Github user vanzin commented on a diff in the pull request:

    https://github.com/apache/spark/pull/17747#discussion_r127023652
  
    --- Diff: 
core/src/main/scala/org/apache/spark/deploy/history/FsHistoryProvider.scala ---
    @@ -728,6 +768,103 @@ private[history] class FsHistoryProvider(conf: 
SparkConf, clock: Clock)
             prevFileSize < latest.fileSize
         }
       }
    +
    +  /**
    +   * Time a closure, returning its output.
    +   * The timer is updated with the duration, and if a counter is supplied, 
it's count
    +   * is incremented by the duration.
    +   * @param timer timer
    +   * @param counter counter: an optional counter of the duration
    +   * @param fn function
    +   * @tparam T type of return value of time
    +   * @return the result of the function.
    +   */
    +  private def time[T](timer: Timer, counter: Option[Counter] = None)(fn: 
=> T): T = {
    +    val timeCtx = timer.time()
    +    try {
    +      fn
    +    } finally {
    +      val duration = timeCtx.stop()
    +      counter.foreach(_.inc(duration))
    +    }
    +  }
    +}
    +
    +/**
    + * Metrics integration: the various counters of activity.
    + */
    +private[history] class FsHistoryProviderMetrics(owner: FsHistoryProvider, 
prefix: String)
    +    extends HistoryMetricSource(prefix) {
    +
    +  /**
    +   * Function to return an average; if the count is 0, so is the average.
    +   * @param value value to average
    +   * @param count event count to divide by
    +   * @return the average, or 0 if the counter is itself 0
    +   */
    +  private def average(value: Long, count: Long): Long = {
    +    if (count> 0) value / count else 0
    +  }
    +
    +  override val sourceName = "history.fs"
    +
    +  private val name = MetricRegistry.name(sourceName)
    +
    +  private val clock = new SystemClock
    +
    +  /** Number of events replayed. */
    +  val appUIEventCount = new Counter()
    +
    +  /** Average time to load a single event in the App UI. */
    +  val appUIEventReplayTime = new LambdaLongGauge(() =>
    +    average(appUITotalLoadTime.getCount, appUIEventCount.getCount))
    +
    +  /** Number of App UI load operations. */
    +  val appUILoadCount = new Counter()
    +
    +  /** Number of App UI load operations that failed due to a 
load/parse/replay problem. */
    +  val appUILoadFailureCount = new Counter()
    +
    +  /** Number of App UI load operations that failed due to an unknown file. 
*/
    +  val appUILoadNotFoundCount = new Counter()
    +
    +  /** Statistics on time to load app UIs. */
    +  val appUiLoadTimer = new Timer()
    +
    +  /** Total load time of all App UIs. */
    +  val appUITotalLoadTime = new Counter()
    +
    +  /** Number of updates. */
    --- End diff --
    
    What's an "update"?


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