MySQL MEM_ROOT详解及实例代码

#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.manongjc. /
#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.manongjc. /
 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;
}