Postgresql检查点调度

CreateCheckPoint：启动检查点
  |
  CheckPointGuts：检查点流程入口
    |
    CheckPointBuffers：刷缓存页入口
      |
      BufferSync：刷缓存页具体函数
        |
        CheckpointWriteDelay：主动延迟函数                <-------- 本篇涉及
          |
          IsCheckpointOnSchedule：主动延迟函数的判断算法    <-------- 本篇涉及

/*
 * CheckpointWriteDelay -- control rate of checkpoint
 *
 * 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) &&
        !shutdown_requested &&
        !ImmediateCheckpointRequested() &&
        IsCheckpointOnSchedule(progress))

    {
        if (got_SIGHUP)
        {
            got_SIGHUP = false;
            ProcessConfigFile(PGC_SIGHUP);
            /* update shmem copies of config variables */
            UpdateSharedMemoryConfig();
        }

        AbsorbFsyncRequests();
        absorb_counter = WRITES_PER_ABSORB;

        CheckArchiveTimeout();

        /*
         * Report interim activity statistics to the stats collector.
         */
        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.
         */
        pg_usleep(100000L);

    }
    else if (--absorb_counter <= 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.
         */
        AbsorbFsyncRequests();
        absorb_counter = WRITES_PER_ABSORB;
    }
}

    {
        {"checkpoint_completion_target", PGC_SIGHUP, WAL_CHECKPOINTS,
            gettext_noop("Time spent flushing dirty buffers during checkpoint, as fraction of checkpoint interval."),
            NULL
        },
        &CheckPointCompletionTarget,
        0.5, 0.0, 1.0,
        NULL, NULL, NULL
    },

/*
 * IsCheckpointOnSchedule -- are we on schedule to finish this checkpoint
 *       (or restartpoint) in time?
 *
 * 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.
 */
static bool
IsCheckpointOnSchedule(double progress)
{
    XLogRecPtr  recptr;
    struct timeval now;
    double      elapsed_xlogs,
                elapsed_time;

    Assert(ckpt_active);

    /* Scale progress according to 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.
     */
    if (progress < ckpt_cached_elapsed)
        return false;

    /*
     * Check progress against WAL segments written and CheckPointSegments.
     *
     * 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
     * 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)) / XLogSegSize) / CheckPointSegments;

    if (progress < elapsed_xlogs)
    {
        ckpt_cached_elapsed = elapsed_xlogs;
        return false;
    }

    /*
     * Check progress against time elapsed and checkpoint_timeout.
     */
    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)
    {
        ckpt_cached_elapsed = elapsed_time;
        return false;
    }

    /* It looks like we're on schedule. */
    return true;
}