大师兄的Python源码学习笔记(五十）: Python的内存管

大师兄的Python源码学习笔记(四十九）: Python的内存管理机制(四）
[大师兄的Python源码学习笔记(五十一）: Python的内存管理机制(六）(https://www.jianshu.com/p/49effa4a619e)]

三、内存池

2. pool的初始化

• Python启动后，在usedpools这个小块空间内存池中并不存在任何可用的pool。
• 这时Python会采用延迟分配策略，当我们确实开始申请小块内存事，才开始建立这个内存池。

• 当申请28个字节的内存时，Python实际会申请32字节的内存。
• Python首先会根据32个字节对应的class size index(3)在usedpools中对应的位置查找。
• 如果发现在对应的位置后并没有链接任何可用的pool，Python会用useable_arenas链表中的第一个pool。
• 需要特别注意，当前获得的arena中包含的这些pool可能并不属于同一个class size index，因为arena和pool不同，没有size class的属性。

Objects\obmalloc.c

static int
pymalloc_alloc(void *ctx, void **ptr_p, size_t nbytes)
{
    block *bp;
    poolp pool;
    poolp next;
    uint size;

    ... ...

    LOCK();
    /*
     * Most frequent paths first
     */
    size = (uint)(nbytes - 1) >> ALIGNMENT_SHIFT;
    pool = usedpools[size + size];
    if (pool != pool->nextpool) {
        ... ...

    }

    /* 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) {
            goto failed;
        }
#endif
        usable_arenas = new_arena();
        if (usable_arenas == NULL) {
            goto failed;
        }
        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:
        ... ...
}

• 可以看到，如果开始时usable_arenas为空，那么Python会通过new_arena()申请一个arena，开始构建usable_arenas链表。
• 在这里，一个脱离了unused_arena_objects并转变为usable的arena被纳入了usable_arenas的控制。
• 随后，Python会尝试从usable_arenas链表中的第一个arena锁维护的pool集合中取出一个可用的pool。
• 如果成功取出了这个pool，则进行一些维护信息的更新工作，甚至在当前arena中可用的pool已经用完后，将该arena从usable_arenas链表中摘除，当然它还在arena数组中。
• 如果取出pool失败的话题放在后面。

2.1 初始化一

• 当我们手中已经有了一块用于32字节内存分配的pool时，为了以后提高内存分配的效率，需要将其放入到usedpools中，这一步叫做init pool:

Objects\obmalloc.c

static int
pymalloc_alloc(void *ctx, void **ptr_p, size_t nbytes)
{
    ... ...
    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;
            assert(bp != NULL);
            pool->freeblock = *(block **)bp;
            goto success;
        }
        /*
         * 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;
        goto success;
   ... ...
}

• 在上面代码中，Python将得到的pool放入到usedpools中。
• 当一个从未被使用的pool被链入usedpools中时，其szidx将被设定为0xFFFF，所以会跳过if (pool->szidx == size)判断。
• 只有当一个pool从empty状态转为used状态之后，由于这时szidx还是其转为empty状态之前的值，所以才会执行if (pool->szidx == size)中的部分。
• 在以下情况下一个pool会从empty状态转换为used状态：

• 假设申请的内存size class index为i，且usedpools[i+i]处没有处于used状态的pool，同时在Python维护的全局变量freepools中还有处于empty的pool。
• 如果满足上面的条件，那么位于freepools所维护的pool链表头部的pool将被取出来，放入usedpools中，并从其内部分配一块block。
• 同时，这个pool也就从empty状态转换到了used状态。

Objects\obmalloc.c

static int
pymalloc_alloc(void *ctx, void **ptr_p, size_t nbytes)
{
    ... ...
    /* 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);
        }
    ... ...
    }

2.2 初始化二

• 当PyObject_Malloc从new_arena中得到一个新的arena后，将初始化其中的pool集合，并最终分配一个block:

Objects\obmalloc.c

static int
pymalloc_alloc(void *ctx, void **ptr_p, size_t nbytes)
{
    block *bp;
    poolp pool;
    poolp next;
    uint size;

... ...

    /* 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 = (uint)(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;

... ...
}

• 这里首先从新申请的arena中取出一个新的pool。
• 然后设置pool中的arenaindex，这个index实际上就是pool所在的arena位于arenas所指的数组中的序号，它可以用来判断一个block是否在某个pool中。
• 随后，Python将新得到的pool的szidx设置为0xffff，用来表示它没管理过block集合。
• 接着，调整刚获得arena中的pools集合，甚至可能调整usable_arenas。
• 在完成上面内容后，Python将goto init_pool，完成将pool放入usedpools中的任务。
• PyObject_Malloc的完整代码如下，其中还有很多细节：

Objects\obmalloc.c

static int
pymalloc_alloc(void *ctx, void **ptr_p, 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)) {
        return 0;
    }
#endif

    if (nbytes == 0) {
        return 0;
    }
    if (nbytes > SMALL_REQUEST_THRESHOLD) {
        return 0;
    }

    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) {
            goto success;
        }

        /*
         * 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;
            goto success;
        }

        /* Pool is full, unlink from used pools. */
        next = pool->nextpool;
        pool = pool->prevpool;
        next->prevpool = pool;
        pool->nextpool = next;
        goto success;
    }

    /* 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) {
            goto failed;
        }
#endif
        usable_arenas = new_arena();
        if (usable_arenas == NULL) {
            goto failed;
        }
        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;
            assert(bp != NULL);
            pool->freeblock = *(block **)bp;
            goto success;
        }
        /*
         * 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;
        goto success;
    }

    /* 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 = (uint)(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;

success:
    UNLOCK();
    assert(bp != NULL);
    *ptr_p = (void *)bp;
    return 1;

failed:
    UNLOCK();
    return 0;
}