Github user JoshRosen commented on the pull request:
https://github.com/apache/spark/pull/4021#issuecomment-76542760
I think I've figured it out: consider the lifecycle of an accumulator in a
task, say ShuffleMapTask: on the executor, each task deserializes its own copy
of the RDD inside of its `runTask` method, so the strong reference to the RDD
disappears at the end of `runTask`. In `Executor.run()`, we call
`Accumulators.values` after `runTask` has exited, so there's a small window in
which the tasks's RDD can be GC'd, causing accumulators to be GC'd as well
because there are no longer any strong references to them.
The fix is to keep strong references in `localAccums`, since we clear this
at the end of each task anyways. I'm glad that I was able to figure out
precisely _why_ this was necessary and sorry that I missed this during review;
I'll submit a fix shortly. In terms of preventative measures, it might be a
good idea to write up the lifetime / lifecycle of objects' strong references
whenever we're using WeakReferences, since the process of explicitly writing
that out would prevent these sorts of mistakes in the future.
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