en.....what I was thinking when I wrote this line?.....

for why eventProcessActor.postStop() is called for two times...I think the first time is that dagScheduler in DAGSchedulerSuite is stopped (in my previous commit, dagScheduler.stop() is called after the end of every case), the second time is SparkContext.stop() is called, because in the constructor of SparkContext, there is another DAGScheduler being constructed, and SparkContext.stop() includes DAGScheduler.stop()