异步不保序，但大作业执行时间后移。
.set("spark.scheduler.mode", "FAIR")
公平调度，充分使用集群资源。
def
collectAsync(): FutureAction[Seq[T]]
Returns a future for retrieving all elements of this RDD.
def
countAsync(): FutureAction[Long]
Returns a future for counting the number of elements in the RDD.
def
foreachAsync(f: (T) ⇒ Unit): FutureAction[Unit]
Applies a function f to all elements of this RDD.
def
foreachPartitionAsync(f: (Iterator[T]) ⇒ Unit): FutureAction[Unit]
Applies a function f to each partition of this RDD.
def
takeAsync(num: Int): FutureAction[Seq[T]]
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Returns a future for retrieving the first num elements of the RDD.