Spark Task

Define of Task

Task是Spark中执行具体计算任务的基本单位(a unit of execution)，task可以分为ShuffleMapTask and ResultTask；Spark Job的最后一个stage包含一组ResultTask，其余的的stage包含ShuffleMapTask；ResultTask在计算完成之后会将结果返回到Driver，而ShuffleMapTask则是在计算完成之后将结果(根据RDD的Partitioner)划分到不同的buckes中。

private[spark] abstract class Task[T](
    val stageId: Int, // task 所属stage ID
    val stageAttemptId: Int, // task  所属 stage's attempt id 
    val partitionId: Int, // index of the number in the RDD
    @transient var localProperties: Properties = new Properties,// copy of thread-local properties set by the user on the driver side
    // The default value is only used in tests.
    serializedTaskMetrics: Array[Byte] =
      SparkEnv.get.closureSerializer.newInstance().serialize(TaskMetrics.registered).array(),
    val jobId: Option[Int] = None,
    val appId: Option[String] = None,
    val appAttemptId: Option[String] = None) extends Serializable

TaskSet

/**
 * A set of tasks submitted together to the low-level TaskScheduler, usually representing
 * missing partitions of a particular stage.
 */
private[spark] class TaskSet(
    val tasks: Array[Task[_]],
    val stageId: Int,
    val stageAttemptId: Int,
    val priority: Int,
    val properties: Properties) {
  val id: String = stageId + "." + stageAttemptId
  override def toString: String = "TaskSet " + id
}

如果每个TaskSet中的Task失败次数达到maxTaskFailures,那么这个TaskSet都会被标记为失败，需要重新调度。

State of Task

在Spark中Task共有7中状态，大体上可以分为3类(Launching,running,[finished,failed,killed,lost])，在Spark源码中定义如下：

private[spark] object TaskState extends Enumeration {
  val LAUNCHING, RUNNING, FINISHED, FAILED, KILLED, LOST = Value
  private val FINISHED_STATES = Set(FINISHED, FAILED, KILLED, LOST)
  type TaskState = Value
  def isFailed(state: TaskState): Boolean = (LOST == state) || (FAILED == state)
  def isFinished(state: TaskState): Boolean = FINISHED_STATES.contains(state)
}