大师兄的Python源码学习笔记(四十七）: Python的内存管理机制(二）

二、小块空间的内存池

3. arena

• 在Python中，多个pool聚合的结果就是一个arena。
• 一个arena里默认有64个pool，如果pool的默认大小为4kb，arena的默认值就是256kb。

Objects/obmalloc.c

#define ARENA_SIZE              (256 << 10)     /* 256KB */

• arena在源码中对应的arena_object结构体如下：

Objects/obmalloc.c

typedef uint8_t 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.
     */
    uintptr_t 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;
};

• arena_object仅仅是arena的一部分，一个完整的arena包括一个arena_object和透过这个arena_object管理着的pool集合。

3.1 未使用的arena和可用的arena

• 在arena_object中可以看到nextarena和prevarena，这是否意味着会有一个或多个arena构成的链表？
• 实际确实会存在多个arena_object构成的集合，但这个集合并不构成链表，而是构成了一个arena数组。
• 这个数组的首个地址由arenas维护，他是Python中通用小块内存的内存池。
• 另一方面nextarena和prevarena也确实是用来连接arena_object组成链表这是为什么哪？
• arena是用来管理一组pool集合的，并且arena_object和pool_header的作用看起来也是一样的。
• 但是pool_header和arena_object管理的内存有一点细微的差别：

• pool_header管理的内存和pool_header自身是一块连续的内存；
• arena_object与其管理的内存是分离的
  

• 这意味着，当pool_header被申请时，它所管理的block内存集合一定也被申请了。
• 而当arena_object被申请时，他所管理的pool集合内存则没有被申请。
• 所以arena_object和pool集合在某一时刻需要建立联系，这个建立联系的时刻是一个关键时刻，会将一个arena_object切分为两种状态。
• 当一个arena的arena_object没有与pool集合建立联系时，处于未使用状态；一旦建立了联系，则转换到可用状态。
• 对于每一种状态，都有一个arena链表，未使用对应****的表头是unused_arena_objects，通过nextarena连接，是一个单向链表。
• 而可用的arena的链表表头是usable_arenas，通过nextarena和prevarena连接，是一个双向链表。
  

3.2 申请arena

• Python使用new_arena来创建arena:

Objects/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;
... ...

/* 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 */
    void *address;
    static int debug_stats = -1;

    if (debug_stats == -1) {
        const char *opt = Py_GETENV("PYTHONMALLOCSTATS");
        debug_stats = (opt != NULL && *opt != '\0');
    }
    if (debug_stats)
        _PyObject_DebugMallocStats(stderr);

    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 > SIZE_MAX / sizeof(*arenas))
            return NULL;                /* overflow */
#endif
        nbytes = numarenas * sizeof(*arenas);
        arenaobj = (struct arena_object *)PyMem_RawRealloc(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);
    address = _PyObject_Arena.alloc(_PyObject_Arena.ctx, ARENA_SIZE);
    if (address == NULL) {
        /* The allocation failed: return NULL after putting the
         * arenaobj back.
         */
        arenaobj->nextarena = unused_arena_objects;
        unused_arena_objects = arenaobj;
        return NULL;
    }
    arenaobj->address = (uintptr_t)address;

    ++narenas_currently_allocated;
    ++ntimes_arena_allocated;
    if (narenas_currently_allocated > narenas_highwater)
        narenas_highwater = narenas_currently_allocated;
    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;
}

• 在这段代码中，收回会检查当前unused_arena_objects链表中是否还有未使用的arena。
• 如果存在未使用的arena：

• 从unused_arena_objects链表中抽取一个arena，并斩断链表与提取arena之间的联系。
• 申请一块大小为ARENA_SIZE的内存，将申请的内存地址赋给arena的address，作为pool集合的容身之处。
• 这时arena_object已经与pool集合建立了联系，具备了成为可用内存的条件。
• 随后，会在arena中设置一些用于维护pool集合的信息。
• 这时，Python会放弃一些申请到的内存，将可使用的内存边界(pool_address)的调整到与系统页对齐。

• 如果不存在未使用的arena：

• Python会将maxarenas的值设置为当前的一倍，用于扩大系统的arena集合(小块内存内存池)数。
• 之后会检查这个新得到的maxarenas是否内存溢出。
• 如果没有溢出则通过realloc扩大arena指向的内存，并对新申请的内存进行设置。
• 这里新申请的内存的address值会被设置为0。
• 实际上address是arena状态的标识，0表示未使用，非0表示可用。
• 最后设置unused_arena_objects链表，这样系统又有了未使用内存，可以跳回上一个分支继续创建arena。