函数mem_pool_create

 

/********************************************************************//**
Creates a memory pool.
@return    memory pool */
UNIV_INTERN
mem_pool_t*
mem_pool_create(
/*============*/
    ulint    size)    /*!< in: pool size in bytes */
{
    mem_pool_t*    pool; 结构体见
    mem_area_t*    area;
    ulint        i;
    ulint        used;

    pool = ut_malloc(sizeof(mem_pool_t));

    pool->buf = ut_malloc_low(size, TRUE);
    pool->size = size;

    mutex_create(mem_pool_mutex_key, &pool->mutex, SYNC_MEM_POOL);

    /* Initialize the free lists */
    /**
 *pool中的free_list可理解为64个元素的数组,数组元素的类型为mem_area_t
 *每个元素同时又是个双循环链表,大小都一样,都是2的N次方
 */
    for (i = 0; i < 64; i++) {

        UT_LIST_INIT(pool->free_list[i]);
    }

    used = 0;
    
  /**
 *假设size为36
 *1) 36-0 > 
 * i=ut_2_log(36-0)=5 计算出不超过36的最大的2的N次方
 * 令pool->buf+0为area,其area->size 为 ut_2_exp(5)=32
 * 再将些area置为pool->free_list[5]中的数组中头结点
 * 此时used=0+32
 *
 *2) 36-32 >
 * i=ut_2_log(36-32)=2
 * 令pool->buf+32为area,其area->size为ut_2_exp(2)=4
 * 再将此area置为pool->free_list[2]中数组的头结点
 * 此时used=32+ut_2_exp(2)=32+4=36
 *
 *3) 36-36 ? 退出循环
 */ while (size - used >= MEM_AREA_MIN_SIZE) {

        i = ut_2_log(size - used);

        if (ut_2_exp(i) > size - used) {

            /* ut_2_log rounds upward */

            i--;
        }

        area = (mem_area_t*)(pool->buf + used);

        mem_area_set_size(area, ut_2_exp(i));
        mem_area_set_free(area, TRUE);
        UNIV_MEM_FREE(MEM_AREA_EXTRA_SIZE + (byte*) area,ut_2_exp(i) - MEM_AREA_EXTRA_SIZE);

        UT_LIST_ADD_FIRST(free_list, pool->free_list[i], area);

        used = used + ut_2_exp(i);
    }

    ut_ad(size >= used);

    pool->reserved = 0;

    return(pool);
}