Jason Lowe created MAPREDUCE-4733:
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Summary: Reducer can fail to make progress during shuffle if too
many reducers complete consecutively
Key: MAPREDUCE-4733
URL: https://issues.apache.org/jira/browse/MAPREDUCE-4733
Project: Hadoop Map/Reduce
Issue Type: Bug
Components: applicationmaster, mrv2
Affects Versions: 0.23.3
Reporter: Jason Lowe
TaskAttemptListenerImpl implements getMapCompletionEvents by calling
Job.getTaskAttemptCompletionEvents with the same fromEvent and maxEvents passed
in from the reducer and then filtering the result for just map events. We can't
filter the task completion event list and expect the caller's "window" into the
list to match up. As soon as a reducer event appears in the list it means we
are redundantly sending map completion events that were already seen by the
reducer.
Worst case the reducer will hang if all of the events in the requested window
are reducer events. In that case zero events will be reported back to the
caller and it won't bump up fromEvent on the next call. Reducer then never
sees the final map completion events needed to complete the shuffle. This could
happen in a case where all maps complete, more than MAX_EVENTS reducers
complete consecutively, but some straggling reducers get fetch failures and
cause a map to be restarted.
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