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

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.

/* * OR-able request flag bits for checkpoints.  The "cause" bits are used only * for logging purposes.  Note: the flags must be defined so that it's * sensible to OR together request flags arising from different requestors. *//* These directly affect the behavior of CreateCheckPoint and subsidiaries */#define CHECKPOINT_IS_SHUTDOWN  0x0001  /* Checkpoint is for shutdown */#define CHECKPOINT_END_OF_RECOVERY  0x0002  /* Like shutdown checkpoint, but                       * issued at end of WAL recovery */#define CHECKPOINT_IMMEDIATE  0x0004  /* Do it without delays */#define CHECKPOINT_FORCE    0x0008  /* Force even if no activity */#define CHECKPOINT_FLUSH_ALL  0x0010  /* Flush all pages, including those                     * belonging to unlogged tables *//* These are important to RequestCheckpoint */#define CHECKPOINT_WAIT     0x0020  /* Wait for completion */#define CHECKPOINT_REQUESTED  0x0040  /* Checkpoint request has been made *//* These indicate the cause of a checkpoint request */#define CHECKPOINT_CAUSE_XLOG 0x0080  /* XLOG consumption */#define CHECKPOINT_CAUSE_TIME 0x0100  /* Elapsed time */

WRITES_PER_ABSORB

/* interval for calling AbsorbSyncRequests in CheckpointWriteDelay *///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.

/* * Calculate CheckPointSegments based on max_wal_size_mb and * checkpoint_completion_target. * 计算CheckPointSegments */static voidCalculateCheckpointSegments(void){  double    target;  /*-------   * Calculate the distance at which to trigger a checkpoint, to avoid   * exceeding max_wal_size_mb. This is based on two assumptions:   *   * a) we keep WAL for only one checkpoint cycle (prior to PG11 we kept   *    WAL for two checkpoint cycles to allow us to recover from the   *    secondary checkpoint if the first checkpoint failed, though we   *    only did this on the master anyway, not on standby. Keeping just   *    one checkpoint simplifies processing and reduces disk space in   *    many smaller databases.)   * b) during checkpoint, we consume checkpoint_completion_target *   *    number of segments consumed between checkpoints.   *-------   */  //#define ConvertToXSegs(x,segsize) (x / ((segsize) / (1024 * 1024)))  target = (double) ConvertToXSegs(max_wal_size_mb, wal_segment_size) /    (1.0 + CheckPointCompletionTarget);  /* round down */  CheckPointSegments = (int) target;  if (CheckPointSegments < 1)    CheckPointSegments = 1;}/* * IsCheckpointOnSchedule -- are we on schedule to finish this checkpoint *     (or restartpoint) in time? * IsCheckpointOnSchedule -- 是否在完成checkpoint的调度中 * * Compares the current progress against the time/segments elapsed since last * checkpoint, and returns true if the progress we've made this far is greater * than the elapsed time/segments. * 当前的进度与消逝的time/xlog segments进行比较,如果进度要早,那么返回T(进入休息状态) */static boolIsCheckpointOnSchedule(double progress){  XLogRecPtr  recptr;  struct timeval now;  double    elapsed_xlogs,        elapsed_time;  Assert(ckpt_active);  /* Scale progress according to checkpoint_completion_target. */  //实际进度调整为progress*checkpoint_completion_target  progress *= CheckPointCompletionTarget;  /*   * Check against the cached value first. Only do the more expensive   * calculations once we reach the target previously calculated. Since   * neither time or WAL insert pointer moves backwards, a freshly   * calculated value can only be greater than or equal to the cached value.   * 如果进度小于缓存值,返回F,需加快进度了!   */  if (progress < ckpt_cached_elapsed)    return false;  /*   * Check progress against WAL segments written and CheckPointSegments.   * 进度 vs WAL   *   * We compare the current WAL insert location against the location   * computed before calling CreateCheckPoint. The code in XLogInsert that   * actually triggers a checkpoint when CheckPointSegments is exceeded   * compares against RedoRecptr, so this is not completely accurate.   * However, it's good enough for our purposes, we're only calculating an   * estimate anyway.   *   * During recovery, we compare last replayed WAL record's location with   * the location computed before calling CreateRestartPoint. That maintains   * the same pacing as we have during checkpoints in normal operation, but   * we might exceed max_wal_size by a fair amount. That's because there can   * be a large gap between a checkpoint's redo-pointer and the checkpoint   * record itself, and we only start the restartpoint after we've seen the   * checkpoint record. (The gap is typically up to CheckPointSegments *   * checkpoint_completion_target where checkpoint_completion_target is the   * value that was in effect when the WAL was generated).   */  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;  }  /*   * Check progress against time elapsed and checkpoint_timeout.   * 比较时间   */  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;  }  /* It looks like we're on schedule. */  //处于调度中,可以休息  return true;}

三、跟踪分析

N/A

四、参考资料

PG Source Code
PgSQL · 特性分析 · 谈谈checkpoint的调度