/* Reference Cycle Garbage Collection ================================== Neil Schemenauer Based on a post on the python-dev list. Ideas from Guido van Rossum, Eric Tiedemann, and various others. http://www.enme.calgary.ca/~nascheme/python/gc.html http://www.python.org/pipermail/python-dev/2000-March/003869.html http://www.python.org/pipermail/python-dev/2000-March/004010.html http://www.python.org/pipermail/python-dev/2000-March/004022.html For a highlevel view of the collection process, read the collect function. TODO: use a different interface for set_debug() (keywords)? tune parameters */ #include "Python.h" #ifdef WITH_CYCLE_GC /* magic gc_refs value */ #define GC_MOVED -1 /*** Global GC state ***/ /* linked lists of container objects */ static PyGC_Head generation0 = {&generation0, &generation0, 0}; static PyGC_Head generation1 = {&generation1, &generation1, 0}; static PyGC_Head generation2 = {&generation2, &generation2, 0}; static int generation = 0; /* current generation being collected */ /* collection frequencies, XXX tune these */ static int enabled = 1; /* automatic collection enabled? */ static int threshold0 = 100; /* net new containers before collection */ static int threshold1 = 10; /* generation0 collections before collecting 1 */ static int threshold2 = 10; /* generation1 collections before collecting 2 */ /* net new objects allocated since last collection */ static int allocated; /* 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_INSTANCES (1<<3) /* print instances */ #define DEBUG_OBJECTS (1<<4) /* print other objects */ #define DEBUG_LEAK DEBUG_COLLECTABLE | \ DEBUG_UNCOLLECTABLE | \ DEBUG_INSTANCES | \ DEBUG_OBJECTS static int debug; /* list of uncollectable objects */ static PyObject *garbage; /* Python string to use if unhandled exception occurs */ static PyObject *gc_str; /*** list functions ***/ static void gc_list_init(PyGC_Head *list) { list->gc_prev = list; list->gc_next = list; } static void gc_list_append(PyGC_Head *node, PyGC_Head *list) { node->gc_next = list; node->gc_prev = list->gc_prev; node->gc_prev->gc_next = node; list->gc_prev = node; } static void gc_list_remove(PyGC_Head *node) { node->gc_prev->gc_next = node->gc_next; node->gc_next->gc_prev = node->gc_prev; #ifdef Py_DEBUG node->gc_prev = NULL; node->gc_next = NULL; #endif } static void gc_list_move(PyGC_Head *from, PyGC_Head *to) { if (from->gc_next == from) { /* empty from list */ gc_list_init(to); } else { to->gc_next = from->gc_next; to->gc_next->gc_prev = to; to->gc_prev = from->gc_prev; to->gc_prev->gc_next = to; } gc_list_init(from); } /* append a list onto another list, from becomes an empty list */ static void gc_list_merge(PyGC_Head *from, PyGC_Head *to) { PyGC_Head *tail; if (from->gc_next != from) { tail = to->gc_prev; tail->gc_next = from->gc_next; tail->gc_next->gc_prev = tail; to->gc_prev = from->gc_prev; to->gc_prev->gc_next = to; } gc_list_init(from); } static long gc_list_size(PyGC_Head *list) { PyGC_Head *gc; long n = 0; for (gc = list->gc_next; gc != list; gc = gc->gc_next) { n++; } return n; } /*** end of list stuff ***/ /* Set all gc_refs = ob_refcnt */ static void update_refs(PyGC_Head *containers) { PyGC_Head *gc = containers->gc_next; for (; gc != containers; gc=gc->gc_next) { gc->gc_refs = PyObject_FROM_GC(gc)->ob_refcnt; } } static int visit_decref(PyObject *op, void *data) { if (op && PyObject_IS_GC(op)) { PyObject_AS_GC(op)->gc_refs--; } return 0; } /* Subtract internal references from gc_refs */ static void subtract_refs(PyGC_Head *containers) { traverseproc traverse; PyGC_Head *gc = containers->gc_next; for (; gc != containers; gc=gc->gc_next) { traverse = PyObject_FROM_GC(gc)->ob_type->tp_traverse; (void) traverse(PyObject_FROM_GC(gc), (visitproc)visit_decref, NULL); } } /* Append objects with gc_refs > 0 to roots list */ static void move_roots(PyGC_Head *containers, PyGC_Head *roots) { PyGC_Head *next; PyGC_Head *gc = containers->gc_next; while (gc != containers) { next = gc->gc_next; if (gc->gc_refs > 0) { gc_list_remove(gc); gc_list_append(gc, roots); gc->gc_refs = GC_MOVED; } gc = next; } } static int visit_reachable(PyObject *op, PyGC_Head *roots) { if (PyObject_IS_GC(op)) { PyGC_Head *gc = PyObject_AS_GC(op); if (gc && gc->gc_refs != GC_MOVED) { gc_list_remove(gc); gc_list_append(gc, roots); gc->gc_refs = GC_MOVED; } } return 0; } /* Move objects referenced from reachable to reachable set. */ static void move_root_reachable(PyGC_Head *reachable) { traverseproc traverse; PyGC_Head *gc = reachable->gc_next; for (; gc != reachable; gc=gc->gc_next) { /* careful, reachable list is growing here */ PyObject *op = PyObject_FROM_GC(gc); traverse = op->ob_type->tp_traverse; (void) traverse(op, (visitproc)visit_reachable, (void *)reachable); } } /* move all objects with finalizers (instances with __del__) */ static void move_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers) { PyGC_Head *next; PyGC_Head *gc = unreachable->gc_next; static PyObject *delstr = NULL; if (delstr == NULL) { delstr = PyString_InternFromString("__del__"); if (delstr == NULL) Py_FatalError("PyGC: can't initialize __del__ string"); } for (; gc != unreachable; gc=next) { PyObject *op = PyObject_FROM_GC(gc); next = gc->gc_next; if (PyInstance_Check(op) && PyObject_HasAttr(op, delstr)) { gc_list_remove(gc); gc_list_append(gc, finalizers); } } } /* called by tp_traverse */ static int visit_finalizer_reachable(PyObject *op, PyGC_Head *finalizers) { if (PyObject_IS_GC(op)) { PyGC_Head *gc = PyObject_AS_GC(op); if (gc && gc->gc_refs != GC_MOVED) { gc_list_remove(gc); gc_list_append(gc, finalizers); gc->gc_refs = GC_MOVED; } } return 0; } /* Move objects referenced from roots to roots */ static void move_finalizer_reachable(PyGC_Head *finalizers) { traverseproc traverse; PyGC_Head *gc = finalizers->gc_next; for (; gc != finalizers; gc=gc->gc_next) { /* careful, finalizers list is growing here */ traverse = PyObject_FROM_GC(gc)->ob_type->tp_traverse; (void) traverse(PyObject_FROM_GC(gc), (visitproc)visit_finalizer_reachable, (void *)finalizers); } } static void debug_instance(char *msg, PyInstanceObject *inst) { char *cname; /* be careful not to create new dictionaries */ PyObject *classname = inst->in_class->cl_name; if (classname != NULL && PyString_Check(classname)) cname = PyString_AsString(classname); else cname = "?"; PySys_WriteStderr("gc: %.100s <%.100s instance at %p>\n", msg, cname, inst); } static void debug_cycle(char *msg, PyObject *op) { if ((debug & DEBUG_INSTANCES) && PyInstance_Check(op)) { debug_instance(msg, (PyInstanceObject *)op); } else if (debug & DEBUG_OBJECTS) { PySys_WriteStderr("gc: %.100s <%.100s %p>\n", msg, op->ob_type->tp_name, op); } } /* Handle uncollectable garbage (cycles with finalizers). */ static void handle_finalizers(PyGC_Head *finalizers, PyGC_Head *old) { PyGC_Head *gc; if (garbage == NULL) { garbage = PyList_New(0); } for (gc = finalizers->gc_next; gc != finalizers; gc = finalizers->gc_next) { PyObject *op = PyObject_FROM_GC(gc); /* Add all instances to a Python accessible list of garbage */ if (PyInstance_Check(op)) { PyList_Append(garbage, op); } /* We assume that all objects in finalizers are reachable from * instances. Once we add the instances to the garbage list * everything is reachable from Python again. */ gc_list_remove(gc); gc_list_append(gc, old); } } /* 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 *unreachable, PyGC_Head *old) { inquiry clear; while (unreachable->gc_next != unreachable) { PyGC_Head *gc = unreachable->gc_next; PyObject *op = PyObject_FROM_GC(gc); /* PyList_Append(garbage, op); */ if ((clear = op->ob_type->tp_clear) != NULL) { Py_INCREF(op); clear((PyObject *)op); Py_DECREF(op); } /* only try to call tp_clear once for each object */ if (unreachable->gc_next == gc) { /* still alive, move it, it may die later */ gc_list_remove(gc); gc_list_append(gc, old); } } } /* This is the main function. Read this to understand how the * collection process works. */ static long collect(PyGC_Head *young, PyGC_Head *old) { long n = 0; long m = 0; PyGC_Head reachable; PyGC_Head unreachable; PyGC_Head finalizers; PyGC_Head *gc; if (debug & DEBUG_STATS) { PySys_WriteStderr( "gc: collecting generation %d...\n" "gc: objects in each generation: %ld %ld %ld\n", generation, gc_list_size(&generation0), gc_list_size(&generation1), gc_list_size(&generation2)); } /* Using ob_refcnt and gc_refs, calculate which objects in the * container set are reachable from outside the set (ie. have a * refcount greater than 0 when all the references within the * set are taken into account */ update_refs(young); subtract_refs(young); /* Move everything reachable from outside the set into the * reachable set (ie. gc_refs > 0). Next, move everything * reachable from objects in the reachable set. */ gc_list_init(&reachable); move_roots(young, &reachable); move_root_reachable(&reachable); /* move unreachable objects to a temporary list, new objects can be * allocated after this point */ gc_list_init(&unreachable); gc_list_move(young, &unreachable); /* move reachable objects to next generation */ gc_list_merge(&reachable, old); /* Move objects reachable from finalizers, we can't safely delete * them. Python programmers should take care not to create such * things. For Python finalizers means instance objects with * __del__ methods. */ gc_list_init(&finalizers); move_finalizers(&unreachable, &finalizers); move_finalizer_reachable(&finalizers); /* Collect statistics on collectable objects found and print * debugging information. */ for (gc = unreachable.gc_next; gc != &unreachable; gc = gc->gc_next) { m++; if (debug & DEBUG_COLLECTABLE) { debug_cycle("collectable", PyObject_FROM_GC(gc)); } } /* call tp_clear on objects in the collectable set. This will cause * the reference cycles to be broken. It may also cause some objects in * finalizers to be freed */ delete_garbage(&unreachable, old); /* Collect statistics on uncollectable objects found and print * debugging information. */ for (gc = finalizers.gc_next; gc != &finalizers; gc = gc->gc_next) { n++; if (debug & DEBUG_UNCOLLECTABLE) { debug_cycle("uncollectable", PyObject_FROM_GC(gc)); } } if (debug & DEBUG_STATS) { if (m == 0 && n == 0) { PySys_WriteStderr("gc: done.\n"); } else { PySys_WriteStderr( "gc: done, %ld unreachable, %ld uncollectable.\n", n+m, n); } } /* 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_finalizers(&finalizers, old); if (PyErr_Occurred()) { PyErr_WriteUnraisable(gc_str); Py_FatalError("unexpected exception during garbage collection"); } allocated = 0; return n+m; } static long collect_generations(void) { static long collections0 = 0; static long collections1 = 0; long n = 0; if (collections1 > threshold2) { generation = 2; gc_list_merge(&generation0, &generation2); gc_list_merge(&generation1, &generation2); if (generation2.gc_next != &generation2) { n = collect(&generation2, &generation2); } collections1 = 0; } else if (collections0 > threshold1) { generation = 1; collections1++; gc_list_merge(&generation0, &generation1); if (generation1.gc_next != &generation1) { n = collect(&generation1, &generation2); } collections0 = 0; } else { generation = 0; collections0++; if (generation0.gc_next != &generation0) { n = collect(&generation0, &generation1); } } return n; } void _PyGC_Insert(PyObject *op) { /* collection lock since collecting may cause allocations */ static int collecting = 0; #ifdef Py_DEBUG if (!PyObject_IS_GC(op)) { abort(); } #endif if (allocated > threshold0 && enabled && threshold0 && !collecting) { collecting++; collect_generations(); collecting--; } allocated++; gc_list_append(PyObject_AS_GC(op), &generation0); } void _PyGC_Remove(PyObject *op) { PyGC_Head *g = PyObject_AS_GC(op); #ifdef Py_DEBUG if (!PyObject_IS_GC(op)) { abort(); } #endif gc_list_remove(g); if (allocated > 0) { allocated--; } } static char gc_enable__doc__[] = "enable() -> None\n" "\n" "Enable automatic garbage collection.\n" ; static PyObject * gc_enable(PyObject *self, PyObject *args) { if (!PyArg_ParseTuple(args, ":enable")) /* check no args */ return NULL; enabled = 1; Py_INCREF(Py_None); return Py_None; } static char gc_disable__doc__[] = "disable() -> None\n" "\n" "Disable automatic garbage collection.\n" ; static PyObject * gc_disable(PyObject *self, PyObject *args) { if (!PyArg_ParseTuple(args, ":disable")) /* check no args */ return NULL; enabled = 0; Py_INCREF(Py_None); return Py_None; } static char gc_isenabled__doc__[] = "isenabled() -> status\n" "\n" "Returns true if automatic garbage collection is enabled.\n" ; static PyObject * gc_isenabled(PyObject *self, PyObject *args) { if (!PyArg_ParseTuple(args, ":isenabled")) /* check no args */ return NULL; return Py_BuildValue("i", enabled); } static char gc_collect__doc__[] = "collect() -> n\n" "\n" "Run a full collection. The number of unreachable objects is returned.\n" ; static PyObject * gc_collect(PyObject *self, PyObject *args) { long n; if (!PyArg_ParseTuple(args, ":collect")) /* check no args */ return NULL; generation = 2; gc_list_merge(&generation0, &generation2); gc_list_merge(&generation1, &generation2); n = collect(&generation2, &generation2); return Py_BuildValue("i", n); } static char gc_set_debug__doc__[] = "set_debug(flags) -> None\n" "\n" "Set the garbage collection debugging flags. Debugging information is\n" "written to sys.stderr.\n" "\n" "flags is an integer and can have the following bits turned on:\n" "\n" " DEBUG_STATS - Print statistics during collection.\n" " DEBUG_COLLECTABLE - Print collectable objects found.\n" " DEBUG_UNCOLLECTABLE - Print unreachable but uncollectable objects found.\n" " DEBUG_INSTANCES - Print instance objects.\n" " DEBUG_OBJECTS - Print objects other than instances.\n" " DEBUG_LEAK - Debug leaking programs (everything but STATS).\n" ; static PyObject * gc_set_debug(PyObject *self, PyObject *args) { if (!PyArg_ParseTuple(args, "l:get_debug", &debug)) return NULL; Py_INCREF(Py_None); return Py_None; } static char gc_get_debug__doc__[] = "get_debug() -> flags\n" "\n" "Get the garbage collection debugging flags.\n" ; static PyObject * gc_get_debug(PyObject *self, PyObject *args) { if (!PyArg_ParseTuple(args, ":get_debug")) /* no args */ return NULL; return Py_BuildValue("i", debug); } static char gc_set_thresh__doc__[] = "set_threshold(threshold0, [threhold1, threshold2]) -> None\n" "\n" "Sets the collection thresholds. Setting threshold0 to zero disables\n" "collection.\n" ; static PyObject * gc_set_thresh(PyObject *self, PyObject *args) { if (!PyArg_ParseTuple(args, "i|ii:set_threshold", &threshold0, &threshold1, &threshold2)) return NULL; Py_INCREF(Py_None); return Py_None; } static char gc_get_thresh__doc__[] = "get_threshold() -> (threshold0, threshold1, threshold2)\n" "\n" "Return the current collection thresholds\n" ; static PyObject * gc_get_thresh(PyObject *self, PyObject *args) { if (!PyArg_ParseTuple(args, ":get_threshold")) /* no args */ return NULL; return Py_BuildValue("(iii)", threshold0, threshold1, threshold2); } static char gc__doc__ [] = "This module provides access to the garbage collector for reference cycles.\n" "\n" "enable() -- Enable automatic garbage collection.\n" "disable() -- Disable automatic garbage collection.\n" "isenabled() -- Returns true if automatic collection is enabled.\n" "collect() -- Do a full collection right now.\n" "set_debug() -- Set debugging flags.\n" "get_debug() -- Get debugging flags.\n" "set_threshold() -- Set the collection thresholds.\n" "get_threshold() -- Return the current the collection thresholds.\n" ; static PyMethodDef GcMethods[] = { {"enable", gc_enable, METH_VARARGS, gc_enable__doc__}, {"disable", gc_disable, METH_VARARGS, gc_disable__doc__}, {"isenabled", gc_isenabled, METH_VARARGS, gc_isenabled__doc__}, {"set_debug", gc_set_debug, METH_VARARGS, gc_set_debug__doc__}, {"get_debug", gc_get_debug, METH_VARARGS, gc_get_debug__doc__}, {"set_threshold", gc_set_thresh, METH_VARARGS, gc_set_thresh__doc__}, {"get_threshold", gc_get_thresh, METH_VARARGS, gc_get_thresh__doc__}, {"collect", gc_collect, METH_VARARGS, gc_collect__doc__}, {NULL, NULL} /* Sentinel */ }; void initgc(void) { PyObject *m; PyObject *d; m = Py_InitModule4("gc", GcMethods, gc__doc__, NULL, PYTHON_API_VERSION); d = PyModule_GetDict(m); if (garbage == NULL) { garbage = PyList_New(0); } if (gc_str == NULL) { gc_str = PyString_FromString("garbage collection"); } PyDict_SetItemString(d, "garbage", garbage); PyDict_SetItemString(d, "DEBUG_STATS", PyInt_FromLong(DEBUG_STATS)); PyDict_SetItemString(d, "DEBUG_COLLECTABLE", PyInt_FromLong(DEBUG_COLLECTABLE)); PyDict_SetItemString(d, "DEBUG_UNCOLLECTABLE", PyInt_FromLong(DEBUG_UNCOLLECTABLE)); PyDict_SetItemString(d, "DEBUG_INSTANCES", PyInt_FromLong(DEBUG_INSTANCES)); PyDict_SetItemString(d, "DEBUG_OBJECTS", PyInt_FromLong(DEBUG_OBJECTS)); PyDict_SetItemString(d, "DEBUG_LEAK", PyInt_FromLong(DEBUG_LEAK)); } #endif /* WITH_CYCLE_GC */