Spark job 的触发

判断是RDD action的操作的一个标志是
其函数实现里得有

sc.runJob

RDD 是怎么触发job的

以 rdd.count 为例

RDD.scala

/**
   * Return the number of elements in the RDD.
   */
  def count(): Long = sc.runJob(this, Utils.getIteratorSize _).sum

SparkContext.scala

/**
   * Run a job on all partitions in an RDD and return the results in an array.
   *
   * @param rdd target RDD to run tasks on
   * @param func a function to run on each partition of the RDD
   * @return in-memory collection with a result of the job (each collection element will contain
   * a result from one partition)
   */
  def runJob[T, U: ClassTag](rdd: RDD[T], func: Iterator[T] => U): Array[U] = {
    runJob(rdd, func, 0 until