MySQL 源码解读 -- innodb 内存

innodb 内存

数据结构

在innodb 内部， 用另外一个内存数据结构， mem_heap_t（也是mem_block_t）来表示

另外有allocator， 

mem_heap_allocator 负责mem_heap_t block的申请和释放， 做一个封装

ut_allocator 负责从_std::* containers申请内存， 内容比较简单， 提供一个封装， 封装alloc/dealloc_
_其中alloc时， 多申请_ut_new_pfx_t自己， 从而在ut_new_pfx_t 保存一些debug 信息

非buffer pool的内存情况（动态内存或者内存小于page_size/2）

mem_heap_t->start 指的位置是（char *）mem_heap_t + MEM_BLOCK_HEADER_SIZE

alignment

申请的（内存大小 + 32 个字节）然后向上取整8个字节

#define MEM_SPACE_NEEDED(N) \
  ut_calc_align(N + 2 * MEM_NO_MANS_LAND, UNIV_MEM_ALIGNMENT)
#define MEM_BLOCK_HEADER_SIZE \
  ut_calc_align(sizeof(mem_block_info_t), UNIV_MEM_ALIGNMENT)
  
#define ut_calc_align(n, m) (((n) + ((m)-1)) & ~((m)-1))
#define UNIV_MEM_ALIGNMENT 8
const int MEM_NO_MANS_LAND = 16;

Block 内部有很多alignment，
比如Block 的大小是  ut_calc_align（sizeof(mem_heap_t)） + MEM_SPACE_NEEDED(用户len)
用户每使用一块内存， 都会前后各空MEM_NO_MANS_LAND 个字节

使用

内存申请

函数mem_heap_alloc负责申请一块内存

mem_heap_add_block 可以参考创建过程中申请流程
assign 内存片段, 用户的内存前后，各保留了16各字节（MEN_NO_MANS_LAND）

free = mem_block_get_free(block);
buf = (byte *)block + free + MEM_NO_MANS_LAND;
mem_block_set_free(block, free + MEM_SPACE_NEEDED(n));
return buf;

创建

mem_heap_t * mem_heap_create_func() {
    block = mem_heap_create_block(NULL, size, type, file_name, line);
    UT_LIST_INIT(block->base, &mem_block_t::list);
    UT_LIST_ADD_FIRST(block->base, block);
    return block;
}

mem_heap_add_block 核心逻辑

mem_block_t *mem_heap_add_block(mem_heap_t *heap, /*!< in: memory heap */
                                ulint n) /*!< in: number of bytes user needs */
{
   //set correct new size
   new_size = xxxxx;
   new_block = mem_heap_create_block(heap, new_size, heap->type, heap->file_name,
                                    heap->line);
   UT_LIST_INSERT_AFTER(heap->base, block, new_block);
	return (new_block);
}

mem_heap_create_block 逻辑

mem_block_t *mem_heap_create_block_func(
    mem_heap_t *heap, /*!< in: memory heap or NULL if first block
                      should be created */
    ulint n,          /*!< in: number of bytes needed for user data */
#ifdef UNIV_DEBUG
    const char *file_name, /*!< in: file name where created */
    ulint line,            /*!< in: line where created */
#endif                     /* UNIV_DEBUG */
    ulint type)            /*!< in: type of heap: MEM_HEAP_DYNAMIC or
                           MEM_HEAP_BUFFER */
{
    len = MEM_BLOCK_HEADER_SIZE + MEM_SPACE_NEEDED(n);
    if (type == MEM_HEAP_DYNAMIC || len < UNIV_PAGE_SIZE / 2) {
    	block = static_cast<mem_block_t *>(ut_malloc_nokey(len));
  	} else {
        block = 从buffer_pool 中取
        len = ((ulint)srv_page_size);
        if ((type & MEM_HEAP_BTR_SEARCH) && heap) {
          /* We cannot allocate the block from the
          buffer pool, but must get the free block from
          the heap header free block field */
          buf_block = static_cast<buf_block_t *>(heap->free_block);
          heap->free_block = NULL;
        } else {
          buf_block = buf_block_alloc(NULL);
        }

        block = (mem_block_t *)buf_block->frame;
    }
    
    // 设置block
    block->buf_block = buf_block;
    block->free_block = NULL;
    block->magic_n = MEM_BLOCK_MAGIC_N;
      mem_block_set_len(block, len);
      mem_block_set_type(block, type);
      mem_block_set_start(block, MEM_BLOCK_HEADER_SIZE);
      mem_block_set_free(block, MEM_BLOCK_HEADER_SIZE);
      
      if (UNIV_UNLIKELY(heap == NULL)) {
        block->total_size = len;
      } else {
        heap->total_size += len;
      }
}

释放heap

先把heap->free_block释放掉， 从最后一个block开始free， 直到整个heap全部释放
mem_heap_block_free(heap, block); 这个函数释放的block不是从buffer pool里面申请的内存

void mem_heap_block_free(mem_heap_t *heap,   /*!< in: heap */
                         mem_block_t *block){
      UT_LIST_REMOVE(heap->base, block);
      heap->total_size -= block->len;
      block->magic_n = MEM_FREED_BLOCK_MAGIC_N;
      if (type == MEM_HEAP_DYNAMIC || len < UNIV_PAGE_SIZE / 2) {
        ut_allocator<byte>(PSI_NOT_INSTRUMENTED) \
      		.deallocate(reinterpret_cast<byte *>(block));
          // 实际上就是把整个ut_new_pfx_t内存块给释放掉， 清掉了trace 字段
      } else {
        /* Make memory available again for buffer pool, as we set parts
        of block to "free" state in heap allocator. */
        UNIV_MEM_ALLOC(block, UNIV_PAGE_SIZE);
        buf_block_free(buf_block);
        ///** Frees a buffer block which does not contain a file page. */
        //buf_LRU_block_free_non_file_page(block);
      }
}