PostgreSQL 源码解读（214）- 后台进程#13（checkpointer-IsCheckpointOnSchedule）

2024-04-02 19:55

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本节介绍了checkpoint中用于控制checkpoint刷盘频率的函数:IsCheckpointOnSchedule.

一、数据结构

宏定义
checkpoints request flag bits
checkpoints request flag bits,检查点请求标记位定义.





#define CHECKPOINT_IS_SHUTDOWN  0x0001  
#define CHECKPOINT_END_OF_RECOVERY  0x0002  
#define CHECKPOINT_IMMEDIATE  0x0004  
#define CHECKPOINT_FORCE    0x0008  
#define CHECKPOINT_FLUSH_ALL  0x0010  

#define CHECKPOINT_WAIT     0x0020  
#define CHECKPOINT_REQUESTED  0x0040  

#define CHECKPOINT_CAUSE_XLOG 0x0080  
#define CHECKPOINT_CAUSE_TIME 0x0100

WRITES_PER_ABSORB




//调用AbsorbSyncRequests的间隔,默认值为1000
#define WRITES_PER_ABSORB   1000

二、源码解读

IsCheckpointOnSchedule
该函数判断是否在完成checkpoint的调度中,如返回T则可以休息,否则返回F则需要干活.




static void
CalculateCheckpointSegments(void)
{
  double    target;
  
  //#define ConvertToXSegs(x,segsize) (x / ((segsize) / (1024 * 1024)))
  target = (double) ConvertToXSegs(max_wal_size_mb, wal_segment_size) /
    (1.0 + CheckPointCompletionTarget);
  
  CheckPointSegments = (int) target;
  if (CheckPointSegments < 1)
    CheckPointSegments = 1;
}

static bool
IsCheckpointOnSchedule(double progress)
{
  XLogRecPtr  recptr;
  struct timeval now;
  double    elapsed_xlogs,
        elapsed_time;
  Assert(ckpt_active);
  
  //实际进度调整为progress*checkpoint_completion_target
  progress *= CheckPointCompletionTarget;
  
  if (progress < ckpt_cached_elapsed)
    return false;
  
  if (RecoveryInProgress())
    recptr = GetXLogReplayRecPtr(NULL);
  else
    recptr = GetInsertRecPtr();
  elapsed_xlogs = (((double) (recptr - ckpt_start_recptr)) /
           wal_segment_size) / CheckPointSegments;
  if (progress < elapsed_xlogs)
  {
    //进度小于产生xlogs的速度,需干活
    ckpt_cached_elapsed = elapsed_xlogs;
    return false;
  }
  
  gettimeofday(&now, NULL);
  elapsed_time = ((double) ((pg_time_t) now.tv_sec - ckpt_start_time) +
          now.tv_usec / 1000000.0) / CheckPointTimeout;
  if (progress < elapsed_time)
  {
    //进度慢于消逝的时间,需干活
    ckpt_cached_elapsed = elapsed_time;
    return false;
  }
  
  //处于调度中,可以休息
  return true;
}