Spark中的排程模式主要有兩種：FIFO和FAIR。預設情況下Spark的排程模式是FIFO（先進先出），誰先提交誰先執行，後面的任務需要等待前面的任務執行。而FAIR（公平排程）模式支援在排程池中為任務進行分組，不同的排程池權重不同，任務可以按照權重來決定執行順序。spark.scheduler.mode來設定，可選的引數有FAIR和FIFO。

1.排程池比較

FIFO建立排程池為空。

FAIR重寫了buildpools的方法，讀取預設路徑 $SPARK_HOME/conf/fairscheduler.xml檔案。addtaskmanager方法把TaskSetManager存入rootPool對應的子pool；

2.排程演算法比較

FIFO:

FIFO模式的排程方式很容易理解，比較stageID，誰小誰先執行；

這也很好理解，stageID小的任務一般來說是遞迴的最底層，是最先提交給排程池的；

private[spark] class FIFOSchedulingAlgorithm extends SchedulingAlgorithm {
  override def comparator(s1: Schedulable, s2: Schedulable): Boolean = {
    val priority1 = s1.priority
    val priority2 = s2.priority
    var res 
 
= math.signum(priority1 - priority2)
    if (res == 0) {
      val stageId1 = s1.stageId
      val stageId2 = s2.stageId
      res = math.signum(stageId1 - stageId2)
    }
    if (res < 0) {
      true
    } else {
      false
    }
  }
}

FAIR:

fair模式來說的話，稍微複雜一點；

但是還是比較容易看懂，

1.先比較兩個stage的 runningtask使用的核數，其實也可以理解為task的數量，誰小誰的優先順序高；

2.比較兩個stage的 runningtask 權重，誰的權重大誰先執行；

3.如果前面都一直，則比較名字了（字串比較），誰大誰先執行；

private[spark] class FairSchedulingAlgorithm extends SchedulingAlgorithm {
  override def comparator(s1: Schedulable, s2: Schedulable): Boolean = {
    val minShare1 = s1.minShare
    val minShare2 = s2.minShare
    val runningTasks1 = s1.runningTasks
    val runningTasks2 = s2.runningTasks
    val s1Needy = runningTasks1 < minShare1
    val s2Needy = runningTasks2 < minShare2
    val minShareRatio1 = runningTasks1.toDouble / math.max(minShare1, 1.0).toDouble
    val minShareRatio2 = runningTasks2.toDouble / math.max(minShare2, 1.0).toDouble
    val taskToWeightRatio1 = runningTasks1.toDouble / s1.weight.toDouble
    val taskToWeightRatio2 = runningTasks2.toDouble / s2.weight.toDouble
    var compare: Int = 0

    if (s1Needy && !s2Needy) {
      return true
    } else if (!s1Needy && s2Needy) {
      return false
    } else if (s1Needy && s2Needy) {
      compare = minShareRatio1.compareTo(minShareRatio2)
    } else {
      compare = taskToWeightRatio1.compareTo(taskToWeightRatio2)
    }

    if (compare < 0) {
      true
    } else if (compare > 0) {
      false
    } else {
      s1.name < s2.name
    }
  }