php的GC机制
在php5.3版本之前, php变量的回收机制只是简单的通过计数来处理(当refcount=0时，会回收内存),但这样会出现一个问题
$a=array("str");
$a[]=&$a;
unset($a);
执行unset之前,$a的refcount 为2,执行unset之后,$a的refcout为1,因为是1不等于0,不能被回收内存,即为垃圾,当然，在php脚本执行完毕后,所分配的内存将全部被回收,但是现在php除了应用于脚本以外，更多的地方用于写守护服务(当然我不推荐),可能长达一个月，两个月才结束脚本,这期间例如上面的程序会产生内存溢出
注：unset并不能释放内存，需要看zval的refcount是否为0
php5.3以后增了GC垃圾回收机制
在分配zval时，以zval_gc_info为单位
#define ALLOC_ZVAL(z) \ do { \ (z) = (zval*)emalloc(sizeof(zval_gc_info)); \ GC_ZVAL_INIT(z); \ } while (0) typedef struct _zval_gc_info { zval z; union { gc_root_buffer *buffered; struct _zval_gc_info *next; } u; } zval_gc_info; #define FREE_ZVAL(z) \ do { \ GC_REMOVE_ZVAL_FROM_BUFFER(z); \ efree(z); \ } while (0)
typedef struct _gc_root_buffer { struct _gc_root_buffer *prev; /* double-linked list */ struct _gc_root_buffer *next; zend_object_handle handle; /* must be 0 for zval */ union { zval *pz; zend_object_handlers *handlers; } u; } gc_root_buffer;
php的GC回收机制启动时,会分配10000个gc_root_buffer的空间
ZEND_API void gc_init(TSRMLS_D)
{
if (GC_G(buf) == NULL && GC_G(gc_enabled)) {
GC_G(buf) = (gc_root_buffer*) malloc(sizeof(gc_root_buffer) * GC_ROOT_BUFFER_MAX_ENTRIES);
GC_G(last_unused) = &GC_G(buf)[GC_ROOT_BUFFER_MAX_ENTRIES];
gc_reset(TSRMLS_C);
}
}
在unset($a)时，详见这里 在active_systom_table找到key为a的zval后，对其value执行析构函数，将其zval的refcount-1，若减1后的值为0，说明可能直接释放内存，若为大于0，放到gc_root_buffer中
ZEND_API void _zval_ptr_dtor(zval **zval_ptr ZEND_FILE_LINE_DC) /* {{{ */
{
Z_DELREF_PP(zval_ptr);
if (Z_REFCOUNT_PP(zval_ptr) == 0) {
TSRMLS_FETCH();
if (*zval_ptr != &EG(uninitialized_zval)) {
GC_REMOVE_ZVAL_FROM_BUFFER(*zval_ptr);
zval_dtor(*zval_ptr);
efree_rel(*zval_ptr);
}
} else {
TSRMLS_FETCH();
if (Z_REFCOUNT_PP(zval_ptr) == 1) {
Z_UNSET_ISREF_PP(zval_ptr);
}
GC_ZVAL_CHECK_POSSIBLE_ROOT(*zval_ptr);
}
}
#define GC_ZVAL_CHECK_POSSIBLE_ROOT(z) /
gc_zval_check_possible_root((z) TSRMLS_CC)
static zend_always_inline void gc_zval_check_possible_root(zval *z TSRMLS_DC)
{
if (z->type == IS_ARRAY || z->type == IS_OBJECT) {
gc_zval_possible_root(z TSRMLS_CC);
}
}
ZEND_API void gc_zval_possible_root(zval *zv TSRMLS_DC)
{
if (UNEXPECTED(GC_G(free_list) != NULL &&
GC_ZVAL_ADDRESS(zv) != NULL &&
GC_ZVAL_GET_COLOR(zv) == GC_BLACK) &&
(GC_ZVAL_ADDRESS(zv) < GC_G(buf) ||
GC_ZVAL_ADDRESS(zv) >= GC_G(last_unused))) {
/* The given zval is a garbage that is going to be deleted by
* currently running GC */
return;
}
if (zv->type == IS_OBJECT) {
GC_ZOBJ_CHECK_POSSIBLE_ROOT(zv);
return;
}
GC_BENCH_INC(zval_possible_root);
//如果zv中的gc_root_buffer最后两位不是紫色,则进行处理
if (GC_ZVAL_GET_COLOR(zv) != GC_PURPLE) {
GC_ZVAL_SET_PURPLE(zv); //设置为紫色
if (!GC_ZVAL_ADDRESS(zv)) {
gc_root_buffer *newRoot = GC_G(unused);
if (newRoot) {
GC_G(unused) = newRoot->prev;
} else if (GC_G(first_unused) != GC_G(last_unused)) {
newRoot = GC_G(first_unused);
GC_G(first_unused)++;
} else {
if (!GC_G(gc_enabled)) {
GC_ZVAL_SET_BLACK(zv);
return;
}
zv->refcount__gc++;
gc_collect_cycles(TSRMLS_C);
zv->refcount__gc--;
newRoot = GC_G(unused);
if (!newRoot) {
return;
}
GC_ZVAL_SET_PURPLE(zv);
GC_G(unused) = newRoot->prev;
}
newRoot->next = GC_G(roots).next;
newRoot->prev = &GC_G(roots);
GC_G(roots).next->prev = newRoot;
GC_G(roots).next = newRoot;
GC_ZVAL_SET_ADDRESS(zv, newRoot); //将gc_root_buffer放到zval_gc_info结构体中
newRoot->handle = 0;
newRoot->u.pz = zv;
GC_BENCH_INC(zval_buffered);
GC_BENCH_INC(root_buf_length);
GC_BENCH_PEAK(root_buf_peak, root_buf_length);
}
}
}
将当前zval放到gc_root_bufer中，每个zval只放一次,依据是该zval所在的zval_gc_info中的gc_root_buffer的颜色 是否是 紫色
#define GC_ZVAL_GET_COLOR(v) \
GC_GET_COLOR(((zval_gc_info*)(v))->u.buffered)
#define GC_GET_COLOR(v) \
(((zend_uintptr_t)(v)) & GC_COLOR)
若不是紫色，则设置为紫色
#define GC_ZVAL_SET_PURPLE(v) \
GC_SET_PURPLE(((zval_gc_info*)(v))->u.buffered)
#define GC_SET_PURPLE(v) \
(v) = ((gc_root_buffer*)(((zend_uintptr_t)(v)) | GC_PURPLE))
宏GC_ZVAL_SET_ADDRESS(zv, newRoot);用来将gc_root_buffer newRoot 放到zv相应位置
#define GC_COLOR 0x03
#define GC_BLACK 0x00
#define GC_WHITE 0x01
#define GC_GREY 0x02
#define GC_PURPLE 0x03
#define GC_ZVAL_SET_ADDRESS(v, a) \
GC_SET_ADDRESS(((zval_gc_info*)(v))->u.buffered, (a))
#define GC_SET_ADDRESS(v, a) \
(v) = ((gc_root_buffer*)((((zend_uintptr_t)(v)) & GC_COLOR) | ((zend_uintptr_t)(a))))
GC_ZVAL_SET_ADDRESS宏中先将v强制转为zval_gc_info类型,本身在为zval分配内存时, 就是以zval_gc_info为单位的,将强转为zval*,因为只需要对zval结构体填充数据,不需要 gc_root_buffer *bufer这个样成员
因为在结构体zval_gc_val中,zval z是第一个成员,那么z的地址也是zval_gc_info本身的内存地址
在PHP GC中,使用颜色来标明垃圾的处理过程
指针无论在32位机或64位机,最后两位均为0,
gc_collect_cycles处理垃圾
ZEND_API int gc_collect_cycles(TSRMLS_D)
{
int count = 0;
if (GC_G(roots).next != &GC_G(roots)) {
zval_gc_info *p, *q, *orig_free_list, *orig_next_to_free;
if (GC_G(gc_active)) {
return 0;
}
GC_G(gc_runs)++;
GC_G(zval_to_free) = FREE_LIST_END;
GC_G(gc_active) = 1;
gc_mark_roots(TSRMLS_C);
gc_scan_roots(TSRMLS_C);
gc_collect_roots(TSRMLS_C);
orig_free_list = GC_G(free_list);
orig_next_to_free = GC_G(next_to_free);
p = GC_G(free_list) = GC_G(zval_to_free);
GC_G(zval_to_free) = NULL;
GC_G(gc_active) = 0;
/* First call destructors */
while (p != FREE_LIST_END) {
if (Z_TYPE(p->z) == IS_OBJECT) {
if (EG(objects_store).object_buckets &&
EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].valid &&
EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].bucket.obj.refcount <= 0 &&
EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].bucket.obj.dtor &&
!EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].destructor_called) {
EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].destructor_called = 1;
EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].bucket.obj.refcount++;
EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].bucket.obj.dtor(EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].bucket.obj.object, Z_OBJ_HANDLE(p->z) TSRMLS_CC);
EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].bucket.obj.refcount--;
}
}
count++;
p = p->u.next;
}
/* Destroy zvals */
p = GC_G(free_list);
while (p != FREE_LIST_END) {
GC_G(next_to_free) = p->u.next;
if (Z_TYPE(p->z) == IS_OBJECT) {
if (EG(objects_store).object_buckets &&
EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].valid &&
EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].bucket.obj.refcount <= 0) {
EG(objects_store).object_buckets[Z_OBJ_HANDLE(p->z)].bucket.obj.refcount = 1;
Z_TYPE(p->z) = IS_NULL;
zend_objects_store_del_ref_by_handle_ex(Z_OBJ_HANDLE(p->z), Z_OBJ_HT(p->z) TSRMLS_CC);
}
} else if (Z_TYPE(p->z) == IS_ARRAY) {
Z_TYPE(p->z) = IS_NULL;
zend_hash_destroy(Z_ARRVAL(p->z));
FREE_HASHTABLE(Z_ARRVAL(p->z));
} else {
zval_dtor(&p->z);
Z_TYPE(p->z) = IS_NULL;
}
p = GC_G(next_to_free);
}
/* Free zvals */
p = GC_G(free_list);
while (p != FREE_LIST_END) {
q = p->u.next;
FREE_ZVAL_EX(&p->z);
p = q;
}
GC_G(collected) += count;
GC_G(free_list) = orig_free_list;
GC_G(next_to_free) = orig_next_to_free;
}
return count;
}
gc_mark_roots(TSRMLS_C); 为垃圾打下颜色标记，遍历gc_root_buffer，其中的u.pz将紫色变更为灰色，遍历u.pz该array zval的每个元素,将其refcount-1,
gc_collect_roots遍历gc_root_buffer，如果refcount==0再设置为白色，表示为垃圾,若refcount >0,表示别人在使用，设置为黑色
static void gc_mark_roots(TSRMLS_D)
{
gc_root_buffer *current = GC_G(roots).next;
while (current != &GC_G(roots)) {
if (current->handle) {
if (EG(objects_store).object_buckets) {
//处理对象,暂时不用看
}
} else {
if (GC_ZVAL_GET_COLOR(current->u.pz) == GC_PURPLE) {
zval_mark_grey(current->u.pz TSRMLS_CC);
} else {
GC_ZVAL_SET_ADDRESS(current->u.pz, NULL);
GC_REMOVE_FROM_BUFFER(current);
}
}
current = current->next;
}
}
static void zval_mark_grey(zval *pz TSRMLS_DC)
{
Bucket *p;
tail_call:
if (GC_ZVAL_GET_COLOR(pz) != GC_GREY) {
p = NULL;
GC_BENCH_INC(zval_marked_grey);
GC_ZVAL_SET_COLOR(pz, GC_GREY);
if (Z_TYPE_P(pz) == IS_OBJECT && EG(objects_store).object_buckets) {
//对象的处理, 暂时不用管
} else if (Z_TYPE_P(pz) == IS_ARRAY) {
if (Z_ARRVAL_P(pz) == &EG(symbol_table)) {
GC_ZVAL_SET_BLACK(pz);
} else {
p = Z_ARRVAL_P(pz)->pListHead;
}
}
while (p != NULL) {
pz = *(zval**)p->pData;
if (Z_TYPE_P(pz) != IS_ARRAY || Z_ARRVAL_P(pz) != &EG(symbol_table)) {
pz->refcount__gc--;
}
if (p->pListNext == NULL) {
goto tail_call;
} else {
zval_mark_grey(pz TSRMLS_CC);
}
p = p->pListNext;
}
}
}
第二次遍历gc_root_buffer,如果zv中的颜色为灰色，且refcount=0,再置为白色，若refcount>0,置为黑色（不是垃圾）
static void gc_scan_roots(TSRMLS_D) { gc_root_buffer *current = GC_G(roots).next; while (current != &GC_G(roots)) { if (current->handle) { zval z; INIT_PZVAL(&z); Z_OBJ_HANDLE(z) = current->handle; Z_OBJ_HT(z) = current->u.handlers; zobj_scan(&z TSRMLS_CC); } else { zval_scan(current->u.pz TSRMLS_CC); } current = current->next; } }
static int zval_scan(zval *pz TSRMLS_DC) { Bucket *p; tail_call: if (GC_ZVAL_GET_COLOR(pz) == GC_GREY) { p = NULL; if (pz->refcount__gc > 0) { zval_scan_black(pz TSRMLS_CC); } else { GC_ZVAL_SET_COLOR(pz, GC_WHITE); if (Z_TYPE_P(pz) == IS_OBJECT && EG(objects_store).object_buckets) { //处理object } else if (Z_TYPE_P(pz) == IS_ARRAY) { if (Z_ARRVAL_P(pz) == &EG(symbol_table)) { GC_ZVAL_SET_BLACK(pz); } else { p = Z_ARRVAL_P(pz)->pListHead; } } } while (p != NULL) { if (p->pListNext == NULL) { pz = *(zval**)p->pData; goto tail_call; } else { zval_scan(*(zval**)p->pData TSRMLS_CC); } p = p->pListNext; } } return 0; }
遍历gc_root_buffer链表，将zv颜色为白色的数据放置单独一个链表，全部回收
static void gc_collect_roots(TSRMLS_D)
{
gc_root_buffer *current = GC_G(roots).next;
while (current != &GC_G(roots)) {
if (current->handle) {
if (EG(objects_store).object_buckets) {
struct _store_object *obj = &EG(objects_store).object_buckets[current->handle].bucket.obj;
zval z;
GC_SET_ADDRESS(obj->buffered, NULL);
INIT_PZVAL(&z);
Z_OBJ_HANDLE(z) = current->handle;
Z_OBJ_HT(z) = current->u.handlers;
zobj_collect_white(&z TSRMLS_CC);
}
} else {
GC_ZVAL_SET_ADDRESS(current->u.pz, NULL);
zval_collect_white(current->u.pz TSRMLS_CC);
}
GC_REMOVE_FROM_BUFFER(current);
current = current->next;
}
}
static void zval_collect_white(zval *pz TSRMLS_DC)
{
Bucket *p;
tail_call:
if (((zval_gc_info*)(pz))->u.buffered == (gc_root_buffer*)GC_WHITE) {
p = NULL;
GC_ZVAL_SET_BLACK(pz);
if (Z_TYPE_P(pz) == IS_OBJECT && EG(objects_store).object_buckets) {
...
} else {
if (Z_TYPE_P(pz) == IS_ARRAY) {
p = Z_ARRVAL_P(pz)->pListHead;
}
}
/* restore refcount and put into list to free */
pz->refcount__gc++;
((zval_gc_info*)pz)->u.next = GC_G(zval_to_free);
GC_G(zval_to_free) = (zval_gc_info*)pz;
while (p != NULL) {
pz = *(zval**)p->pData;
if (Z_TYPE_P(pz) != IS_ARRAY || Z_ARRVAL_P(pz) != &EG(symbol_table)) {
pz->refcount__gc++;
}
if (p->pListNext == NULL) {
goto tail_call;
} else {
zval_collect_white(pz TSRMLS_CC);
}
p = p->pListNext;
}
}
}
