本节介绍了checkpoint中用于控制checkpoint刷盘频率的函数:CheckpointWriteDelay. ### 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 **CheckpointWriteDelay** CheckpointWriteDelay,控制checkpoint的频率,逻辑不复杂,判断checkpoint flags非CHECKPOINT_IMMEDIATE/非shutdown_requested/非CHECKPOINT_IMMEDIATE并且checkpoint处于调度中,如满足上述条件,则调用AbsorbSyncRequests处理sync,休眠100ms;不如满足上述条件,则absorb_counter计数器减一,如计数器≤0,则调用AbsorbSyncRequests处理sync. ``` /* * CheckpointWriteDelay -- control rate of checkpoint * 控制checkpoint的频率 * * This function is called after each page write performed by BufferSync(). * It is responsible for throttling BufferSync()'s write rate to hit * checkpoint_completion_target. * * The checkpoint request flags should be passed in; currently the only one * examined is CHECKPOINT_IMMEDIATE, which disables delays between writes. * * 'progress' is an estimate of how much of the work has been done, as a * fraction between 0.0 meaning none, and 1.0 meaning all done. */ void CheckpointWriteDelay(int flags, double progress) { static int absorb_counter = WRITES_PER_ABSORB; /* Do nothing if checkpoint is being executed by non-checkpointer process */ if (!AmCheckpointerProcess()) return; /* * Perform the usual duties and take a nap, unless we're behind schedule, * in which case we just try to catch up as quickly as possible. */ if (!(flags & CHECKPOINT_IMMEDIATE) &&//非CHECKPOINT_IMMEDIATE !shutdown_requested &&//非关闭请求 !ImmediateCheckpointRequested() &&//非CHECKPOINT_IMMEDIATE IsCheckpointOnSchedule(progress))//处于checkpoint调度中 { if (got_SIGHUP) { got_SIGHUP = false; ProcessConfigFile(PGC_SIGHUP); /* update shmem copies of config variables */ UpdateSharedMemoryConfig(); } //Absorb吸收(处理)同步请求 AbsorbSyncRequests(); //重置为WRITES_PER_ABSORB(1000) absorb_counter = WRITES_PER_ABSORB; //检查归档是否超时 CheckArchiveTimeout(); /* * Report interim activity statistics to the stats collector. */ //统计信息 pgstat_send_bgwriter(); /* * This sleep used to be connected to bgwriter_delay, typically 200ms. * That resulted in more frequent wakeups if not much work to do. * Checkpointer and bgwriter are no longer related so take the Big * Sleep. */ //休眠100ms pg_usleep(100000L); } else if (--absorb_counter <= 0)//Absorb计数器减一 { //如计数器小于等于0 /* * Absorb pending fsync requests after each WRITES_PER_ABSORB write * operations even when we don't sleep, to prevent overflow of the * fsync request queue. */ //Absorb同步请求 AbsorbSyncRequests(); //重置计数器 absorb_counter = WRITES_PER_ABSORB; } } ``` ### 三、跟踪分析 N/A ### 四、参考资料 [PG Source Code](https://doxygen.postgresql.org) [PgSQL · 特性分析 · 谈谈checkpoint的调度](http://mysql.taobao.org/monthly/2015/09/06/)