Nginx源碼閱讀（ngx_pool_t）

// src/core/ngx_palloc.h

struct ngx_pool_s {
    ngx_pool_data_t       d;
    size_t                max; /* 全部可使用內存的大小，包含已使用和未使用內存。
                                  區別小塊和大塊內存的標準，小于等于max為小塊內存，大于max為大塊內存。 */
    ngx_pool_t           *current; // 在多個ngx_pool_t連成的鏈表中，current指向分配內存時遍歷的第一個ngx_pool_t
    ngx_chain_t          *chain;
    ngx_pool_large_t     *large; // 指向多個ngx_pool_large_t連成的鏈表
    ngx_pool_cleanup_t   *cleanup; // 指向多個ngx_pool_cleanup_t連成的鏈表
    ngx_log_t            *log;
};


typedef struct {
    u_char               *last; // 指向未使用內存的首部
    u_char               *end; // 指向未使用內存的尾部
    ngx_pool_t           *next; // 多個ngx_pool_t通過next連成鏈表
    ngx_uint_t            failed; /* 每當剩余空間不足以分配出小塊內存時，failed成員就會加1。
                                     failed成員大于4后，ngx_pool_t的current將移向下一個小塊內存池。 */
} ngx_pool_data_t;


struct ngx_pool_large_s {
    ngx_pool_large_t     *next; // 多個ngx_pool_large_t通過next連成鏈表
    void                 *alloc; // 指向ngx_alloc分配出的大塊內存
};

struct ngx_pool_cleanup_s {
    ngx_pool_cleanup_pt   handler; // typedef void (*ngx_pool_cleanup_pt)(void *data);
    void                 *data;
    ngx_pool_cleanup_t   *next; // 多個ngx_pool_cleanup_t通過next連成鏈表
};

// src/core/ngx_palloc.c

//創建內存池
ngx_pool_t *
ngx_create_pool(size_t size, ngx_log_t *log)
{
    ngx_pool_t  *p;

    p = ngx_memalign(NGX_POOL_ALIGNMENT, size, log);
    if (p == NULL) {
        return NULL;
    }

    p->d.last = (u_char *) p + sizeof(ngx_pool_t);
    p->d.end = (u_char *) p + size;
    p->d.next = NULL;
    p->d.failed = 0;

    // max = min{size - sizeof(ngx_pool_t), NGX_MAX_ALLOC_FROM_POOL}
    size = size - sizeof(ngx_pool_t);
    p->max = (size < NGX_MAX_ALLOC_FROM_POOL) ? size : NGX_MAX_ALLOC_FROM_POOL;

    p->current = p;
    p->chain = NULL;
    p->large = NULL;
    p->cleanup = NULL;
    p->log = log;

    return p;
}


// 銷毀內存池
void
ngx_destroy_pool(ngx_pool_t *pool)
{
    ngx_pool_t          *p, *n;
    ngx_pool_large_t    *l;
    ngx_pool_cleanup_t  *c;

    for (c = pool->cleanup; c; c = c->next) {
        if (c->handler) {
            ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0,
                           "run cleanup: %p", c);
            c->handler(c->data);
        }
    }

#if (NGX_DEBUG)

    /*
     * we could allocate the pool->log from this pool
     * so we cannot use this log while free()ing the pool
     */

    for (l = pool->large; l; l = l->next) {
        ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0, "free: %p", l->alloc);
    }

    for (p = pool, n = pool->d.next; /* void */; p = n, n = n->d.next) {
        ngx_log_debug2(NGX_LOG_DEBUG_ALLOC, pool->log, 0,
                       "free: %p, unused: %uz", p, p->d.end - p->d.last);

        if (n == NULL) {
            break;
        }
    }

#endif


    for (l = pool->large; l; l = l->next) {
        if (l->alloc) {
            ngx_free(l->alloc);
        }
    }

    for (p = pool, n = pool->d.next; /* void */; p = n, n = n->d.next) {
        ngx_free(p);

        if (n == NULL) {
            break;
        }
    }
}


// 重置內存池。把大塊內存釋放給操作系統，而小塊內存則在不釋放的情況下復用
void
ngx_reset_pool(ngx_pool_t *pool)
{
    ngx_pool_t        *p;
    ngx_pool_large_t  *l;

    for (l = pool->large; l; l = l->next) {
        if (l->alloc) {
            ngx_free(l->alloc);
        }
    }

    for (p = pool; p; p = p->d.next) {
        p->d.last = (u_char *) p + sizeof(ngx_pool_t);
        p->d.failed = 0;
    }

    pool->current = pool;
    pool->chain = NULL;
    pool->large = NULL;
}


/* ngx_palloc和ngx_pnalloc都是根據pool->max和size的大小關系，從pool中分配大小為size的內存塊。
   兩者的區別在于分配小塊內存時是否對齊d.last。 */
void *
ngx_palloc(ngx_pool_t *pool, size_t size)
{
#if !(NGX_DEBUG_PALLOC)
    if (size <= pool->max) {
        return ngx_palloc_small(pool, size, 1); // 對齊
    }
#endif

    return ngx_palloc_large(pool, size);
}


void *
ngx_pnalloc(ngx_pool_t *pool, size_t size)
{
#if !(NGX_DEBUG_PALLOC)
    if (size <= pool->max) {
        return ngx_palloc_small(pool, size, 0); // 不對齊
    }
#endif

    return ngx_palloc_large(pool, size);
}


// 分配小塊內存
static ngx_inline void *
ngx_palloc_small(ngx_pool_t *pool, size_t size, ngx_uint_t align)
{
    u_char      *m;
    ngx_pool_t  *p;

    p = pool->current;

    // 遍歷ngx_pool_t鏈表，分配大小為size的小塊內存
    do {
        m = p->d.last;

        if (align) {
            m = ngx_align_ptr(m, NGX_ALIGNMENT); // 對齊d.last
        }

        if ((size_t) (p->d.end - m) >= size) {
            p->d.last = m + size;

            return m;
        }

        p = p->d.next;

    } while (p);

    // 沒有找到大小為size的小塊內存，在鏈表尾部插入一個ngx_pool_t，分配大小為size的小塊內存
    return ngx_palloc_block(pool, size);
}


static void *
ngx_palloc_block(ngx_pool_t *pool, size_t size)
{
    u_char      *m;
    size_t       psize;
    ngx_pool_t  *p, *new;

    psize = (size_t) (pool->d.end - (u_char *) pool);

    m = ngx_memalign(NGX_POOL_ALIGNMENT, psize, pool->log);
    if (m == NULL) {
        return NULL;
    }

    new = (ngx_pool_t *) m;

    new->d.end = m + psize;
    new->d.next = NULL;
    new->d.failed = 0;

    m += sizeof(ngx_pool_data_t);
    m = ngx_align_ptr(m, NGX_ALIGNMENT);
    new->d.last = m + size;

    // 鏈表中d.failed的值依次為6, ..., 6, 5, 4, 3, 2, 1, 0，其中current指向5
    for (p = pool->current; p->d.next; p = p->d.next) {
        if (p->d.failed++ > 4) {
            pool->current = p->d.next;
        }
    }

    p->d.next = new;

    return m;
}


// 分配大塊內存
static void *
ngx_palloc_large(ngx_pool_t *pool, size_t size)
{
    void              *p;
    ngx_uint_t         n;
    ngx_pool_large_t  *large;

    p = ngx_alloc(size, pool->log);
    if (p == NULL) {
        return NULL;
    }

    n = 0;

    for (large = pool->large; large; large = large->next) {
        if (large->alloc == NULL) {
            large->alloc = p;
            return p;
        }

        if (n++ > 3) {
            break;
        }
    }

    // 沒有找到alloc為NULL的ngx_pool_large_t，從小塊內存池中分配一個新的ngx_pool_large_t
    large = ngx_palloc_small(pool, sizeof(ngx_pool_large_t), 1);
    if (large == NULL) {
        ngx_free(p);
        return NULL;
    }

    large->alloc = p;
    // 在鏈表頭部插入新建的ngx_pool_large_t
    large->next = pool->large;
    pool->large = large;

    return p;
}


// 這樣分配出的內存不管申請的size有多小，都是不會使用小塊內存池的，它會從進程的堆中分配內存，并掛在大塊內存組成的large單鏈表中
void *
ngx_pmemalign(ngx_pool_t *pool, size_t size, size_t alignment)
{
    void              *p;
    ngx_pool_large_t  *large;

    p = ngx_memalign(alignment, size, pool->log);
    if (p == NULL) {
        return NULL;
    }

    large = ngx_palloc_small(pool, sizeof(ngx_pool_large_t), 1);
    if (large == NULL) {
        ngx_free(p);
        return NULL;
    }

    large->alloc = p;
    large->next = pool->large;
    pool->large = large;

    return p;
}


// 釋放地址為p的大塊內存， 注意需要置alloc為NULL
ngx_int_t
ngx_pfree(ngx_pool_t *pool, void *p)
{
    ngx_pool_large_t  *l;

    for (l = pool->large; l; l = l->next) {
        if (p == l->alloc) {
            ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0,
                           "free: %p", l->alloc);
            ngx_free(l->alloc);
            l->alloc = NULL;

            return NGX_OK;
        }
    }

    return NGX_DECLINED;
}


// 在ngx_palloc的基礎上增加了清零分配的內存
void *
ngx_pcalloc(ngx_pool_t *pool, size_t size)
{
    void *p;

    p = ngx_palloc(pool, size);
    if (p) {
        ngx_memzero(p, size);
    }

    return p;
}