's tp_dealloc will decref op->wr_callback too. Note

* that the refcount probably will hit 0 now, and because this

* weakref was reachable to begin with, gc didn't already

* add it to its count of freed objects. Example: a reachable

* weak value dict maps some key to this reachable weakref.

* The callback removes this key->weakref mapping from the

* dict, leaving no other references to the weakref (excepting

* ours).

Py_DECREF(op);

if (wrcb_to_call.gc.gc_next == gc) {

/* object is still alive -- move it */

gc_list_move(gc, old);

}

else

++num_freed;

}

return num_freed;

}

static void

debug_cycle(char *msg, PyObject *op)

{

PySys_FormatStderr("gc: %s <%s %p>\n",

msg, Py_TYPE(op)->tp_name, op);

}

/* Handle uncollectable garbage (cycles with tp_del slots, and stuff reachable

* only from such cycles).

* If DEBUG_SAVEALL, all objects in finalizers are appended to the module

* garbage list (a Python list), else only the objects in finalizers with

* __del__ methods are appended to garbage. All objects in finalizers are

* merged into the old list regardless.

* Returns 0 if all OK, <0 on error (out of memory to grow the garbage list).

* The finalizers list is made empty on a successful return.

static int

handle_legacy_finalizers(PyGC_Head *finalizers, PyGC_Head *old)

{

PyGC_Head *gc = finalizers->gc.gc_next;

if (garbage == NULL) {

garbage = PyList_New(0);

if (garbage == NULL)

Py_FatalError("gc couldn't create gc.garbage list");

}

for (; gc != finalizers; gc = gc->gc.gc_next) {

PyObject *op = FROM_GC(gc);

if ((debug & DEBUG_SAVEALL) || has_legacy_finalizer(op)) {

if (PyList_Append(garbage, op) < 0)

return -1;

}

gc_list_merge(finalizers, old);

return 0;

}

/* Run first-time finalizers (if any) on all the objects in collectable.

* Note that this may remove some (or even all) of the objects from the

* list, due to refcounts falling to 0.

static void

finalize_garbage(PyGC_Head *collectable)

{

destructor finalize;

PyGC_Head seen;

/* While we're going through the loop, `finalize(op)` may cause op, or

* other objects, to be reclaimed via refcounts falling to zero. So

* there's little we can rely on about the structure of the input

* `collectable` list across iterations. For safety, we always take the

* first object in that list and move it to a temporary `seen` list.

* If objects vanish from the `collectable` and `seen` lists we don't

* care.

gc_list_init(&seen);

while (!gc_list_is_empty(collectable)) {

PyGC_Head *gc = collectable->gc.gc_next;

PyObject *op = FROM_GC(gc);

gc_list_move(gc, &seen);

if (!_PyGCHead_FINALIZED(gc) &&

PyType_HasFeature(Py_TYPE(op), Py_TPFLAGS_HAVE_FINALIZE) &&

(finalize = Py_TYPE(op)->tp_finalize) != NULL) {

_PyGCHead_SET_FINALIZED(gc, 1);

Py_INCREF(op);

finalize(op);

Py_DECREF(op);

}

gc_list_merge(&seen, collectable);

}

/* Walk the collectable list and check that they are really unreachable

from the outside (some objects could have been resurrected by a

finalizer). */

static int

check_garbage(PyGC_Head *collectable)

{

PyGC_Head *gc;

for (gc = collectable->gc.gc_next; gc != collectable;

gc = gc->gc.gc_next) {

_PyGCHead_SET_REFS(gc, Py_REFCNT(FROM_GC(gc)));

assert(_PyGCHead_REFS(gc) != 0);

}

subtract_refs(collectable);

for (gc = collectable->gc.gc_next; gc != collectable;

gc = gc->gc.gc_next) {

assert(_PyGCHead_REFS(gc) >= 0);

if (_PyGCHead_REFS(gc) != 0)

return -1;

}

return 0;

}

static void

revive_garbage(PyGC_Head *collectable)

{

PyGC_Head *gc;

for (gc = collectable->gc.gc_next; gc != collectable;

gc = gc->gc.gc_next) {

_PyGCHead_SET_REFS(gc, GC_REACHABLE);

}

/* Break reference cycles by clearing the containers involved. This is

* tricky business as the lists can be changing and we don't know which

* objects may be freed. It is possible I screwed something up here.

static void

delete_garbage(PyGC_Head *collectable, PyGC_Head *old)

{

inquiry clear;

while (!gc_list_is_empty(collectable)) {

PyGC_Head *gc = collectable->gc.gc_next;

PyObject *op = FROM_GC(gc);

if (debug & DEBUG_SAVEALL) {

PyList_Append(garbage, op);

}

else {

if ((clear = Py_TYPE(op)->tp_clear) != NULL) {

Py_INCREF(op);

clear(op);

Py_DECREF(op);

}

if (collectable->gc.gc_next == gc) {

/* object is still alive, move it, it may die later */

gc_list_move(gc, old);

_PyGCHead_SET_REFS(gc, GC_REACHABLE);

}

/* Clear all free lists

* All free lists are cleared during the collection of the highest generation.

* Allocated items in the free list may keep a pymalloc arena occupied.

* Clearing the free lists may give back memory to the OS earlier.

static void

clear_freelists(void)

{

(void)PyMethod_ClearFreeList();

(void)PyFrame_ClearFreeList();

(void)PyCFunction_ClearFreeList();

(void)PyTuple_ClearFreeList();

(void)PyUnicode_ClearFreeList();

(void)PyFloat_ClearFreeList();

(void)PyList_ClearFreeList();

(void)PyDict_ClearFreeList();

(void)PySet_ClearFreeList();

}

static double

get_time(void)

{

double result = 0;

if (tmod != NULL) {

_Py_IDENTIFIER(time);

PyObject *f = _PyObject_CallMethodId(tmod, &PyId_time, NULL);

if (f == NULL) {

PyErr_Clear();

}

else {

if (PyFloat_Check(f))

result = PyFloat_AsDouble(f);

Py_DECREF(f);

}

return result;

}

/* This is the main function. Read this to understand how the

* collection process works. */

static Py_ssize_t

collect(int generation, Py_ssize_t *n_collected, Py_ssize_t *n_uncollectable,

int nofail)

{

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;

double t1 = 0.0;

struct gc_generation_stats *stats = &generation_stats[generation];

if (debug & DEBUG_STATS) {

PySys_WriteStderr("gc: collecting generation %d...\n",

generation);

PySys_WriteStderr("gc: objects in each generation:");

for (i = 0; i < NUM_GENERATIONS; i++)

PySys_WriteStderr(" %" PY_FORMAT_SIZE_T "d",

gc_list_size(GEN_HEAD(i)));

t1 = get_time();

PySys_WriteStderr("\n");

}

/* update collection and allocation counters */

if (generation+1 < NUM_GENERATIONS)

generations[generation+1].count += 1;

for (i = 0; i <= generation; i++)

generations[i].count = 0;

/* merge younger generations with one we are currently collecting */

for (i = 0; i < generation; i++) {

gc_list_merge(GEN_HEAD(i), GEN_HEAD(generation));

}

/* handy references */

young = GEN_HEAD(generation);

if (generation < NUM_GENERATIONS-1)

old = GEN_HEAD(generation+1);

else

old = young;

/* Using ob_refcnt and gc_refs, calculate which objects in the

* container set are reachable from outside the set (i.e., have a

* refcount greater than 0 when all the references within the

* set are taken into account).

update_refs(young);

subtract_refs(young);

/* Leave everything reachable from outside young in young, and move

* everything else (in young) to unreachable.

* NOTE: This used to move the reachable objects into a reachable

* set instead. But most things usually turn out to be reachable,

* so it's more efficient to move the unreachable things.

gc_list_init(&unreachable);

move_unreachable(young, &unreachable);

/* Move reachable objects to next generation. */

if (young != old) {

if (generation == NUM_GENERATIONS - 2) {

long_lived_pending += gc_list_size(young);

}

gc_list_merge(young, old);

}

else {

/* We only untrack dicts in full collections, to avoid quadratic

dict build-up. See issue #14775. */

untrack_dicts(young);

long_lived_pending = 0;

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.

gc_list_init(&finalizers);

move_legacy_finalizers(&unreachable, &finalizers);

/* finalizers contains the unreachable objects with a legacy finalizer;

View remainder of file in raw view

cpython/Modules/gcmodule.c at 3.4 · python/cpython · GitHub

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