Github user liyezhang556520 commented on the pull request:
https://github.com/apache/spark/pull/2956#issuecomment-61047597
Since checkpoint will be done recursively on RDD parents, we need to avoid
one RDD traversed for multiple times. E.g. for following lineage:
A -- B -- C -- D -- E
`-- F --'
When we call E.count(), we shall avoid traverse A and B for twice, since
there are two path to these two nodes: EDCBA and EDFBA. Or else the traverse
time will increase exponentially.
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