Spark2.x（六十三）：（Spark2.4）Driver如何把Task(闭包等)分配给Executor

在Spark中一个appliation可能包含多个job，每个job都是由SparkContext#runJob(。。。)触发的，一个Job下包含1个或多个Stage，Job的最后一个stage为ResultStage，其余的stage都为ShuffleMapStage。ResultStage会生成一组ResultTask，ResultTask在计算完成之后会将结果返回给Drive；而ShuffleMapStage会生成一组ShuffleMapTask，ShuffleMapTask则是在计算完成之后将结果（根据RDD的Partitioner）划分到不同的buckets中。

Spark代码如何被解析为RDD？

1）spark程序（dataframe,dataset,spark.sql(),rdd）经过catalyst优化解析后，把spark的程序都转化为了层级关联的rdd，经过DAG划分为的stage时，实际上就是根据rddshuffle的依赖关系来划分的（依赖关系分为窄依赖、宽依赖，如果遇到两个RDD（父子）依赖是宽依赖，那么会把RDD拆分为2个Stage，父类RDD在一个Stage，子类在一个Stage），并且map,reduce等算子转化为RDD时，将算子的实现函数（“闭包”或者“自定义函数、自定义类”）赋值到对应的RDD#f属性下。

2）在DAGScheduler#submitMissingTasks中会把stage划分为两种task：ShuffleMapTask,ResultTask，这两个Task会被传递给Executor，Executor会使用TaskRunner来运行它们。

在运行时，会调用ShuffleMapTask,ResultTask#runTask()方法，该方法内部都有rdd.iterator(...)的调用代码，rdd#iterator(..,)内部调用了rdd.compute(...)。如果RDDA的子是RDDB，RDDB的子是RDDC，执行时：

--------RDDA'compute.

---------------RDDB'compute.

----------------------RDDC'compute。

如果使用SparkSQL（dataset,dataframe,spark.sql('')）编写的代码经过catalyst优化解析后的代码后你会发现，实际上它就是把代码解析后层级关联RDD。

taskBinary中序列化的就是解析后RDD和（RDD依赖关系、ResultStage的话会把ResultTask的最后一个算子实现函数），其中非ResultTask的RDD属性中包含了算子业务函数，在算子转化为RDD时，会将算子的实现函数（“闭包”或者“自定义函数、自定义类”）赋值到对应的RDD#f属性下，并被RDD#compute()使用。

• 如果是“闭包”一般就是把一些常量定义到函数内部；
• 如果是“自定义函数、自定义类”可能会引用了外部包中的子函数，这时候在TaskRunner运行时会通过反射把jar加载到当前线程中，供调用使用。

算子如何转化为RDD（map算子为例）？

RDD有很多种：MapPartitoinRDD,ShuffleRDD等，但是每一种rdd都有一个compute()和iterator()方法，这个compute()方法就是循环某个partition下所有数据并调用“程序员调用算子时编写的算子内部业务代码函数”。

以RDD的map算子为例来分析，RDD#map()内部是把map算子转化为MapPartitionRDD，

  /**
   * Return a new RDD by applying a function to all elements of this RDD.
   */
  def map[U: ClassTag](f: T => U): RDD[U] = withScope {
    val cleanF = sc.clean(f)
    new MapPartitionsRDD[U, T](this, (context, pid, iter) => iter.map(cleanF))
  }

备注：一般我们调用RDD的map算子时会实现 f 函数。

 其中MapPartitionRDD的触发依赖iterator()、compute()。compute的实现就是循环RDD下某个partition下所有元素并执行 f() 函数，RDD#map()的 f() 函数被封装传递给MapPartitionRDD。

/**
 * An RDD that applies the provided function to every partition of the parent RDD.
 *
 * @param prev the parent RDD.
 * @param f The function used to map a tuple of (TaskContext, partition index, input iterator) to
 *          an output iterator.
 * @param preservesPartitioning Whether the input function preserves the partitioner, which should
 *                              be `false` unless `prev` is a pair RDD and the input function
 *                              doesn't modify the keys.
 * @param isFromBarrier Indicates whether this RDD is transformed from an RDDBarrier, a stage
 *                      containing at least one RDDBarrier shall be turned into a barrier stage.
 * @param isOrderSensitive whether or not the function is order-sensitive. If it's order
 *                         sensitive, it may return totally different result when the input order
 *                         is changed. Mostly stateful functions are order-sensitive.
 */
private[spark] class MapPartitionsRDD[U: ClassTag, T: ClassTag](
    var prev: RDD[T],
    f: (TaskContext, Int, Iterator[T]) => Iterator[U],  // (TaskContext, partition index, iterator)
    preservesPartitioning: Boolean = false,
    isFromBarrier: Boolean = false,
    isOrderSensitive: Boolean = false)
  extends RDD[U](prev) {

  override val partitioner = if (preservesPartitioning) firstParent[T].partitioner else None

  override def getPartitions: Array[Partition] = firstParent[T].partitions

  override def compute(split: Partition, context: TaskContext): Iterator[U] =
    f(context, split.index, firstParent[T].iterator(split, context))

  override def clearDependencies() {
    super.clearDependencies()
    prev = null
  }

  @transient protected lazy override val isBarrier_ : Boolean =
    isFromBarrier || dependencies.exists(_.rdd.isBarrier())

  override protected def getOutputDeterministicLevel = {
    if (isOrderSensitive && prev.outputDeterministicLevel == DeterministicLevel.UNORDERED) {
      DeterministicLevel.INDETERMINATE
    } else {
      super.getOutputDeterministicLevel
    }
  }
}

MapPartitionRDD的父类RDD类中定义了iterator()函数：

  /**
   * Internal method to this RDD; will read from cache if applicable, or otherwise compute it.
   * This should ''not'' be called by users directly, but is available for implementors of custom
   * subclasses of RDD.
   */
  final def iterator(split: Partition, context: TaskContext): Iterator[T] = {
    if (storageLevel != StorageLevel.NONE) {
      getOrCompute(split, context) // 会调用compute()
    } else {
      computeOrReadCheckpoint(split, context) // 会调用compute()
    }
  }

ShuflleMapTask和ResultTask

ShuffleMapTask类

ShuffleMapTask将RDD的元素分为多个存储桶（基于 ShuffleDependency 中指定的分区器）。
https://github.com/apache/spark/blob/branch-2.4/core/src/main/scala/org/apache/spark/scheduler/ShuffleMapTask.scala

private[spark] class ShuffleMapTask(
    stageId: Int,
    stageAttemptId: Int,
    taskBinary: Broadcast[Array[Byte]],
    partition: Partition,
    @transient private var locs: Seq[TaskLocation],
    localProperties: Properties,
    serializedTaskMetrics: Array[Byte],
    jobId: Option[Int] = None,
    appId: Option[String] = None,
    appAttemptId: Option[String] = None,
    isBarrier: Boolean = false)
  extends Task[MapStatus](stageId, stageAttemptId, partition.index, localProperties,
    serializedTaskMetrics, jobId, appId, appAttemptId, isBarrier)
  with Logging {

  /** A constructor used only in test suites. This does not require passing in an RDD. */
  def this(partitionId: Int) {
    this(0, 0, null, new Partition { override def index: Int = 0 }, null, new Properties, null)
  }

  @transient private val preferredLocs: Seq[TaskLocation] = {
    if (locs == null) Nil else locs.toSet.toSeq
  }

  override def runTask(context: TaskContext): MapStatus = {
    // Deserialize the RDD using the broadcast variable.
    val threadMXBean = ManagementFactory.getThreadMXBean
    val deserializeStartTime = System.currentTimeMillis()
    val deserializeStartCpuTime = if (threadMXBean.isCurrentThreadCpuTimeSupported) {
      threadMXBean.getCurrentThreadCpuTime
    } else 0L
    val ser = SparkEnv.get.closureSerializer.newInstance()
    val (rdd, dep) = ser.deserialize[(RDD[_], ShuffleDependency[_, _, _])](ByteBuffer.wrap(taskBinary.value), Thread.currentThread.getContextClassLoader)
      
    _executorDeserializeTime = System.currentTimeMillis() - deserializeStartTime
    _executorDeserializeCpuTime = if (threadMXBean.isCurrentThreadCpuTimeSupported) {
      threadMXBean.getCurrentThreadCpuTime - deserializeStartCpuTime
    } else 0L

    var writer: ShuffleWriter[Any, Any] = null
    try {
      val manager = SparkEnv.get.shuffleManager
      writer = manager.getWriter[Any, Any](dep.shuffleHandle, partitionId, context)
      writer.write(rdd.iterator(partition, context).asInstanceOf[Iterator[_ <: Product2[Any, Any]]])
      writer.stop(success = true).get
    } catch {
      case e: Exception =>
        try {
          if (writer != null) {
            writer.stop(success = false)
          }
        } catch {
          case e: Exception =>
            log.debug("Could not stop writer", e)
        }
        throw e
    }
  }

  override def preferredLocations: Seq[TaskLocation] = preferredLocs

  override def toString: String = "ShuffleMapTask(%d, %d)".format(stageId, partitionId)
}

@param stageId 此Task所属Stage的ID（id of the stage this task belongs to）
@param stageAttemptId 此Task所属Stage的尝试ID (attempt id of the stage this task belongs to)
@param taskBinary RDD和ShuffleDependency的广播版本。反序列化后，类型应为(RDD[_], ShuffleDependency[_, _, _])。（broadcast version of the RDD and the ShuffleDependency. Once deserialized,the type should be (RDD[_], ShuffleDependency[_, _, _])）
@param partition 与此Task关联的RDD分区（partition of the RDD this task is associated with）
@param locs 区域调度的首选Task执行位置 (preferred task execution locations for locality scheduling)
@param localProperties 用户在driver端设置的线程本地属性的副本。 (copy of thread-local properties set by the user on the driver side.)
@param serializedTaskMetrics 在driver端创建并序列化并发送到executor端的“TaskMetrics”。 (a `TaskMetrics` that is created and serialized on the driver side and sent to executor side.)
 以下参数是可选的：
@param jobId 此Task所属Job的id(id of the job this task belongs to)
@param appId 此Task所属application的id(id of the app this task belongs to)
@param appAttemptId 此Task所属application的尝试id（attempt id of the app this task belongs to）
@param isBarrier 此Task是否属于屏障Stage。Spark必须同时启动所有Task以进入屏障Stage(whether this task belongs to a barrier stage. Spark must launch all the tasks at the same time for a barrier stage.)

TaskMetrics就是对task的执行信息的一个描述类

class TaskMetrics private[spark] () extends Serializable {
  // Each metric is internally represented as an accumulator
  private val _executorDeserializeTime = new LongAccumulator    // executor端反序列化耗时
  private val _executorDeserializeCpuTime = new LongAccumulator // executor端反序列化CPU耗时
  private val _executorRunTime = new LongAccumulator   // executor端运行时间
  private val _executorCpuTime = new LongAccumulator   // executor端CPU耗时
  private val _resultSize = new LongAccumulator        // 结果大小
  private val _jvmGCTime = new LongAccumulator         // JVM GC耗时
  private val _resultSerializationTime = new LongAccumulator // 结果序列化耗时
  private val _memoryBytesSpilled = new LongAccumulator      // 溢出的内存字节
  private val _diskBytesSpilled = new LongAccumulator        // 溢出的磁盘字节
  private val _peakExecutionMemory = new LongAccumulator     // 峰值执行内存
  private val _updatedBlockStatuses = new CollectionAccumulator[(BlockId, BlockStatus)] // 修改的block状态信息集合
}

ResultTask类

将输出发送回Driver应用程序的Task。
https://github.com/apache/spark/blob/branch-2.4/core/src/main/scala/org/apache/spark/scheduler/ResultTask.scala

private[spark] class ResultTask[T, U](
    stageId: Int,
    stageAttemptId: Int,
    taskBinary: Broadcast[Array[Byte]],
    partition: Partition,
    locs: Seq[TaskLocation],
    val outputId: Int,
    localProperties: Properties,
    serializedTaskMetrics: Array[Byte],
    jobId: Option[Int] = None,
    appId: Option[String] = None,
    appAttemptId: Option[String] = None,
    isBarrier: Boolean = false)
  extends Task[U](stageId, stageAttemptId, partition.index, localProperties, serializedTaskMetrics,
    jobId, appId, appAttemptId, isBarrier)
  with Serializable {

  @transient private[this] val preferredLocs: Seq[TaskLocation] = {
    if (locs == null) Nil else locs.toSet.toSeq
  }

  override def runTask(context: TaskContext): U = {
    // Deserialize the RDD and the func using the broadcast variables.
    val threadMXBean = ManagementFactory.getThreadMXBean
    val deserializeStartTime = System.currentTimeMillis()
    val deserializeStartCpuTime = if (threadMXBean.isCurrentThreadCpuTimeSupported) {
      threadMXBean.getCurrentThreadCpuTime
    } else 0L
    val ser = SparkEnv.get.closureSerializer.newInstance()
    val (rdd, func) = ser.deserialize[(RDD[T], (TaskContext, Iterator[T]) => U)](ByteBuffer.wrap(taskBinary.value), Thread.currentThread.getContextClassLoader)
    
    _executorDeserializeTime = System.currentTimeMillis() - deserializeStartTime
    _executorDeserializeCpuTime = if (threadMXBean.isCurrentThreadCpuTimeSupported) {
      threadMXBean.getCurrentThreadCpuTime - deserializeStartCpuTime
    } else 0L

    func(context, rdd.iterator(partition, context))
  }

  // This is only callable on the driver side.
  override def preferredLocations: Seq[TaskLocation] = preferredLocs

  override def toString: String = "ResultTask(" + stageId + ", " + partitionId + ")"
}

断点监控Task序列化携带信息

使用RDD来编写一个测试程序：

package com.dx.test;

import org.apache.spark.SparkConf;
import org.apache.spark.api.java.JavaRDD;
import org.apache.spark.api.java.JavaSparkContext;
import org.apache.spark.api.java.function.Function;
import org.apache.spark.broadcast.Broadcast;
import org.apache.spark.sql.Row;
import org.apache.spark.sql.RowFactory;
import org.apache.spark.sql.catalyst.encoders.ExpressionEncoder;
import org.apache.spark.sql.catalyst.encoders.RowEncoder;
import org.apache.spark.sql.types.DataTypes;
import org.apache.spark.sql.types.StructType;

import java.util.Map;

public class TestBroadcast {
   public static void main(String[] args) {
      SparkConf conf = new SparkConf();
      conf.setMaster("local[*]");
      conf.setAppName("test application");
      JavaSparkContext javaSparkContext = new JavaSparkContext(conf);
      Map<String, String> resource = new java.util.HashMap<String, String>();
      for (int i = 0; i < 10000; i++) {
         resource.put(String.valueOf(i), String.valueOf(i));
      }
      final Broadcast<Map<String, String>> broadcastMap = javaSparkContext.broadcast(resource);

      StructType resulStructType = new StructType();
      resulStructType = resulStructType.add("int_id", DataTypes.StringType, false);
      resulStructType = resulStructType.add("job_result", DataTypes.StringType, true);
      ExpressionEncoder<Row> resultEncoder = RowEncoder.apply(resulStructType);

      JavaRDD<String> sourceDataset = javaSparkContext.textFile("E:\test2");
      JavaRDD<Row> dataset = sourceDataset.map(new Function<String, Row>() {
         public Row call(String line) throws Exception {

            String[] fields = line.split(",");
            int int_id = Integer.valueOf(fields[1]);

            Map<String, String> resources = broadcastMap.getValue();
            String job_result = resources.get(int_id);

            Object[] values = new Object[2];
            values[0] = int_id;
            values[1] = job_result;

            return RowFactory.create(values);
         }
      });
      dataset.saveAsTextFile("E:\test3");
   }
}

端点设置到DAGScheduler#submitMissingTasks(stage: Stage, jobId: Int)

  /** Called when stage's parents are available and we can now do its task. */
  private def submitMissingTasks(stage: Stage, jobId: Int) {
    logDebug("submitMissingTasks(" + stage + ")")

    // First figure out the indexes of partition ids to compute.
    val partitionsToCompute: Seq[Int] = stage.findMissingPartitions()

    // Use the scheduling pool, job group, description, etc. from an ActiveJob associated
    // with this Stage
    val properties = jobIdToActiveJob(jobId).properties

    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 match {
      case s: ShuffleMapStage =>
        outputCommitCoordinator.stageStart(stage = s.id, maxPartitionId = s.numPartitions - 1)
      case s: ResultStage =>
        outputCommitCoordinator.stageStart(
          stage = s.id, maxPartitionId = s.rdd.partitions.length - 1)
    }
    val taskIdToLocations: Map[Int, Seq[TaskLocation]] = try {
      stage match {
        case s: ShuffleMapStage =>
          partitionsToCompute.map { id => (id, getPreferredLocs(stage.rdd, id))}.toMap
        case s: ResultStage =>
          partitionsToCompute.map { id =>
            val p = s.partitions(id)
            (id, getPreferredLocs(stage.rdd, p))
          }.toMap
      }
    } catch {
      case NonFatal(e) =>
        stage.makeNewStageAttempt(partitionsToCompute.size)
        listenerBus.post(SparkListenerStageSubmitted(stage.latestInfo, properties))
        abortStage(stage, s"Task creation failed: $e
${Utils.exceptionString(e)}", Some(e))
        runningStages -= stage
        return
    }

    stage.makeNewStageAttempt(partitionsToCompute.size, taskIdToLocations.values.toSeq)

    // If there are tasks to execute, record the submission time of the stage. Otherwise,
    // post the even without the submission time, which indicates that this stage was
    // skipped.
    if (partitionsToCompute.nonEmpty) {
      stage.latestInfo.submissionTime = Some(clock.getTimeMillis())
    }
    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
    var partitions: Array[Partition] = null
    try {
      // For ShuffleMapTask, serialize and broadcast (rdd, shuffleDep).
      // For ResultTask, serialize and broadcast (rdd, func).
      var taskBinaryBytes: Array[Byte] = null
      // taskBinaryBytes and partitions are both effected by the checkpoint status. We need
      // this synchronization in case another concurrent job is checkpointing this RDD, so we get a
      // consistent view of both variables.
      RDDCheckpointData.synchronized {
        taskBinaryBytes = stage match {
          case stage: ShuffleMapStage =>
            JavaUtils.bufferToArray(
              closureSerializer.serialize((stage.rdd, stage.shuffleDep): AnyRef))
          case stage: ResultStage =>
            JavaUtils.bufferToArray(closureSerializer.serialize((stage.rdd, stage.func): AnyRef))
        }

        partitions = stage.rdd.partitions
      }

      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, Some(e))
        runningStages -= stage

        // Abort execution
        return
      case e: Throwable =>
        abortStage(stage, s"Task serialization failed: $e
${Utils.exceptionString(e)}", Some(e))
        runningStages -= stage

        // Abort execution
        return
    }

    val tasks: Seq[Task[_]] = try {
      val serializedTaskMetrics = closureSerializer.serialize(stage.latestInfo.taskMetrics).array()
      stage match {
        case stage: ShuffleMapStage =>
          stage.pendingPartitions.clear()
          partitionsToCompute.map { id =>
            val locs = taskIdToLocations(id)
            val part = partitions(id)
            stage.pendingPartitions += id
            new ShuffleMapTask(stage.id, stage.latestInfo.attemptNumber,
              taskBinary, part, locs, properties, serializedTaskMetrics, Option(jobId),
              Option(sc.applicationId), sc.applicationAttemptId, stage.rdd.isBarrier())
          }

        case stage: ResultStage =>
          partitionsToCompute.map { id =>
            val p: Int = stage.partitions(id)
            val part = partitions(p)
            val locs = taskIdToLocations(id)
            new ResultTask(stage.id, stage.latestInfo.attemptNumber,
              taskBinary, part, locs, id, properties, serializedTaskMetrics,
              Option(jobId), Option(sc.applicationId), sc.applicationAttemptId,
              stage.rdd.isBarrier())
          }
      }
    } catch {
      case NonFatal(e) =>
        abortStage(stage, s"Task creation failed: $e
${Utils.exceptionString(e)}", Some(e))
        runningStages -= stage
        return
    }

    if (tasks.size > 0) {
      logInfo(s"Submitting ${tasks.size} missing tasks from $stage (${stage.rdd}) (first 15 " +
        s"tasks are for partitions ${tasks.take(15).map(_.partitionId)})")
      taskScheduler.submitTasks(new TaskSet(
        tasks.toArray, stage.id, stage.latestInfo.attemptNumber, jobId, properties))
    } else {
      // Because we posted SparkListenerStageSubmitted earlier, we should mark
      // the stage as completed here in case there are no tasks to run
      markStageAsFinished(stage, None)

      stage match {
        case stage: ShuffleMapStage =>
          logDebug(s"Stage ${stage} is actually done; " +
              s"(available: ${stage.isAvailable}," +
              s"available outputs: ${stage.numAvailableOutputs}," +
              s"partitions: ${stage.numPartitions})")
          markMapStageJobsAsFinished(stage)
        case stage : ResultStage =>
          logDebug(s"Stage ${stage} is actually done; (partitions: ${stage.numPartitions})")
      }
      submitWaitingChildStages(stage)
    }
  }

中的

  if (tasks.size > 0) { // 断点

IDEA下Debug运行 TestBroadcast ，之后能拿到断点：

从Stage下可以看到stage#rdd属性，该rdd就是当前stage中包含执行逻辑代码的解析结果。stage 里边就是一层的rdd，不管是spark sql，dataframe,dataset还是rdd编程，最终程序都被解析（spark sql,dataframe,dataset经过catalyst解析后）为RDD，每个rdd包含了都有可以接收实现函数，比如map算子被转化为 MapPartitionRDD，转化后，把实现函数转化为 mapPartitionRDD实例的一个属性函数。

一个rdd,在执行过程中属性列表

 其中f就是我们的实现的函数，该函数被当做RDD的一个属性

 

 当stage转化为Task后，Task内部包含的操作数据实际上就是RDD的某一个分区，该RDD依然携带了RDD#f 函数属性，因此当task被序列化为Task时，这些实现函数也被序列化。等到达了Executor后会被反序列化加载到TaskRunner中去执行。

TaskRunner执行时，会加载driver传递给container的application.jar等jar，如果Task的反序列化的RDD的f依赖jar包的会从加载jar包中读取依赖函数等。

ResultTask的func从哪里来？

以上边例子来说

dataset.saveAsTextFile("E:\test3");

的调用信息为：

CallSite(
runJob at 
SparkHadoopWriter.scala:78,org.apache.spark.SparkContext.runJob(SparkContext.scala:2114)
org.apache.spark.internal.io.SparkHadoopWriter$.write(SparkHadoopWriter.scala:78)
org.apache.spark.rdd.PairRDDFunctions$$anonfun$saveAsHadoopDataset$1.apply$mcV$sp(PairRDDFunctions.scala:1096)
org.apache.spark.rdd.PairRDDFunctions$$anonfun$saveAsHadoopDataset$1.apply(PairRDDFunctions.scala:1094)
org.apache.spark.rdd.PairRDDFunctions$$anonfun$saveAsHadoopDataset$1.apply(PairRDDFunctions.scala:1094)
org.apache.spark.rdd.RDDOperationScope$.withScope(RDDOperationScope.scala:151)
org.apache.spark.rdd.RDDOperationScope$.withScope(RDDOperationScope.scala:112)
org.apache.spark.rdd.RDD.withScope(RDD.scala:363)
org.apache.spark.rdd.PairRDDFunctions.saveAsHadoopDataset(PairRDDFunctions.scala:1094)
org.apache.spark.rdd.PairRDDFunctions$$anonfun$saveAsHadoopFile$4.apply$mcV$sp(PairRDDFunctions.scala:1067)
org.apache.spark.rdd.PairRDDFunctions$$anonfun$saveAsHadoopFile$4.apply(PairRDDFunctions.scala:1032)
org.apache.spark.rdd.PairRDDFunctions$$anonfun$saveAsHadoopFile$4.apply(PairRDDFunctions.scala:1032)
org.apache.spark.rdd.RDDOperationScope$.withScope(RDDOperationScope.scala:151)
org.apache.spark.rdd.RDDOperationScope$.withScope(RDDOperationScope.scala:112)
org.apache.spark.rdd.RDD.withScope(RDD.scala:363)
org.apache.spark.rdd.PairRDDFunctions.saveAsHadoopFile(PairRDDFunctions.scala:1032)
org.apache.spark.rdd.PairRDDFunctions$$anonfun$saveAsHadoopFile$1.apply$mcV$sp(PairRDDFunctions.scala:958)
org.apache.spark.rdd.PairRDDFunctions$$anonfun$saveAsHadoopFile$1.apply(PairRDDFunctions.scala:958)
org.apache.spark.rdd.PairRDDFunctions$$anonfun$saveAsHadoopFile$1.apply(PairRDDFunctions.scala:958)
org.apache.spark.rdd.RDDOperationScope$.withScope(RDDOperationScope.scala:151))

Dataset#saveAsTextFile(。。。)内部实现是调用SparkContext.runJob(...)来提交任务：

org.apache.spark.internal.io.SparkHadoopWriter

  def write[K, V: ClassTag](
      rdd: RDD[(K, V)],
      config: HadoopWriteConfigUtil[K, V]): Unit = {
    // Extract context and configuration from RDD.
    val sparkContext = rdd.context
    val commitJobId = rdd.id

    // Set up a job.
    val jobTrackerId = createJobTrackerID(new Date())
    val jobContext = config.createJobContext(jobTrackerId, commitJobId)
    config.initOutputFormat(jobContext)

    // Assert the output format/key/value class is set in JobConf.
    config.assertConf(jobContext, rdd.conf)

    val committer = config.createCommitter(commitJobId)
    committer.setupJob(jobContext)

    // Try to write all RDD partitions as a Hadoop OutputFormat.
    try {
      val ret = sparkContext.runJob(rdd, (context: TaskContext, iter: Iterator[(K, V)]) => {
        // SPARK-24552: Generate a unique "attempt ID" based on the stage and task attempt numbers.
        // Assumes that there won't be more than Short.MaxValue attempts, at least not concurrently.
        val attemptId = (context.stageAttemptNumber << 16) | context.attemptNumber

        executeTask(
          context = context,
          config = config,
          jobTrackerId = jobTrackerId,
          commitJobId = commitJobId,
          sparkPartitionId = context.partitionId,
          sparkAttemptNumber = attemptId,
          committer = committer,
          iterator = iter)
      })

      committer.commitJob(jobContext, ret)
      logInfo(s"Job ${jobContext.getJobID} committed.")
    } catch {
      case cause: Throwable =>
        logError(s"Aborting job ${jobContext.getJobID}.", cause)
        committer.abortJob(jobContext)
        throw new SparkException("Job aborted.", cause)
    }
  }

其中SparkContext#runJob(...)传递的第二个参数就是func。SparkContext#runJob(...)内部调用DAGScheduler#runJob(...)

  def runJob[T, U: ClassTag](
      rdd: RDD[T],
      func: (TaskContext, Iterator[T]) => U,
      partitions: Seq[Int],
      resultHandler: (Int, U) => Unit): Unit = {
    if (stopped.get()) {
      throw new IllegalStateException("SparkContext has been shutdown")
    }
    val callSite = getCallSite
    val cleanedFunc = clean(func)
    logInfo("Starting job: " + callSite.shortForm)
    if (conf.getBoolean("spark.logLineage", false)) {
      logInfo("RDD's recursive dependencies:
" + rdd.toDebugString)
    }
    dagScheduler.runJob(rdd, cleanedFunc, partitions, callSite, resultHandler, localProperties.get)
    progressBar.foreach(_.finishAll())
    rdd.doCheckpoint()
  }

在DAGScheduler内部会将func作为ResultStage的属性，

  /**
   * Create a ResultStage associated with the provided jobId.
   */
  private def createResultStage(
      rdd: RDD[_],
      func: (TaskContext, Iterator[_]) => _,
      partitions: Array[Int],
      jobId: Int,
      callSite: CallSite): ResultStage = {
    checkBarrierStageWithDynamicAllocation(rdd)
    checkBarrierStageWithNumSlots(rdd)
    checkBarrierStageWithRDDChainPattern(rdd, partitions.toSet.size)
    val parents = getOrCreateParentStages(rdd, jobId)
    val id = nextStageId.getAndIncrement()
    val stage = new ResultStage(id, rdd, func, partitions, parents, jobId, callSite)
    stageIdToStage(id) = stage
    updateJobIdStageIdMaps(jobId, stage)
    stage
  }

在DAGScheduler#submitMissingTasks(...)中生成序列化的taskBinary时，如果stage为ResultStage时，将stage#func也和stage#rdd一起序列化，最终跟随Task一起被发送到executor上。

      var taskBinaryBytes: Array[Byte] = null
      // taskBinaryBytes and partitions are both effected by the checkpoint status. We need
      // this synchronization in case another concurrent job is checkpointing this RDD, so we get a
      // consistent view of both variables.
      RDDCheckpointData.synchronized {
        taskBinaryBytes = stage match {
          case stage: ShuffleMapStage =>
            JavaUtils.bufferToArray(
              closureSerializer.serialize((stage.rdd, stage.shuffleDep): AnyRef))
          case stage: ResultStage =>
            JavaUtils.bufferToArray(closureSerializer.serialize((stage.rdd, stage.func): AnyRef))
        }

        partitions = stage.rdd.partitions
      }

      taskBinary = sc.broadcast(taskBinaryBytes)

在Executor中启动Task是会调用org.apache.spark.executor.Executor#launchTask()加载task进行反序列化，在org.apache.spark.executor.Executor.TaskRunner中对Task执行，如果Task是ResultTask时，会调用ResultTask#runTask()。

在ResultTask#runTask()中会反序列化taskBinary，反序列化出func和rdd，之后调动func函数，函数内部进行RDD迭代执行。

  override def runTask(context: TaskContext): U = {
    // Deserialize the RDD and the func using the broadcast variables.
    val threadMXBean = ManagementFactory.getThreadMXBean
    val deserializeStartTime = System.currentTimeMillis()
    val deserializeStartCpuTime = if (threadMXBean.isCurrentThreadCpuTimeSupported) {
      threadMXBean.getCurrentThreadCpuTime
    } else 0L
    val ser = SparkEnv.get.closureSerializer.newInstance()
    val (rdd, func) = ser.deserialize[(RDD[T], (TaskContext, Iterator[T]) => U)](
      ByteBuffer.wrap(taskBinary.value), Thread.currentThread.getContextClassLoader)
    _executorDeserializeTime = System.currentTimeMillis() - deserializeStartTime
    _executorDeserializeCpuTime = if (threadMXBean.isCurrentThreadCpuTimeSupported) {
      threadMXBean.getCurrentThreadCpuTime - deserializeStartCpuTime
    } else 0L

    func(context, rdd.iterator(partition, context))
  }

到这里，应该可以清楚的知道ResultTask中从taskBinary中反序列化的func就是SparkContext#runJob(...)的第二个参数。在ResutlTask中rdd#compute()在func内部迭代被调用，这也是真正算子触发的地方。

参考：

《[译]Spark编程指南（二）》

《[spark] Task执行流程》

《Apache Spark源码走读之3 -- Task运行期之函数调用关系分析》