php 垃圾处理机制
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; } } }