pg内存管理

buffer

• 全局变量

  int			nbufs = num_temp_buffers;
  Block	   *LocalBufferBlockPointers = NULL;
  BufferDesc *LocalBufferDescriptors = NULL;

• 初始化

  static void InitLocalBuffers(void){
      int			nbufs = num_temp_buffers;
      LocalBufferDescriptors = (BufferDesc *) calloc(nbufs, sizeof(BufferDesc));
      LocalBufferBlockPointers = (Block *) calloc(nbufs, sizeof(Block));
      ......
  }

• Block

  typedef void *Block;

• BufferDesc

  typedef struct BufferDesc
  {
    BufferTag	tag;			/* ID of page contained in buffer */
    int			buf_id;			/* buffer's index number (from 0) */
  
    /* state of the tag, containing flags, refcount and usagecount */
    pg_atomic_uint32 state;
  
    int			wait_backend_pgprocno;	/* backend of pin-count waiter */
    int			freeNext;		/* link in freelist chain */
    LWLock		content_lock;	/* to lock access to buffer contents */
  } BufferDesc;
  
  typedef struct buftag
  {
    RelFileNode rnode;			/* physical relation identifier */
    ForkNumber	forkNum;
    BlockNumber blockNum;		/* blknum relative to begin of reln */
  } BufferTag;

page

• 定义

  typedef char *Pointer;
  typedef Pointer Page;

• pageheader

  typedef struct PageHeaderData
  {
    /* XXX LSN is member of *any* block, not only page-organized ones */
    PageXLogRecPtr pd_lsn;		/* LSN: next byte after last byte of xlog
                   * record for last change to this page */
    uint16		pd_checksum;	/* checksum */
    uint16		pd_flags;		/* flag bits, see below */
    LocationIndex pd_lower;		/* offset to start of free space */
    LocationIndex pd_upper;		/* offset to end of free space */
    LocationIndex pd_special;	/* offset to start of special space */
    uint16		pd_pagesize_version;
    TransactionId pd_prune_xid; /* oldest prunable XID, or zero if none */
    ItemIdData	pd_linp[FLEXIBLE_ARRAY_MEMBER]; /* line pointer array */
  } PageHeaderData;
  
  typedef PageHeaderData *PageHeader;

• page与pageheader的关系

  page是一段内存的指针，pageheaderData是page内存最前面的数据,pageheader是指向pageheaderData的指针。

  PageHeader	phdr = (PageHeader) page;