闲来无事，研究一下Spark Rdd的处理过程。
以一个简单的例子看看：

val textRDD = sc.textFile("/home/ubuntu/people.txt")
val filterRDD = textRDD.filter(_.startsWith("123"))
val mapRDD = filterRDD.map(line => (line, 1))
val reduceRDD = mapRDD.reduceByKey(_+_)
reduceRDD.foreach(println)

首先看第一行代码。
val textRDD = sc.textFile("/home/ubuntu/people.txt")
看一下textFile的代码：

def filter(f: T => Boolean): RDD[T] = withScope {
  val cleanF = sc.clean(f)
  new MapPartitionsRDD[T, T](
    this,
    (context, pid, iter) => iter.filter(cleanF),
    preservesPartitioning = true)
}

可以看到，经过filter，重新生成了一个RDD。

再看第二行：
val mapRDD = filterRDD.map(line => (line, 1))
看一下map的代码：

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))}

同样是生成一个RDD。
再看第三行：
val reduceRDD = mapRDD.reduceByKey(_+_)
这次reduceByKey的代码定位到了PairRDDFunctions，也是重新生成了一个RDD

def reduceByKey(func: (V, V) => V): RDD[(K, V)] = self.withScope {
  reduceByKey(defaultPartitioner(self), func)
}

在看最后一行
reduceRDD.foreach(println)
代码定位到RDD.scala

def foreach(f: T => Unit): Unit = withScope {
  val cleanF = sc.clean(f)
  sc.runJob(this, (iter: Iterator[T]) => iter.foreach(cleanF))
}

可以看到，最终调用了SparkContext的runJob，看一下runJob的代码：

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(fund)
 logInfo("Starting job: " + callSite.shortForm)
  if (conf.getBoolean("spark.logLineage", false)) {
    logInfo("RDD's recursive dependencies:\n" + rdd.toDebugString)
  }
  dagScheduler.runJob(rdd, cleanedFunc, partitions, callSite, resultHandler, localProperties.get)
  progressBar.foreach(_.finishAll())
  rdd.doCheckpoint()}

大致可以看出，最终由DAGScheduler来执行了任务。
再看看DAGScheduler的runJob

def runJob[T, U](
    rdd: RDD[T],
    func: (TaskContext, Iterator[T]) => U,
    partitions: Seq[Int],
    callSite: CallSite,
    resultHandler: (Int, U) => Unit,
    properties: Properties): Unit = {
  val start = System.nanoTime
  val waiter = submitJob(rdd, func, partitions, callSite, resultHandler, properties)
  // Note: Do not call Await.ready(future) because that calls `scala.concurrent.blocking`,
  // which causes concurrent SQL executions to fail if a fork-join pool is used. Note that
  // due to idiosyncrasies in Scala, `awaitPermission` is not actually used anywhere so it's
  // safe to pass in null here. For more detail, see SPARK-13747.
  val awaitPermission = null.asInstanceOf[scala.concurrent.CanAwait]
  waiter.completionFuture.ready(Duration.Inf)(awaitPermission)
  waiter.completionFuture.value.get match {
    case scala.util.Success(_) =>
      logInfo("Job %d finished: %s, took %f s".format
        (waiter.jobId, callSite.shortForm, (System.nanoTime - start) / 1e9))
    case scala.util.Failure(exception) =>
      logInfo("Job %d failed: %s, took %f s".format
        (waiter.jobId, callSite.shortForm, (System.nanoTime - start) / 1e9))
      // SPARK-8644: Include user stack trace in exceptions coming from DAGScheduler.
      val callerStackTrace = Thread.currentThread().getStackTrace.tail
      exception.setStackTrace(exception.getStackTrace ++ callerStackTrace)
      throw exception
  }
}

可以看到，DAGScheduler.runJob将任务提交后，就等待任务执行完成。
再看看是如何提交任务的:

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)
  eventProcessLoop.post(JobSubmitted(
    jobId, rdd, func2, partitions.toArray, callSite, waiter,
    SerializationUtils.clone(properties)))
  waiter
}

大致逻辑，找到空闲的partitions，然后将任务提交上去。
暂时就看到这里，下一篇再做更深入的研究。