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https://issues.apache.org/jira/browse/MAPREDUCE-4733?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13480337#comment-13480337
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Vinod Kumar Vavilapalli commented on MAPREDUCE-4733:
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Looks good, checking it in.
> 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
> Assignee: Jason Lowe
> Attachments: MAPREDUCE-4733.patch, MAPREDUCE-4733.patch,
> MAPREDUCE-4733.patch
>
>
> 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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