MySQL MEM_ROOT结构体详解

#define MALLOC_OVERHEAD 8 //分配过程中，需要保留一部分额外的空间
#define ALLOC_MAX_BLOCK_TO_DROP 4096 //后续会继续分析该宏的用途
#define ALLOC_MAX_BLOCK_USAGE_BEFORE_DROP 10 //后续会继续分析该宏的用途
 
#define ALIGN_SIZE(A) MY_ALIGN((A),sizeof(double))
#define MY_ALIGN(A,L) (((A) + (L) - 1) & ~((L) - 1))
 
#define ALLOC_ROOT_MIN_BLOCK_SIZE (MALLOC_OVERHEAD + sizeof(USED_MEM) + 8)
/* Define some useful general macros (should be done after all headers). */
/*作者：www.manongjc.com  */
#define MY_MAX(a, b) ((a) > (b) ? (a) : (b)) //求两个数值之间的最大值
#define MY_MIN(a, b) ((a) < (b) ? (a) : (b)) //求两个数值之间的最小值

typedef struct st_mem_root
{USED_MEM    *free;                  /* free block link list 的链表头指针 */
    USED_MEM    *used;                  /* used block link list 的链表头指针 */
    USED_MEM    *pre_alloc;             /* 预先分配的 block */
    size_t        min_malloc;             /* 如果 block 剩下的可用空间小于该值，将会从 free list 移动到 used list */
    size_t        block_size;             /* 每次初始化的空间大小 */
    unsigned int    block_num;              /* 记录实际的 block 数量, 初始化为 4 */
    unsigned int    first_block_usage;      /* free list 中的第一个 block 测试不满足分配空间大小的次数 */
    void (*error_handler)(void);          /* 分配失败的错误处理函数 */
} MEM_ROOT;

typedef struct st_used_mem
{struct st_used_mem *next; //指向下一个分配的 block
    unsigned int left; //该 block 剩余的空间大小
    unsigned int size; //该 block 的总大小
} USED_MEM;

void init_alloc_root(MEM_ROOT *mem_root, size_t block_size, size_t pre_alloc_size __attribute__((unused) ) )
{mem_root->free            = mem_root->used = mem_root->pre_alloc = 0;
    mem_root->min_malloc        = 32;
    mem_root->block_size        = block_size - ALLOC_ROOT_MIN_BLOCK_SIZE;
    mem_root->error_handler        = 0;
    mem_root->block_num        = 4; /* We shift this with >>2 */
    mem_root->first_block_usage    = 0;
}

void *alloc_root(MEM_ROOT *mem_root, size_t length )
{
    size_t        get_size, block_size;
    uchar        * point;
    reg1 USED_MEM    *next = 0;
    reg2 USED_MEM    **prev;
 
    length = ALIGN_SIZE(length);
    if ((*(prev = &mem_root->free) ) != NULL )
    {if ((*prev)->left < length &&
             mem_root->first_block_usage++ >= ALLOC_MAX_BLOCK_USAGE_BEFORE_DROP &&
             (*prev)->left < ALLOC_MAX_BLOCK_TO_DROP )
        {next                = *prev;
            *prev                = next->next; /* Remove block from list */
            next->next            = mem_root->used;
            mem_root->used            = next;
            mem_root->first_block_usage    = 0;
        }
        for (next = *prev; next && next->left < length; next = next->next )
            prev = &next->next;
    }
    if (!next )
    {/* Time to alloc new block */
        block_size    = mem_root->block_size * (mem_root->block_num >> 2);
        get_size    = length + ALIGN_SIZE(sizeof(USED_MEM) );
        get_size    = MY_MAX(get_size, block_size);
 
        if (!(next = (USED_MEM *) my_malloc(get_size, MYF( MY_WME | ME_FATALERROR ) ) ) )
        {if (mem_root->error_handler )
                (*mem_root->error_handler)();
            DBUG_RETURN((void *) 0 );                              /* purecov: inspected */
        }
        mem_root->block_num++;
        next->next    = *prev;
        next->size    = get_size;
        next->left    = get_size - ALIGN_SIZE(sizeof(USED_MEM) );    /* bug: 如果该 block 是通过 mem_root->block_size * (mem_root->block_num >> 2)计算出来的，则已经去掉了 ALIGN_SIZE(sizeof(USED_MEM)，这里重复了。*/
        *prev        = next;
    }
 
    point = (uchar *) ((char *) next + (next->size - next->left) );
/*TODO: next part may be unneded due to mem_root->first_block_usage counter*/
/* 作者：www.manongjc.com */
    if ((next->left -= length) < mem_root->min_malloc )
    {/* Full block */
        *prev                = next->next;                   /* Remove block from list */
        next->next            = mem_root->used;
        mem_root->used            = next;
        mem_root->first_block_usage    = 0;
    }
}

for (next= *prev ; next && next->left < length ; next= next->next)
prev= &next->next;
}