//提交stage，為stage創(chuàng)建一批task，task數(shù)量和partition數(shù)量相同

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 private def submitMissingTasks(stage: Stage, jobId: Int) {

  logDebug("submitMissingTasks(" + stage + ")")

  // Get our pending tasks and remember them in our pendingTasks entry

  stage.pendingTasks.clear()

  // First figure out the indexes of partition ids to compute.

//獲取要創(chuàng)建的task的數(shù)量

  val partitionsToCompute: Seq[Int] = {

   if (stage.isShuffleMap) {

    (0 until stage.numPartitions).filter(id => stage.outputLocs(id) == Nil)

   } else {

    val job = stage.resultOfJob.get

    (0 until job.numPartitions).filter(id => !job.finished(id))

   }

  }

  val properties = if (jobIdToActiveJob.contains(jobId)) {

   jobIdToActiveJob(stage.jobId).properties

  } else {

   // this stage will be assigned to "default" pool

   null

  }

//將stage加入runningstage隊列

  runningStages += stage

  // SparkListenerStageSubmitted should be posted before testing whether tasks are

  // serializable. If tasks are not serializable, a SparkListenerStageCompleted event

  // will be posted, which should always come after a corresponding SparkListenerStageSubmitted

  // event.

  stage.latestInfo = StageInfo.fromStage(stage, Some(partitionsToCompute.size))

  outputCommitCoordinator.stageStart(stage.id)

  listenerBus.post(SparkListenerStageSubmitted(stage.latestInfo, properties))

  // TODO: Maybe we can keep the taskBinary in Stage to avoid serializing it multiple times.

  // Broadcasted binary for the task, used to dispatch tasks to executors. Note that we broadcast

  // the serialized copy of the RDD and for each task we will deserialize it, which means each

  // task gets a different copy of the RDD. This provides stronger isolation between tasks that

  // might modify state of objects referenced in their closures. This is necessary in Hadoop

  // where the JobConf/Configuration object is not thread-safe.

  var taskBinary: Broadcast[Array[Byte]] = null

  try {

   // For ShuffleMapTask, serialize and broadcast (rdd, shuffleDep).

   // For ResultTask, serialize and broadcast (rdd, func).

   val taskBinaryBytes: Array[Byte] =

    if (stage.isShuffleMap) {

     closureSerializer.serialize((stage.rdd, stage.shuffleDep.get) : AnyRef).array()

    } else {

     closureSerializer.serialize((stage.rdd, stage.resultOfJob.get.func) : AnyRef).array()

    }

   taskBinary = sc.broadcast(taskBinaryBytes)

  } catch {

   // In the case of a failure during serialization, abort the stage.

   case e: NotSerializableException =>

    abortStage(stage, "Task not serializable: " + e.toString)

    runningStages -= stage

    return

   case NonFatal(e) =>

    abortStage(stage, s"Task serialization failed: $e\n${e.getStackTraceString}")

    runningStages -= stage

    return

  }

//為stage創(chuàng)建指定數(shù)量的task

  val tasks: Seq[Task[_]] = if (stage.isShuffleMap) {

   partitionsToCompute.map { id =>

//給每個partition創(chuàng)建一個task

//給每個task計算最佳位置

    val locs = getPreferredLocs(stage.rdd, id)

    val part = stage.rdd.partitions(id)

//對于finalstage之外的stage的isShuffleMap都是true

//所以會創(chuàng)建ShuffleMapTask

    new ShuffleMapTask(stage.id, taskBinary, part, locs)

   }

  } else {

//如果不是ShuffleMap，就會創(chuàng)建finalstage

//finalstage是穿件resultTask

   val job = stage.resultOfJob.get

   partitionsToCompute.map { id =>

    val p: Int = job.partitions(id)

    val part = stage.rdd.partitions(p)

//獲取task計算的最佳位置的方法 getPreferredLocs

    val locs = getPreferredLocs(stage.rdd, p)

    new ResultTask(stage.id, taskBinary, part, locs, id)

   }

  }

  if (tasks.size > 0) {

   logInfo("Submitting " + tasks.size + " missing tasks from " + stage + " (" + stage.rdd + ")")

   stage.pendingTasks ++= tasks

   logDebug("New pending tasks: " + stage.pendingTasks)

   taskScheduler.submitTasks(

    new TaskSet(tasks.toArray, stage.id, stage.newAttemptId(), stage.jobId, properties))

   stage.latestInfo.submissionTime = Some(clock.getTimeMillis())

  } else {

   // Because we posted SparkListenerStageSubmitted earlier, we should post

   // SparkListenerStageCompleted here in case there are no tasks to run.

   outputCommitCoordinator.stageEnd(stage.id)

   listenerBus.post(SparkListenerStageCompleted(stage.latestInfo))

   logDebug("Stage " + stage + " is actually done; %b %d %d".format(

    stage.isAvailable, stage.numAvailableOutputs, stage.numPartitions))

   runningStages -= stage

  }

 }

 def getPreferredLocs(rdd: RDD[_], partition: Int): Seq[TaskLocation] = {

  getPreferredLocsInternal(rdd, partition, new HashSet)

 }

//task對應(yīng)partition的最佳位置

//就是從stage的最后一個RDD開始，找哪個RDD是被持久化了或者checkpoint

//那么task的最佳位置就是緩存的/checkpoint 的 partition的位置

//因為這樣的話，task就在那個節(jié)點上執(zhí)行，不需要計算之前的RDD

 private def getPreferredLocsInternal(

   rdd: RDD[_],

   partition: Int,

   visited: HashSet[(RDD[_],Int)])

  : Seq[TaskLocation] =

 {

  // If the partition has already been visited, no need to re-visit.

  // This avoids exponential path exploration.  SPARK-695

  if (!visited.add((rdd,partition))) {

   // Nil has already been returned for previously visited partitions.

   return Nil

  }

  // If the partition is cached, return the cache locations

//尋找當(dāng)前RDD是否緩存了

  val cached = getCacheLocs(rdd)(partition)

  if (!cached.isEmpty) {

   return cached

  }

  // If the RDD has some placement preferences (as is the case for input RDDs), get those

//尋找當(dāng)前RDD是否checkpoint了

  val rddPrefs = rdd.preferredLocations(rdd.partitions(partition)).toList

  if (!rddPrefs.isEmpty) {

   return rddPrefs.map(TaskLocation(_))

  }

  // If the RDD has narrow dependencies, pick the first partition of the first narrow dep

  // that has any placement preferences. Ideally we would choose based on transfer sizes,

  // but this will do for now.

//遞歸調(diào)用，看看父RDD是否緩存或者checkpoint

  rdd.dependencies.foreach {

   case n: NarrowDependency[_] =>

    for (inPart <- n.getParents(partition)) {

     val locs = getPreferredLocsInternal(n.rdd, inPart, visited)

     if (locs != Nil) {

      return locs

     }

    }

   case _ =>

  }

//如果從第一個RDD到最后一個RDD都沒有緩存或者checkpoint，那最佳位置就是Nil，也就是沒有最佳位置

//那他的位置就要由taskscheduler來分配

  Nil

 }

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