Spark自动监视每个节点上的缓存使用情况，并以最近最少使用(LRU)的方式丢弃旧的数据分区.如果要手动删除RDD而不是等待它脱离缓存，请使用RDD.unpersist()方法.

Spark automatically monitors cache usage on each node and drops out old data partitions in a least-recently-used (LRU) fashion. If you would like to manually remove an RDD instead of waiting for it to fall out of the cache, use the RDD.unpersist() method.

引用来自: http://spark. apache.org/docs/latest/rdd-programming-guide.html#rdd-persistence