Spark 源码解析 : DAGScheduler中的DAG划分

一、Spark 运行架构

Spark 运行架构如下图：
各个RDD之间存在着依赖关系，这些依赖关系形成有向无环图DAG，DAGScheduler对这些依赖关系形成的DAG，进行Stage划分，划分的规则很简单，从后往前回溯，遇到窄依赖加入本stage，遇见宽依赖进行Stage切分。完成了Stage的划分,DAGScheduler基于每个Stage生成TaskSet,并将TaskSet提交给TaskScheduler。TaskScheduler 负责具体的task调度,在Worker节点上启动task。

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二、源码解析：DAGScheduler中的DAG划分
当RDD触发一个Action操作（如：colllect）后，导致SparkContext.runJob的执行。而在SparkContext的run方法中会调用DAGScheduler的run方法最终调用了DAGScheduler的submit方法：

  def submitJob[T, U](
      rdd: RDD[T],
      func: (TaskContext, Iterator[T]) => U,
      partitions: Seq[Int],
      callSite: CallSite,
      resultHandler: (Int, U) => Unit,
      properties: Properties): JobWaiter[U] = {
    // Check to make sure we are not launching a task on a partition that does not exist.
    val maxPartitions = rdd.partitions.length
    partitions.find(p => p >= maxPartitions || p < 0).foreach { p =>
      throw new IllegalArgumentException(
        "Attempting to access a non-existent partition: " + p + ". " +
          "Total number of partitions: " + maxPartitions)
    }
    val jobId = nextJobId.getAndIncrement()
    if (partitions.size == 0) {
      // Return immediately if the job is running 0 tasks
      return new JobWaiter[U](this, jobId, 0, resultHandler)
    }
    assert(partitions.size > 0)
    val func2 = func.asInstanceOf[(TaskContext, Iterator[_]) => _]
    val waiter = new JobWaiter(this, jobId, partitions.size, resultHandler)
    //给eventProcessLoop发送JobSubmitted消息
    eventProcessLoop.post(JobSubmitted(
      jobId, rdd, func2, partitions.toArray, callSite, waiter,
      SerializationUtils.clone(properties)))
    waiter
  }

DAGScheduler的submit方法中，像eventProcessLoop对象发送了JobSubmitted消息。eventProcessLoop是DAGSchedulerEventProcessLoop类的对象

private[scheduler] val eventProcessLoop = new DAGSchedulerEventProcessLoop(this)

DAGSchedulerEventProcessLoop，接收各种消息并进行处理，处理的逻辑在其doOnReceive方法中：

  private def doOnReceive(event: DAGSchedulerEvent): Unit = event match {
   //Job提交
    case JobSubmitted(jobId, rdd, func, partitions, callSite, listener, properties) =>
      dagScheduler.handleJobSubmitted(jobId, rdd, func, partitions, callSite, listener, properties)
    case MapStageSubmitted(jobId, dependency, callSite, listener, properties) =>
      dagScheduler.handleMapStageSubmitted(jobId, dependency, callSite, listener, properties)
    case StageCancelled(stageId) =>
      dagScheduler.handleStageCancellation(stageId)
    case JobCancelled(jobId) =>
      dagScheduler.handleJobCancellation(jobId)
    case JobGroupCancelled(groupId) =>
      dagScheduler.handleJobGroupCancelled(groupId)
    case AllJobsCancelled =>
      dagScheduler.doCancelAllJobs()
    case ExecutorAdded(execId, host) =>
      dagScheduler.handleExecutorAdded(execId, host)
    case ExecutorLost(execId) =>
      dagScheduler.handleExecutorLost(execId, fetchFailed = false)
    case BeginEvent(task, taskInfo) =>
      dagScheduler.handleBeginEvent(task, taskInfo)
    case GettingResultEvent(taskInfo) =>
      dagScheduler.handleGetTaskResult(taskInfo)
    case completion: CompletionEvent =>
      dagScheduler.handleTaskCompletion(completion)
    case TaskSetFailed(taskSet, reason, exception) =>
      dagScheduler.handleTaskSetFailed(taskSet, reason, exception)
    case ResubmitFailedStages =>
      dagScheduler.resubmitFailedStages()
  }

可以把DAGSchedulerEventProcessLoop理解成DAGScheduler的对外的功能接口。它对外隐藏了自己内部实现的细节。无论是内部还是外部消息，DAGScheduler可以共用同一消息处理代码，逻辑清晰，处理方式统一。

接下来分析DAGScheduler的Stage划分，handleJobSubmitted方法首先创建ResultStage

    try {
      //创建新stage可能出现异常，比如job运行依赖hdfs文文件被删除
      finalStage = newResultStage(finalRDD, func, partitions, jobId, callSite)
    } catch {
      case e: Exception =>
        logWarning("Creating new stage failed due to exception - job: " + jobId, e)
        listener.jobFailed(e)
        return
    }

然后调用submitStage方法，进行stage的划分。

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首先由finalRDD获取它的父RDD依赖，判断依赖类型，如果是窄依赖，则将父RDD压入栈中，如果是宽依赖，则作为父Stage。

看一下源码的具体过程:

 private def getMissingParentStages(stage: Stage): List[Stage] = {
    val missing = new HashSet[Stage] //存储需要返回的父Stage
    val visited = new HashSet[RDD[_]] //存储访问过的RDD
    //自己建立栈，以免函数的递归调用导致
    val waitingForVisit = new Stack[RDD[_]]

    def visit(rdd: RDD[_]) {
      if (!visited(rdd)) {
        visited += rdd
        val rddHasUncachedPartitions = getCacheLocs(rdd).contains(Nil)
        if (rddHasUncachedPartitions) {
          for (dep <- rdd.dependencies) {
            dep match {
              case shufDep: ShuffleDependency[_, _, _] =>
                val mapStage = getShuffleMapStage(shufDep, stage.firstJobId)
                if (!mapStage.isAvailable) {
                  missing += mapStage //遇到宽依赖，加入父stage
                }
              case narrowDep: NarrowDependency[_] =>
                waitingForVisit.push(narrowDep.rdd) //窄依赖入栈,
            }
          }
        }
      }
    }

   //回溯的起始RDD入栈
   waitingForVisit.push(stage.rdd)
    while (waitingForVisit.nonEmpty) {
      visit(waitingForVisit.pop())
    }
    missing.toList
  }

getMissingParentStages方法是由当前stage，返回他的父stage，父stage的创建由getShuffleMapStage返回，最终会调用newOrUsedShuffleStage方法返回ShuffleMapStage

  private def newOrUsedShuffleStage(
      shuffleDep: ShuffleDependency[_, _, _],
      firstJobId: Int): ShuffleMapStage = {
    val rdd = shuffleDep.rdd
    val numTasks = rdd.partitions.length
    val stage = newShuffleMapStage(rdd, numTasks, shuffleDep, firstJobId, rdd.creationSite)
    if (mapOutputTracker.containsShuffle(shuffleDep.shuffleId)) {
      //Stage已经被计算过，从MapOutputTracker中获取计算结果
      val serLocs = mapOutputTracker.getSerializedMapOutputStatuses(shuffleDep.shuffleId)
      val locs = MapOutputTracker.deserializeMapStatuses(serLocs)
      (0 until locs.length).foreach { i =>
        if (locs(i) ne null) {
          // locs(i) will be null if missing
          stage.addOutputLoc(i, locs(i))
        }
      }
    } else {
      // Kind of ugly: need to register RDDs with the cache and map output tracker here
      // since we can't do it in the RDD constructor because # of partitions is unknown
      logInfo("Registering RDD " + rdd.id + " (" + rdd.getCreationSite + ")")
      mapOutputTracker.registerShuffle(shuffleDep.shuffleId, rdd.partitions.length)
    }
    stage
  }

现在父Stage已经划分好，下面看看你Stage的提交逻辑

  /** Submits stage, but first recursively submits any missing parents. */
  private def submitStage(stage: Stage) {
    val jobId = activeJobForStage(stage)
    if (jobId.isDefined) {
      logDebug("submitStage(" + stage + ")")
      if (!waitingStages(stage) && !runningStages(stage) && !failedStages(stage)) {
        val missing = getMissingParentStages(stage).sortBy(_.id)
        logDebug("missing: " + missing)
        if (missing.isEmpty) {
          logInfo("Submitting " + stage + " (" + stage.rdd + "), which has no missing parents")
          //如果没有父stage，则提交当前stage
          submitMissingTasks(stage, jobId.get)
        } else {
          for (parent <- missing) {
            //如果有父stage，则递归提交父stage
            submitStage(parent)
          }
          waitingStages += stage
        }
      }
    } else {
      abortStage(stage, "No active job for stage " + stage.id, None)
    }
  }

提交的过程很简单，首先当前stage获取父stage，如果父stage为空，则当前Stage为起始stage，交给submitMissingTasks处理，如果当前stage不为空，则递归调用submitStage进行提交。

到这里，DAGScheduler中的DAG划分与提交就讲完了，下次解析这些stage是如果封装成TaskSet交给TaskScheduler以及TaskSchedule的调度过程。