https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/deploy/master/ApplicationInfo.scala#L84, coreLeft implementation, 

https://github.com/CodingCat/spark/blob/SPARK-1706/core/src/main/scala/org/apache/spark/scheduler/cluster/SparkDeploySchedulerBackend.scala#L63,