六、内存管理机制
1、内存管理架构
2、小块空间的内存池
3、循环引用的垃圾收集
4、python中的垃圾收集
1、内存管理架构
Python内存管理机制有两套实现，由编译符号PYMALLOC_DEBUG控制，当该符号被定义时，开启debug模式下的内存管理机制，这套机制在正常内存管理动作外还记录许多关于内存的信息，方便调试。
Python内存管理机制被抽象成分层设计：
[obmalloc.c]
Object-specific allocators
_____ ______ ______ ________
[ int ] [ dict ] [ list ] ... [ string ] Python core |
+3 | <----- Object-specific memory -----> | <-- Non-object memory --> |
_______________________________ | |
[ Python's object allocator ] | |
+2 | ####### Object memory ####### | <------ Internal buffers ------> |
______________________________________________________________ |
[ Python's raw memory allocator (PyMem_ API) ] |
+1 | <----- Python memory (under PyMem manager's control) ------> | |
__________________________________________________________________
[ Underlying general-purpose allocator (ex: C library malloc) ]
0 | <------ Virtual memory allocated for the python process -------> |
=========================================================================
_______________________________________________________________________
[ OS-specific Virtual Memory Manager (VMM) ]
-1 | <--- Kernel dynamic storage allocation & management (page-based) ---> |
__________________________________ __________________________________
[ ] [ ]
-2 | <-- Physical memory: ROM/RAM --> | | <-- Secondary storage (swap) --> |
-1层、-2层是虚拟机或操作系统和物理硬盘等的级别，我们不管。
0层是操作系统提供的内存管理接口，python用的是C运行时提供的malloc接口和free接口，这一层由操作系统实现并管理，python无法干涉这一层的行为。上面三层则是由Python实现并维护。
1层时python基于0层的包装，为Python提供一层统一的 raw memory 管理接口：
[pymem.h] PyAPI_FUNC(void *) PyMem_Malloc(size_t); PyAPI_FUNC(void *) PyMem_Realloc(void *, size_t); PyAPI_FUNC(void) PyMem_Free(void *); [object.c] void * PyMem_Malloc(size_t nbytes) { return PyMem_MALLOC(nbytes); } void * PyMem_Realloc(void *p, size_t nbytes) { return PyMem_REALLOC(p, nbytes); } void PyMem_Free(void *p) { PyMem_FREE(p); }
对应宏实现：
[pymem.h] #define PyMem_MALLOC(n) ((size_t)(n) > (size_t)PY_SSIZE_T_MAX ? NULL : malloc((n) ? (n) : 1)) #define PyMem_REALLOC(p, n) ((size_t)(n) > (size_t)PY_SSIZE_T_MAX ? NULL : realloc((p), (n) ? (n) : 1)) #define PyMem_FREE free
使用宏可减少一次函数调用提高运行效率；另一方面，对于用户使用C编写python扩展模块来说，使用宏是危险的，python内存管理的宏可能会变，导致旧版与新版python产生二进制不兼容。故使用C编写Python扩展时，使用函数接口是好习惯。
1层还提供Python中类型的内存分配器：
[pymem.h] #define PyMem_New(type, n) ( ((size_t)(n) > PY_SSIZE_T_MAX / sizeof(type)) ? NULL : ( (type *) PyMem_Malloc((n) * sizeof(type)) ) ) #define PyMem_NEW(type, n) ( ((size_t)(n) > PY_SSIZE_T_MAX / sizeof(type)) ? NULL : ( (type *) PyMem_MALLOC((n) * sizeof(type)) ) ) #define PyMem_Resize(p, type, n) ( (p) = ((size_t)(n) > PY_SSIZE_T_MAX / sizeof(type)) ? NULL : (type *) PyMem_Realloc((p), (n) * sizeof(type)) ) #define PyMem_RESIZE(p, type, n) ( (p) = ((size_t)(n) > PY_SSIZE_T_MAX / sizeof(type)) ? NULL : (type *) PyMem_REALLOC((p), (n) * sizeof(type)) ) #define PyMem_Del PyMem_Free #define PyMem_DEL PyMem_FREE
PyMem_Malloc需要提供所需申请空间的大小，而PyMem_New只需提供类型和数量。
1层提供的功能是有限的，故需要2层；2层提供创建Python对象的接口（Pymalloc机制），gc管理就在其中。
3层则是Python的一些常用对象，如整数对象，字符串对象等。
2、小块空间的内存池
对于Python中小块内存管理（不为创建对象而申请），Python2.5中启用内存池机制，通过PyObject_Malloc、PyObject_Realloc、PyObject_Free提供。小块内存内存池也可视为一个层次结构，下到上分为：block、pool、arena和内存池。
2.1、Block
最底层有一个确定大小的内存块block。不同的block有不同的内存大小（size class），并且是8字节对齐：
[obmalloc.c] #define ALIGNMENT 8 /* must be 2^N */ #define ALIGNMENT_SHIFT 3 #define ALIGNMENT_MASK (ALIGNMENT - 1)
block大小上限为256，申请内存超过时就使用PyMem函数族处理：
[obmalloc.h] #define SMALL_REQUEST_THRESHOLD 256 #define NB_SMALL_SIZE_CLASSES (SMALL_REQUEST_THRESHOLD / ALIGNMENT)
根据以上设定可得：
[obmalloc.c]
* Request in bytes Size of allocated block Size class idx
* ----------------------------------------------------------------
* 1-8 8 0
* 9-16 16 1
* 17-24 24 2
* 25-32 32 3
* 33-40 40 4
* 41-48 48 5
* 49-56 56 6
* 57-64 64 7
* 65-72 72 8
* ... ... ...
* 241-248 248 30
* 249-256 256 31
*
* 0, 257 and up: routed to the underlying allocator.
size class index 到 size class 的转换：
[obmalloc.c] /* Return the number of bytes in size class I, as a uint. */ #define INDEX2SIZE(I) (((uint)(I) + 1) << ALIGNMENT_SHIFT)
block只是一个概念，在Python源码中并无实体。而管理block的则是pool。
2.2、pool
pool管理着一堆固定大小的block块，是block的集合。pool的大小通常为系统内存页（4K）：
[obmalloc.c] #define SYSTEM_PAGE_SIZE (4 * 1024) #define SYSTEM_PAGE_SIZE_MASK (SYSTEM_PAGE_SIZE - 1) #define POOL_SIZE SYSTEM_PAGE_SIZE /* must be 2^N */ #define POOL_SIZE_MASK SYSTEM_PAGE_SIZE_MASK
python中pool相关的结构：
[obmalloc.h] /* When you say memory, my mind reasons in terms of (pointers to) blocks */ typedef uchar block; /* Pool for small blocks. */ struct pool_header { union { block *_padding; uint count; } ref; /* number of allocated blocks */ block *freeblock; /* pool's free list head */ struct pool_header *nextpool; /* next pool of this size class */ struct pool_header *prevpool; /* previous pool "" */ uint arenaindex; /* index into arenas of base adr */ uint szidx; /* block size class index */ uint nextoffset; /* bytes to virgin block */ uint maxnextoffset; /* largest valid nextoffset */ };
这是一个pool的头部，4KB除去这头部剩下的就是pool管理的内存了。
一个pool管理着一堆同样大小的block，由szidx（size class index）决定。
将4KB内存改造成pool：
申请block：
if (pool != pool->nextpool) { /* * There is a used pool for this size class. * Pick up the head block of its free list. */ ++pool->ref.count; bp = pool->freeblock; assert(bp != NULL); if ((pool->freeblock = *(block **)bp) != NULL) { UNLOCK(); return (void *)bp; } /* * Reached the end of the free list, try to extend it. */ if (pool->nextoffset <= pool->maxnextoffset) { /* There is room for another block. */ pool->freeblock = (block*)pool + pool->nextoffset; pool->nextoffset += INDEX2SIZE(size); *(block **)(pool->freeblock) = NULL; UNLOCK(); return (void *)bp; } /* Pool is full, unlink from used pools. */ next = pool->nextpool; pool = pool->prevpool; next->prevpool = pool; pool->nextpool = next; UNLOCK(); return (void *)bp; }
释放block：
void PyObject_Free(void *p) { poolp pool; block *lastfree; poolp next, prev; uint size; #ifndef Py_USING_MEMORY_DEBUGGER uint arenaindex_temp; #endif if (p == NULL) /* free(NULL) has no effect */ return; #ifdef WITH_VALGRIND if (UNLIKELY(running_on_valgrind > 0)) goto redirect; #endif pool = POOL_ADDR(p); if (Py_ADDRESS_IN_RANGE(p, pool)) { /* We allocated this address. */ LOCK(); /* Link p to the start of the pool's freeblock list. Since * the pool had at least the p block outstanding, the pool * wasn't empty (so it's already in a usedpools[] list, or * was full and is in no list -- it's not in the freeblocks * list in any case). */ assert(pool->ref.count > 0); /* else it was empty */ *(block **)p = lastfree = pool->freeblock; pool->freeblock = (block *)p; if (lastfree) { struct arena_object* ao; uint nf; /* ao->nfreepools */ /* freeblock wasn't NULL, so the pool wasn't full, * and the pool is in a usedpools[] list. */ if (--pool->ref.count != 0) { /* pool isn't empty: leave it in usedpools */ UNLOCK(); return; } /* Pool is now empty: unlink from usedpools, and * link to the front of freepools. This ensures that * previously freed pools will be allocated later * (being not referenced, they are perhaps paged out). */ next = pool->nextpool; prev = pool->prevpool; next->prevpool = prev; prev->nextpool = next; /* Link the pool to freepools. This is a singly-linked * list, and pool->prevpool isn't used there. */ ao = &arenas[pool->arenaindex]; pool->nextpool = ao->freepools; ao->freepools = pool; nf = ++ao->nfreepools; /* All the rest is arena management. We just freed * a pool, and there are 4 cases for arena mgmt: * 1. If all the pools are free, return the arena to * the system free(). * 2. If this is the only free pool in the arena, * add the arena back to the `usable_arenas` list. * 3. If the "next" arena has a smaller count of free * pools, we have to "slide this arena right" to * restore that usable_arenas is sorted in order of * nfreepools. * 4. Else there's nothing more to do. */ if (nf == ao->ntotalpools) { /* Case 1. First unlink ao from usable_arenas. */ assert(ao->prevarena == NULL || ao->prevarena->address != 0); assert(ao ->nextarena == NULL || ao->nextarena->address != 0); /* Fix the pointer in the prevarena, or the * usable_arenas pointer. */ if (ao->prevarena == NULL) { usable_arenas = ao->nextarena; assert(usable_arenas == NULL || usable_arenas->address != 0); } else { assert(ao->prevarena->nextarena == ao); ao->prevarena->nextarena = ao->nextarena; } /* Fix the pointer in the nextarena. */ if (ao->nextarena != NULL) { assert(ao->nextarena->prevarena == ao); ao->nextarena->prevarena = ao->prevarena; } /* Record that this arena_object slot is * available to be reused. */ ao->nextarena = unused_arena_objects; unused_arena_objects = ao; /* Free the entire arena. */ free((void *)ao->address); ao->address = 0; /* mark unassociated */ --narenas_currently_allocated; UNLOCK(); return; } if (nf == 1) { /* Case 2. Put ao at the head of * usable_arenas. Note that because * ao->nfreepools was 0 before, ao isn't * currently on the usable_arenas list. */ ao->nextarena = usable_arenas; ao->prevarena = NULL; if (usable_arenas) usable_arenas->prevarena = ao; usable_arenas = ao; assert(usable_arenas->address != 0); UNLOCK(); return; } /* If this arena is now out of order, we need to keep * the list sorted. The list is kept sorted so that * the "most full" arenas are used first, which allows * the nearly empty arenas to be completely freed. In * a few un-scientific tests, it seems like this * approach allowed a lot more memory to be freed. */ if (ao->nextarena == NULL || nf <= ao->nextarena->nfreepools) { /* Case 4. Nothing to do. */ UNLOCK(); return; } /* Case 3: We have to move the arena towards the end * of the list, because it has more free pools than * the arena to its right. * First unlink ao from usable_arenas. */ if (ao->prevarena != NULL) { /* ao isn't at the head of the list */ assert(ao->prevarena->nextarena == ao); ao->prevarena->nextarena = ao->nextarena; } else { /* ao is at the head of the list */ assert(usable_arenas == ao); usable_arenas = ao->nextarena; } ao->nextarena->prevarena = ao->prevarena; /* Locate the new insertion point by iterating over * the list, using our nextarena pointer. */ while (ao->nextarena != NULL && nf > ao->nextarena->nfreepools) { ao->prevarena = ao->nextarena; ao->nextarena = ao->nextarena->nextarena; } /* Insert ao at this point. */ assert(ao->nextarena == NULL || ao->prevarena == ao->nextarena->prevarena); assert(ao->prevarena->nextarena == ao->nextarena); ao->prevarena->nextarena = ao; if (ao->nextarena != NULL) ao->nextarena->prevarena = ao; /* Verify that the swaps worked. */ assert(ao->nextarena == NULL || nf <= ao->nextarena->nfreepools); assert(ao->prevarena == NULL || nf > ao->prevarena->nfreepools); assert(ao->nextarena == NULL || ao->nextarena->prevarena == ao); assert((usable_arenas == ao && ao->prevarena == NULL) || ao->prevarena->nextarena == ao); UNLOCK(); return; } /* Pool was full, so doesn't currently live in any list: * link it to the front of the appropriate usedpools[] list. * This mimics LRU pool usage for new allocations and * targets optimal filling when several pools contain * blocks of the same size class. */ --pool->ref.count; assert(pool->ref.count > 0); /* else the pool is empty */ size = pool->szidx; next = usedpools[size + size]; prev = next->prevpool; /* insert pool before next: prev <-> pool <-> next */ pool->nextpool = next; pool->prevpool = prev; next->prevpool = pool; prev->nextpool = pool; UNLOCK(); return; } #ifdef WITH_VALGRIND redirect: #endif /* We didn't allocate this address. */ free(p); }
释放后freeblock会调整指到释放了的blobk上，有效利用空闲block。
block分配的一般行为：
[obmalloc.c]-[allocate block] ... if (pool != pool->nextpool) { /* * There is a used pool for this size class. * Pick up the head block of its free list. */ ++pool->ref.count; bp = pool->freeblock; assert(bp != NULL); if ((pool->freeblock = *(block **)bp) != NULL) { UNLOCK(); return (void *)bp; } ... if (pool->nextoffset <= pool->maxnextoffset) { ... } ... }
freeblock为空证明pool满了，会提供另一个pool。而pool的集合则是arena。
2.3、arena
arena是多个pool的聚合。Pyhton中arena的默认大小为256KB（可装64个pool）：
[obmalloc.c] #define ARENA_SIZE (256 << 10) /* 256KB */
Python中的arena：
[obmalloc.c] typedef uchar block; /* Record keeping for arenas. */ struct arena_object { /* The address of the arena, as returned by malloc. Note that 0 * will never be returned by a successful malloc, and is used * here to mark an arena_object that doesn't correspond to an * allocated arena. */ uptr address; /* Pool-aligned pointer to the next pool to be carved off. */ block* pool_address; /* The number of available pools in the arena: free pools + never- * allocated pools. */ uint nfreepools; /* The total number of pools in the arena, whether or not available. */ uint ntotalpools; /* Singly-linked list of available pools. */ struct pool_header* freepools; /* Whenever this arena_object is not associated with an allocated * arena, the nextarena member is used to link all unassociated * arena_objects in the singly-linked `unused_arena_objects` list. * The prevarena member is unused in this case. * * When this arena_object is associated with an allocated arena * with at least one available pool, both members are used in the * doubly-linked `usable_arenas` list, which is maintained in * increasing order of `nfreepools` values. * * Else this arena_object is associated with an allocated arena * all of whose pools are in use. `nextarena` and `prevarena` * are both meaningless in this case. */ struct arena_object* nextarena; struct arena_object* prevarena; };
一个完整的arena是 一个arena_object和其管理的pool集合；
一个完整的pool时一个 pool_header 和其管理的block集合。
pool_header和其管理的block内存上是连续的，而arena则是分离：
差别体现在申请pool_header时其所管理的内存被申请了，而arena_object则没有。
当一个arena_object没与pool集合建立联系时，arena处于“未使用”状态，否则进入“可用”状态。未使用的单向连接（unused_arena_objects），可用的双向连接（usable_arenas）。
arena的申请new_arena：
[obmalloc.c] /* Array of objects used to track chunks of memory (arenas). */ static struct arena_object* arenas = NULL; /* Number of slots currently allocated in the `arenas` vector. */ static uint maxarenas = 0; /* The head of the singly-linked, NULL-terminated list of available * arena_objects. */ static struct arena_object* unused_arena_objects = NULL; /* The head of the doubly-linked, NULL-terminated at each end, list of * arena_objects associated with arenas that have pools available. */ static struct arena_object* usable_arenas = NULL; /* How many arena_objects do we initially allocate? * 16 = can allocate 16 arenas = 16 * ARENA_SIZE = 4MB before growing the * `arenas` vector. */ #define INITIAL_ARENA_OBJECTS 16 /* Number of arenas allocated that haven't been free()'d. */ static size_t narenas_currently_allocated = 0; #ifdef PYMALLOC_DEBUG /* Total number of times malloc() called to allocate an arena. */ static size_t ntimes_arena_allocated = 0; /* High water mark (max value ever seen) for narenas_currently_allocated. */ static size_t narenas_highwater = 0; #endif /* Allocate a new arena. If we run out of memory, return NULL. Else * allocate a new arena, and return the address of an arena_object * describing the new arena. It's expected that the caller will set * `usable_arenas` to the return value. */ static struct arena_object* new_arena(void) { struct arena_object* arenaobj; uint excess; /* number of bytes above pool alignment */ #ifdef PYMALLOC_DEBUG if (Py_GETENV("PYTHONMALLOCSTATS")) _PyObject_DebugMallocStats(); #endif if (unused_arena_objects == NULL) { uint i; uint numarenas; size_t nbytes; /* Double the number of arena objects on each allocation. * Note that it's possible for `numarenas` to overflow. */ numarenas = maxarenas ? maxarenas << 1 : INITIAL_ARENA_OBJECTS; if (numarenas <= maxarenas) return NULL; /* overflow */ #if SIZEOF_SIZE_T <= SIZEOF_INT if (numarenas > PY_SIZE_MAX / sizeof(*arenas)) return NULL; /* overflow */ #endif nbytes = numarenas * sizeof(*arenas); arenaobj = (struct arena_object *)realloc(arenas, nbytes); if (arenaobj == NULL) return NULL; arenas = arenaobj; /* We might need to fix pointers that were copied. However, * new_arena only gets called when all the pages in the * previous arenas are full. Thus, there are *no* pointers * into the old array. Thus, we don't have to worry about * invalid pointers. Just to be sure, some asserts: */ assert(usable_arenas == NULL); assert(unused_arena_objects == NULL); /* Put the new arenas on the unused_arena_objects list. */ for (i = maxarenas; i < numarenas; ++i) { arenas[i].address = 0; /* mark as unassociated */ arenas[i].nextarena = i < numarenas - 1 ? &arenas[i+1] : NULL; } /* Update globals. */ unused_arena_objects = &arenas[maxarenas]; maxarenas = numarenas; } /* Take the next available arena object off the head of the list. */ assert(unused_arena_objects != NULL); arenaobj = unused_arena_objects; unused_arena_objects = arenaobj->nextarena; assert(arenaobj->address == 0); arenaobj->address = (uptr)malloc(ARENA_SIZE); if (arenaobj->address == 0) { /* The allocation failed: return NULL after putting the * arenaobj back. */ arenaobj->nextarena = unused_arena_objects; unused_arena_objects = arenaobj; return NULL; } ++narenas_currently_allocated; #ifdef PYMALLOC_DEBUG ++ntimes_arena_allocated; if (narenas_currently_allocated > narenas_highwater) narenas_highwater = narenas_currently_allocated; #endif arenaobj->freepools = NULL; /* pool_address <- first pool-aligned address in the arena nfreepools <- number of whole pools that fit after alignment */ arenaobj->pool_address = (block*)arenaobj->address; arenaobj->nfreepools = ARENA_SIZE / POOL_SIZE; assert(POOL_SIZE * arenaobj->nfreepools == ARENA_SIZE); excess = (uint)(arenaobj->address & POOL_SIZE_MASK); if (excess != 0) { --arenaobj->nfreepools; arenaobj->pool_address += POOL_SIZE - excess; } arenaobj->ntotalpools = arenaobj->nfreepools; return arenaobj; }
先检查unused_arena_objects中是否有“未使用”的arena，有则从中取；否则新增arenas（并调整maxarenas的值）；
先申请ARENA_SIZE（256KB）的内存块，将其变成“可用”，然后设置一些维护pool的信息，后被usable_arenas接收；
address标记arena_object状态（“未使用”还是“可用”）。
2.4、内存池
小块内存池大小限制由SMALL_MEMORY_LIMIT控制，默认不限制：
/* * Maximum amount of memory managed by the allocator for small requests. */ #ifdef WITH_MEMORY_LIMITS #ifndef SMALL_MEMORY_LIMIT #define SMALL_MEMORY_LIMIT (64 * 1024 * 1024) /* 64 MB -- more? */ #endif #endif /* * The allocator sub-allocates <Big> blocks of memory (called arenas) aligned * on a page boundary. This is a reserved virtual address space for the * current process (obtained through a malloc call). In no way this means * that the memory arenas will be used entirely. A malloc(<Big>) is usually * an address range reservation for <Big> bytes, unless all pages within this * space are referenced subsequently. So malloc'ing big blocks and not using * them does not mean "wasting memory". It's an addressable range wastage... * * Therefore, allocating arenas with malloc is not optimal, because there is * some address space wastage, but this is the most portable way to request * memory from the system across various platforms. */ #define ARENA_SIZE (256 << 10) /* 256KB */ #ifdef WITH_MEMORY_LIMITS #define MAX_ARENAS (SMALL_MEMORY_LIMIT / ARENA_SIZE) #endif
虽然arena是Python小块内存池的最上层结构，但申请内存时不与它打交道，而是直接以pool作为基本操作单元。同一个arena里面可能管理着 管理不同大小block的pool。
pool在python运行时处于used状态、full状态或empty状态中的一种。arena包含三种状态pool的集合的一个可能状态：
看下维护used状态pool的usedpools：
[obmalloc.c] typedef uchar block; #define PTA(x) ((poolp )((uchar *)&(usedpools[2*(x)]) - 2*sizeof(block *))) #define PT(x) PTA(x), PTA(x) static poolp usedpools[2 * ((NB_SMALL_SIZE_CLASSES + 7) / 8) * 8] = { PT(0), PT(1), PT(2), PT(3), PT(4), PT(5), PT(6), PT(7) #if NB_SMALL_SIZE_CLASSES > 8 , PT(8), PT(9), PT(10), PT(11), PT(12), PT(13), PT(14), PT(15) #if NB_SMALL_SIZE_CLASSES > 16 , PT(16), PT(17), PT(18), PT(19), PT(20), PT(21), PT(22), PT(23) #if NB_SMALL_SIZE_CLASSES > 24 , PT(24), PT(25), PT(26), PT(27), PT(28), PT(29), PT(30), PT(31) #if NB_SMALL_SIZE_CLASSES > 32 , PT(32), PT(33), PT(34), PT(35), PT(36), PT(37), PT(38), PT(39) #if NB_SMALL_SIZE_CLASSES > 40 , PT(40), PT(41), PT(42), PT(43), PT(44), PT(45), PT(46), PT(47) #if NB_SMALL_SIZE_CLASSES > 48 , PT(48), PT(49), PT(50), PT(51), PT(52), PT(53), PT(54), PT(55) #if NB_SMALL_SIZE_CLASSES > 56 , PT(56), PT(57), PT(58), PT(59), PT(60), PT(61), PT(62), PT(63) #endif /* NB_SMALL_SIZE_CLASSES > 56 */ #endif /* NB_SMALL_SIZE_CLASSES > 48 */ #endif /* NB_SMALL_SIZE_CLASSES > 40 */ #endif /* NB_SMALL_SIZE_CLASSES > 32 */ #endif /* NB_SMALL_SIZE_CLASSES > 24 */ #endif /* NB_SMALL_SIZE_CLASSES > 16 */ #endif /* NB_SMALL_SIZE_CLASSES > 8 */ };
其中 NB_SMALL_SIZE_CLASSES 指明一共有多少个size class：
[obmalloc.c] #define SMALL_REQUEST_THRESHOLD 256 #define NB_SMALL_SIZE_CLASSES (SMALL_REQUEST_THRESHOLD / ALIGNMENT)
Python启动后usedpools中无可用pool。Python采用延迟分配策略，当我们申请小块内存时才分配。
初始分配空间代码PyObject_Malloc：
#undef PyObject_Malloc void * PyObject_Malloc(size_t nbytes) { block *bp; poolp pool; poolp next; uint size; #ifdef WITH_VALGRIND if (UNLIKELY(running_on_valgrind == -1)) running_on_valgrind = RUNNING_ON_VALGRIND; if (UNLIKELY(running_on_valgrind)) goto redirect; #endif /* * Limit ourselves to PY_SSIZE_T_MAX bytes to prevent security holes. * Most python internals blindly use a signed Py_ssize_t to track * things without checking for overflows or negatives. * As size_t is unsigned, checking for nbytes < 0 is not required. */ if (nbytes > PY_SSIZE_T_MAX) return NULL; /* * This implicitly redirects malloc(0). */ if ((nbytes - 1) < SMALL_REQUEST_THRESHOLD) { LOCK(); /* * Most frequent paths first */ size = (uint)(nbytes - 1) >> ALIGNMENT_SHIFT; pool = usedpools[size + size]; if (pool != pool->nextpool) { /* * There is a used pool for this size class. * Pick up the head block of its free list. */ ++pool->ref.count; bp = pool->freeblock; assert(bp != NULL); if ((pool->freeblock = *(block **)bp) != NULL) { UNLOCK(); return (void *)bp; } /* * Reached the end of the free list, try to extend it. */ if (pool->nextoffset <= pool->maxnextoffset) { /* There is room for another block. */ pool->freeblock = (block*)pool + pool->nextoffset; pool->nextoffset += INDEX2SIZE(size); *(block **)(pool->freeblock) = NULL; UNLOCK(); return (void *)bp; } /* Pool is full, unlink from used pools. */ next = pool->nextpool; pool = pool->prevpool; next->prevpool = pool; pool->nextpool = next; UNLOCK(); return (void *)bp; } /* There isn't a pool of the right size class immediately * available: use a free pool. */ if (usable_arenas == NULL) { /* No arena has a free pool: allocate a new arena. */ #ifdef WITH_MEMORY_LIMITS if (narenas_currently_allocated >= MAX_ARENAS) { UNLOCK(); goto redirect; } #endif usable_arenas = new_arena(); if (usable_arenas == NULL) { UNLOCK(); goto redirect; } usable_arenas->nextarena = usable_arenas->prevarena = NULL; } assert(usable_arenas->address != 0); /* Try to get a cached free pool. */ pool = usable_arenas->freepools; if (pool != NULL) { /* Unlink from cached pools. */ usable_arenas->freepools = pool->nextpool; /* This arena already had the smallest nfreepools * value, so decreasing nfreepools doesn't change * that, and we don't need to rearrange the * usable_arenas list. However, if the arena has * become wholly allocated, we need to remove its * arena_object from usable_arenas. */ --usable_arenas->nfreepools; if (usable_arenas->nfreepools == 0) { /* Wholly allocated: remove. */ assert(usable_arenas->freepools == NULL); assert(usable_arenas->nextarena == NULL || usable_arenas->nextarena->prevarena == usable_arenas); usable_arenas = usable_arenas->nextarena; if (usable_arenas != NULL) { usable_arenas->prevarena = NULL; assert(usable_arenas->address != 0); } } else { /* nfreepools > 0: it must be that freepools * isn't NULL, or that we haven't yet carved * off all the arena's pools for the first * time. */ assert(usable_arenas->freepools != NULL || usable_arenas->pool_address <= (block*)usable_arenas->address + ARENA_SIZE - POOL_SIZE); } init_pool: /* Frontlink to used pools. */ next = usedpools[size + size]; /* == prev */ pool->nextpool = next; pool->prevpool = next; next->nextpool = pool; next->prevpool = pool; pool->ref.count = 1; if (pool->szidx == size) { /* Luckily, this pool last contained blocks * of the same size class, so its header * and free list are already initialized. */ bp = pool->freeblock; pool->freeblock = *(block **)bp; UNLOCK(); return (void *)bp; } /* * Initialize the pool header, set up the free list to * contain just the second block, and return the first * block. */ pool->szidx = size; size = INDEX2SIZE(size); bp = (block *)pool + POOL_OVERHEAD; pool->nextoffset = POOL_OVERHEAD + (size << 1); pool->maxnextoffset = POOL_SIZE - size; pool->freeblock = bp + size; *(block **)(pool->freeblock) = NULL; UNLOCK(); return (void *)bp; } /* Carve off a new pool. */ assert(usable_arenas->nfreepools > 0); assert(usable_arenas->freepools == NULL); pool = (poolp)usable_arenas->pool_address; assert((block*)pool <= (block*)usable_arenas->address + ARENA_SIZE - POOL_SIZE); pool->arenaindex = usable_arenas - arenas; assert(&arenas[pool->arenaindex] == usable_arenas); pool->szidx = DUMMY_SIZE_IDX; usable_arenas->pool_address += POOL_SIZE; --usable_arenas->nfreepools; if (usable_arenas->nfreepools == 0) { assert(usable_arenas->nextarena == NULL || usable_arenas->nextarena->prevarena == usable_arenas); /* Unlink the arena: it is completely allocated. */ usable_arenas = usable_arenas->nextarena; if (usable_arenas != NULL) { usable_arenas->prevarena = NULL; assert(usable_arenas->address != 0); } } goto init_pool; } /* The small block allocator ends here. */ redirect: /* Redirect the original request to the underlying (libc) allocator. * We jump here on bigger requests, on error in the code above (as a * last chance to serve the request) or when the max memory limit * has been reached. */ if (nbytes == 0) nbytes = 1; return (void *)malloc(nbytes); }
开始如果usable_arenas为空，则从new_arena申请一个arena，再构建usable_arenas链表，从usable_arenas取一个可用pool。取完后如arena无可用pool，将其移出usable_arenas。
取到pool后将其放到usedpools中，然后对pool进行初始化，返回相应block。
python2.5后，将arena内存泄漏问题修复（arena申请pool但从不释放），回收代码PyObject_Free：
#undef PyObject_Free void PyObject_Free(void *p) { poolp pool; block *lastfree; poolp next, prev; uint size; #ifndef Py_USING_MEMORY_DEBUGGER uint arenaindex_temp; #endif if (p == NULL) /* free(NULL) has no effect */ return; #ifdef WITH_VALGRIND if (UNLIKELY(running_on_valgrind > 0)) goto redirect; #endif pool = POOL_ADDR(p); if (Py_ADDRESS_IN_RANGE(p, pool)) { /* We allocated this address. */ LOCK(); /* Link p to the start of the pool's freeblock list. Since * the pool had at least the p block outstanding, the pool * wasn't empty (so it's already in a usedpools[] list, or * was full and is in no list -- it's not in the freeblocks * list in any case). */ assert(pool->ref.count > 0); /* else it was empty */ *(block **)p = lastfree = pool->freeblock; pool->freeblock = (block *)p; if (lastfree) { struct arena_object* ao; uint nf; /* ao->nfreepools */ /* freeblock wasn't NULL, so the pool wasn't full, * and the pool is in a usedpools[] list. */ if (--pool->ref.count != 0) { /* pool isn't empty: leave it in usedpools */ UNLOCK(); return; } /* Pool is now empty: unlink from usedpools, and * link to the front of freepools. This ensures that * previously freed pools will be allocated later * (being not referenced, they are perhaps paged out). */ next = pool->nextpool; prev = pool->prevpool; next->prevpool = prev; prev->nextpool = next; /* Link the pool to freepools. This is a singly-linked * list, and pool->prevpool isn't used there. */ ao = &arenas[pool->arenaindex]; pool->nextpool = ao->freepools; ao->freepools = pool; nf = ++ao->nfreepools; /* All the rest is arena management. We just freed * a pool, and there are 4 cases for arena mgmt: * 1. If all the pools are free, return the arena to * the system free(). * 2. If this is the only free pool in the arena, * add the arena back to the `usable_arenas` list. * 3. If the "next" arena has a smaller count of free * pools, we have to "slide this arena right" to * restore that usable_arenas is sorted in order of * nfreepools. * 4. Else there's nothing more to do. */ if (nf == ao->ntotalpools) { /* Case 1. First unlink ao from usable_arenas. */ assert(ao->prevarena == NULL || ao->prevarena->address != 0); assert(ao ->nextarena == NULL || ao->nextarena->address != 0); /* Fix the pointer in the prevarena, or the * usable_arenas pointer. */ if (ao->prevarena == NULL) { usable_arenas = ao->nextarena; assert(usable_arenas == NULL || usable_arenas->address != 0); } else { assert(ao->prevarena->nextarena == ao); ao->prevarena->nextarena = ao->nextarena; } /* Fix the pointer in the nextarena. */ if (ao->nextarena != NULL) { assert(ao->nextarena->prevarena == ao); ao->nextarena->prevarena = ao->prevarena; } /* Record that this arena_object slot is * available to be reused. */ ao->nextarena = unused_arena_objects; unused_arena_objects = ao; /* Free the entire arena. */ free((void *)ao->address); ao->address = 0; /* mark unassociated */ --narenas_currently_allocated; UNLOCK(); return; } if (nf == 1) { /* Case 2. Put ao at the head of * usable_arenas. Note that because * ao->nfreepools was 0 before, ao isn't * currently on the usable_arenas list. */ ao->nextarena = usable_arenas; ao->prevarena = NULL; if (usable_arenas) usable_arenas->prevarena = ao; usable_arenas = ao; assert(usable_arenas->address != 0); UNLOCK(); return; } /* If this arena is now out of order, we need to keep * the list sorted. The list is kept sorted so that * the "most full" arenas are used first, which allows * the nearly empty arenas to be completely freed. In * a few un-scientific tests, it seems like this * approach allowed a lot more memory to be freed. */ if (ao->nextarena == NULL || nf <= ao->nextarena->nfreepools) { /* Case 4. Nothing to do. */ UNLOCK(); return; } /* Case 3: We have to move the arena towards the end * of the list, because it has more free pools than * the arena to its right. * First unlink ao from usable_arenas. */ if (ao->prevarena != NULL) { /* ao isn't at the head of the list */ assert(ao->prevarena->nextarena == ao); ao->prevarena->nextarena = ao->nextarena; } else { /* ao is at the head of the list */ assert(usable_arenas == ao); usable_arenas = ao->nextarena; } ao->nextarena->prevarena = ao->prevarena; /* Locate the new insertion point by iterating over * the list, using our nextarena pointer. */ while (ao->nextarena != NULL && nf > ao->nextarena->nfreepools) { ao->prevarena = ao->nextarena; ao->nextarena = ao->nextarena->nextarena; } /* Insert ao at this point. */ assert(ao->nextarena == NULL || ao->prevarena == ao->nextarena->prevarena); assert(ao->prevarena->nextarena == ao->nextarena); ao->prevarena->nextarena = ao; if (ao->nextarena != NULL) ao->nextarena->prevarena = ao; /* Verify that the swaps worked. */ assert(ao->nextarena == NULL || nf <= ao->nextarena->nfreepools); assert(ao->prevarena == NULL || nf > ao->prevarena->nfreepools); assert(ao->nextarena == NULL || ao->nextarena->prevarena == ao); assert((usable_arenas == ao && ao->prevarena == NULL) || ao->prevarena->nextarena == ao); UNLOCK(); return; } /* Pool was full, so doesn't currently live in any list: * link it to the front of the appropriate usedpools[] list. * This mimics LRU pool usage for new allocations and * targets optimal filling when several pools contain * blocks of the same size class. */ --pool->ref.count; assert(pool->ref.count > 0); /* else the pool is empty */ size = pool->szidx; next = usedpools[size + size]; prev = next->prevpool; /* insert pool before next: prev <-> pool <-> next */ pool->nextpool = next; pool->prevpool = prev; next->prevpool = pool; prev->nextpool = pool; UNLOCK(); return; } #ifdef WITH_VALGRIND redirect: #endif /* We didn't allocate this address. */ free(p); }
1、如果arena中所有pool都是empty，释放pool集合所占内存；
2、如果之前arena没有empty的pool，多一个后将arena移到usable_arenas中；
3、usable_arenas时一个有序链表，nfreepools个数递增，保证一个arena empty pool个数越多被使用机会越少。从而保证多余内存被释放并归还系统；
4、其他情况不对arena进行处理；
3、循环引用的垃圾收集
python通过引用计数实时内存管理，优点是具有实时性，缺点是带来维护引用计数额外操作、更多的内存分配与释放。python设计了大量内存池，除了第2节提到的小块内存的内存池，对其他python对象也有内存池机制，以此弥补引用计数软肋。
引用计数还有一致命弱点----循环引用，python引入标记-清除以及分代回收填补此漏洞。
垃圾回收分两阶段：垃圾检测和垃圾回收。
python垃圾收集的过程：
4、python中的垃圾收集
4.1、可收集对象链表
python中循环引用发生在container对象间，用PyGC_Head变成可收集对象（进入可收集对象链表）：
[objimpl.h] /* GC information is stored BEFORE the object structure. */ typedef union _gc_head { struct { union _gc_head *gc_next; union _gc_head *gc_prev; Py_ssize_t gc_refs; } gc; long double dummy; /* force worst-case alignment */ } PyGC_Head;
container创建过程：
[gcmodule.c] PyObject * _PyObject_GC_New(PyTypeObject *tp) { PyObject *op = _PyObject_GC_Malloc(_PyObject_SIZE(tp)); if (op != NULL) op = PyObject_INIT(op, tp); return op; }
PyObject * _PyObject_GC_Malloc(size_t basicsize) { PyObject *op; PyGC_Head *g; if (basicsize > PY_SSIZE_T_MAX - sizeof(PyGC_Head)) return PyErr_NoMemory(); g = (PyGC_Head *)PyObject_MALLOC( sizeof(PyGC_Head) + basicsize); if (g == NULL) return PyErr_NoMemory(); g->gc.gc_refs = GC_UNTRACKED; generations[0].count++; /* number of allocated GC objects */ if (generations[0].count > generations[0].threshold && enabled && generations[0].threshold && !collecting && !PyErr_Occurred()) { collecting = 1; collect_generations(); collecting = 0; } op = FROM_GC(g); return op; }
创建后第一部分是用于垃圾收集的PyGC_Head，接着是python所有对象都有的PyObject_HEAD，最后是属于container对象自身的数据。
PyGC_Head和PyObject_HEAD地址转换：
[gcmodule.c] /* 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)) [objimpl.h] #define _Py_AS_GC(o) ((PyGC_Head *)(o)-1)
在创建某个container对象最后一步会链接到可收集对象链表中：
[objimpl.h] /* Tell the GC to track this object. NB: While the object is tracked the * collector it must be safe to call the ob_traverse method. */ #define _PyObject_GC_TRACK(o) do { PyGC_Head *g = _Py_AS_GC(o); if (g->gc.gc_refs != _PyGC_REFS_UNTRACKED) Py_FatalError("GC object already tracked"); g->gc.gc_refs = _PyGC_REFS_REACHABLE; g->gc.gc_next = _PyGC_generation0; g->gc.gc_prev = _PyGC_generation0->gc.gc_prev; g->gc.gc_prev->gc.gc_next = g; _PyGC_generation0->gc.gc_prev = g; } while (0);
从链表摘除container对象：
[objimpl.h] /* Tell the GC to stop tracking this object. * gc_next doesn't need to be set to NULL, but doing so is a good * way to provoke memory errors if calling code is confused. */ #define _PyObject_GC_UNTRACK(o) do { PyGC_Head *g = _Py_AS_GC(o); assert(g->gc.gc_refs != _PyGC_REFS_UNTRACKED); g->gc.gc_refs = _PyGC_REFS_UNTRACKED; g->gc.gc_prev->gc.gc_next = g->gc.gc_next; g->gc.gc_next->gc.gc_prev = g->gc.gc_prev; g->gc.gc_next = NULL; } while (0);
4.2、分代的垃圾收集
python中引入分代的垃圾收集机制，共有3代，每一代都是一个链表，在之前的链表基础上加上一个表头：
[gcmodule.c] struct gc_generation { PyGC_Head head; int threshold; /* collection threshold */ int count; /* count of allocations or collections of younger generations */ };
python中维护 了三个gc_generation结构的数组，通过这数组控制三条可收集对象链表，即三个“代”：
[gcmodule.c] #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);
count表示有多少个可收集对象，threshold表示该链可容纳收集对象个数，当超过这个值时会触发垃圾回收机制：
[gcmodule.c] static Py_ssize_t collect_generations(void) { int i; Py_ssize_t n = 0; /* Find the oldest generation (highest numbered) where the count * exceeds the threshold. Objects in the that generation and * generations younger than it will be collected. */ for (i = NUM_GENERATIONS-1; i >= 0; i--) { if (generations[i].count > generations[i].threshold) { /* Avoid quadratic performance degradation in number of tracked objects. See comments at the beginning of this file, and issue #4074. */ if (i == NUM_GENERATIONS - 1 && long_lived_pending < long_lived_total / 4) continue; n = collect(i); break; } } return n; }
4.3、Python中的标记——清除方法
开始垃圾收集前，会将收集的代及更年轻的代合并，再进行收集：
[gcmodule.c] static void gc_list_init(PyGC_Head *list) { list->gc.gc_prev = list; list->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); }
为了得出真正的引用计数，引入有效引入计数，使用计数副本计算，即PyGC_Head中的gc.gc_ref：
[gcmodule.c] static void update_refs(PyGC_Head *containers) { PyGC_Head *gc = containers->gc.gc_next; for (; gc != containers; gc = gc->gc.gc_next) { assert(gc->gc.gc_refs == GC_REACHABLE); gc->gc.gc_refs = 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(gc->gc.gc_refs != 0); } }
先将对象gc.gc_ref设置为ob_refcnt的值，再将循环引用摘除：
[gcmodule.c] 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); } }
traverse与特定的container对象相关，用于遍历container对象中的每一个引用，对引用作某种动作，在subtract_refs中动作就是visit_dec_ref。完成后摘除了container对象间的环引用，得出root object（用于开始标记--清除算法）集合。
得出root object集合后，开始标记垃圾，用move_unreachable将可回收对象从root object链表中移到unreachable链表中：
[gcmodule.c] 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 (gc->gc.gc_refs) { /* 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(gc->gc.gc_refs > 0); gc->gc.gc_refs = 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); gc->gc.gc_refs = GC_TENTATIVELY_UNREACHABLE; } gc = next; } } 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 = gc->gc.gc_refs; 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. */ gc->gc.gc_refs = 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); gc->gc.gc_refs = 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; }
分割完就得到垃圾回收目标对象，unreachable链表中的对象。
但是，并不是所有在unreachable链表中的对象都能安全回收。
当一个container对象，从类对象实例化出来的实例对象，定义了__del__方法时（python中称为finalizer）。当一个拥有finalizer的实例对象被销毁时，首先调用finalizer，因为__del__是python在对象销毁时进行资源释放的Hook机制。问题是，unreachable链表中都是循环引用对象，需要被销毁，其中有对象的finalizer引用了另一对象，python又不能保证销毁顺序。python将unreachable链表中拥有finalizer的PyInstanceObject都移到garbage的PyListObject对象中。
回收unreachable链表中的垃圾对象：
[gcmodule.c] static int gc_list_is_empty(PyGC_Head *list) { return (list->gc.gc_next == list); } /* 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); assert(IS_TENTATIVELY_UNREACHABLE(op)); 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); gc->gc.gc_refs = GC_REACHABLE; } } }
对ob_refcnt下手，将unreachable链表中所有对象ob_refcnt变为0，引发对象销毁。
其中调用container对象的tp_clear操作，调整container对象中每个引用所用的对象的引用计数值，从而打破循环。
实际完成垃圾收集的collect：
[gcmodule.c] /* This is the main function. Read this to understand how the * collection process works. */ static Py_ssize_t collect(int generation) { 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; if (delstr == NULL) { delstr = PyString_InternFromString("__del__"); if (delstr == NULL) Py_FatalError("gc couldn't allocate "__del__""); } if (debug & DEBUG_STATS) { PySys_WriteStderr("gc: collecting generation %d... ", 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(" "); } /* 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 * finalizers can't safely be deleted. Python programmers should take * care not to create such things. For Python, finalizers means * instance objects with __del__ methods. Weakrefs with callbacks * can also call arbitrary Python code but they will be dealt with by * handle_weakrefs(). */ gc_list_init(&finalizers); move_finalizers(&unreachable, &finalizers); /* finalizers contains the unreachable objects with a finalizer; * unreachable objects reachable *from* those are also uncollectable, * and we move those into the finalizers list too. */ move_finalizer_reachable(&finalizers); /* Collect statistics on collectable objects found and print * debugging information. */ for (gc = unreachable.gc.gc_next; gc != &unreachable; gc = gc->gc.gc_next) { m++; if (debug & DEBUG_COLLECTABLE) { debug_cycle("collectable", FROM_GC(gc)); } } /* Clear weakrefs and invoke callbacks as necessary. */ m += handle_weakrefs(&unreachable, old); /* Call tp_clear on objects in the unreachable 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.gc_next; gc != &finalizers; gc = gc->gc.gc_next) { n++; if (debug & DEBUG_UNCOLLECTABLE) debug_cycle("uncollectable", FROM_GC(gc)); } if (debug & DEBUG_STATS) { double t2 = get_time(); if (m == 0 && n == 0) PySys_WriteStderr("gc: done"); else PySys_WriteStderr( "gc: done, " "%" PY_FORMAT_SIZE_T "d unreachable, " "%" PY_FORMAT_SIZE_T "d uncollectable", n+m, n); if (t1 && t2) { PySys_WriteStderr(", %.4fs elapsed", t2-t1); } PySys_WriteStderr(". "); } /* 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. */ (void)handle_finalizers(&finalizers, old); /* Clear free list only during the collection of the highest * generation */ if (generation == NUM_GENERATIONS-1) { clear_freelists(); } if (PyErr_Occurred()) { if (gc_str == NULL) gc_str = PyString_FromString("garbage collection"); PyErr_WriteUnraisable(gc_str); Py_FatalError("unexpected exception during garbage collection"); } return n+m; }
python中的垃圾收集机制完全是为了处理循环引用而设计的，几乎大多数对象创建时都会被纳入垃圾收集机制的监控中。并且，正常的引用计数就能销毁一个被纳入垃圾收集机制监控的对象。
python很多对象挂在垃圾收集监控的链表上，但大多情况是引用计数在维护这些对象。对引用计数无能为力的循环引用，垃圾收集机制才起作用。而垃圾收集机制只处理引用计数不为0的情况：一是被程序使用的对象（不能被回收），二是循环引用对象。因此垃圾回收机制只能处理循环引用中的对象。
还有一点，PyObject_GC_New底层是以之前剖析的PyObject_Malloc作为真正申请内存的接口的，大多数情况下Python都在使用内存池。而本书中剖析过得最大的对象PyTypeObject也不超过200个字节，小于256个字节，故也使用内存池。因此可将垃圾收集和内存管理融为一体。
4.5、python 中 的gc模块
python中通过gc模块提供了观察和手动实用gc机制的接口。
具体打开python，动手实验。