细说MySQL 之MEM_ROOT

<ol style="margin:0 1px 0 0px;padding-left:40px;" start="1" class="dp-css"><li>#define malloc_overhead 8 //分配过程中，需要保留一部分额外的空间<br /> </li><li>#define alloc_max_block_to_drop 4096 //后续会继续分析该宏的用途<br /></li><li>#define alloc_max_block_usage_before_drop 10 //后续会继续分析该宏的用途<br /></li><li><br /></li><li>#define align_size(a) my_align((a),sizeof(double))<br /></li><li>#define my_align(a,l) (((a) + (l) - 1) & ~((l) - 1))<br /></li><li><br /></li><li>#define alloc_root_min_block_size (malloc_overhead + sizeof(used_mem) + 8)<br /></li><li>/* define some useful general macros (should be done after all headers). */<br /></li><li>#define my_max(a, b) ((a) > (b) ? (a) : (b)) //求两个数值之间的最大值<br /></li><li>#define my_min(a, b) ((a) < (b) ? (a) : (b)) //求两个数值之间的最小值</li></ol>

<ol style="margin:0 1px 0 0px;padding-left:40px;" start="1" class="dp-css"><li>typedef struct st_mem_root<br /> </li><li>{<br /></li><li>used_mem *free; //free block link list的链表头指针<br /></li><li>used_mem *used;//used block link list的链表头指针<br /></li><li>used_mem *pre_alloc; //预先分配的block<br /></li><li>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<br /></li><li>size_t block_size; //每次初始化的空间大小<br /></li><li>unsigned int block_num; //记录实际的block数量,初始化为4<br /></li><li>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<br /></li><li>void (*error_handler)(void);//分配失败的错误处理函数<br /></li><li>} mem_root; </li></ol>

<ol style="margin:0 1px 0 0px;padding-left:40px;" start="1" class="dp-css"><li>typedef struct st_used_mem<br /> </li><li>{ <br /></li><li>struct st_used_mem *next; //指向下一个分配的block<br /></li><li>unsigned int left; //该block剩余的空间大小<br /></li><li>unsigned int size; //该block的总大小<br /></li><li>} used_mem; </li></ol>

<ol style="margin:0 1px 0 0px;padding-left:40px;" start="1" class="dp-css"><li>void init_alloc_root(mem_root *mem_root, size_t block_size,size_t pre_alloc_size __attribute__((unused)))<br /> </li><li>{<br /></li><li>mem_root->free= mem_root->used= mem_root->pre_alloc= 0;<br /></li><li>mem_root->min_malloc= 32;<br /></li><li>mem_root->block_size= block_size - alloc_root_min_block_size;<br /></li><li>mem_root->error_handler= 0;<br /></li><li>mem_root->block_num= 4; /* we shift this with >>2 */<br /></li><li>mem_root->first_block_usage= 0;<br /></li><li>} </li></ol>

<ol style="margin:0 1px 0 0px;padding-left:40px;" start="1" class="dp-css"><li>void *alloc_root(mem_root *mem_root, size_t length)<br /> </li><li>{<br /></li><li>size_t get_size, block_size;<br /></li><li>uchar* point;<br /></li><li>reg1 used_mem *next= 0;<br /></li><li>reg2 used_mem **prev;<br /></li><li><br /></li><li>length= align_size(length);<br /></li><li>if ((*(prev= &mem_root->free)) != null)<br /></li><li>{<br /></li><li>if ((*prev)->left < length &&<br /></li><li>mem_root->first_block_usage++ >= alloc_max_block_usage_before_drop &&<br /></li><li>(*prev)->left < alloc_max_block_to_drop)<br /></li><li>{<br /></li><li>next= *prev;<br /></li><li>*prev= next->next; /* remove block from list */<br /></li><li>next->next= mem_root->used;<br /></li><li>mem_root->used= next;<br /></li><li>mem_root->first_block_usage= 0;<br /></li><li>}<br /></li><li>for (next= *prev ; next && next->left < length ; next= next->next)<br /></li><li>prev= &next->next;<br /></li><li>}<br /></li><li>if (! next)<br /></li><li>{ /* time to alloc new block */<br /></li><li>block_size= mem_root->block_size * (mem_root->block_num >> 2);<br /></li><li>get_size= length+align_size(sizeof(used_mem));<br /></li><li>get_size= my_max(get_size, block_size);<br /></li><li><br /></li><li>if (!(next = (used_mem*) my_malloc(get_size,myf(my_wme | me_fatalerror))))<br /></li><li>{<br /></li><li>if (mem_root->error_handler)<br /></li><li>(*mem_root->error_handler)();<br /></li><li>dbug_return((void*) 0); /* purecov: inspected */<br /></li><li>}<br /></li><li>mem_root->block_num++;<br /></li><li>next->next= *prev;<br /></li><li>next->size= get_size;<br /></li><li>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。<br /></li><li>*prev=next;<br /></li><li>}<br /></li><li><br /></li><li>point= (uchar*) ((char*) next+ (next->size-next->left));<br /></li><li>/*todo: next part may be unneded due to mem_root->first_block_usage counter*/<br /></li><li>if ((next->left-= length) < mem_root->min_malloc)<br /></li><li>{ /* full block */<br /></li><li>*prev= next->next; /* remove block from list */<br /></li><li>next->next= mem_root->used;<br /></li><li>mem_root->used= next;<br /></li><li>mem_root->first_block_usage= 0;<br /></li><li>}<br /></li><li>} </li></ol>

<ol style="margin:0 1px 0 0px;padding-left:40px;" start="1" class="dp-css"><li>for (next= *prev ; next && next->left < length ; next= next->next)<br /> </li><li>prev= &next->next;<br /></li><li>} </li></ol>