MySQL MEM_ROOT详细讲解

#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 intleft; //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;
}