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From "Jason Lowe (JIRA)" <j...@apache.org>
Subject [jira] [Created] (MAPREDUCE-4733) Reducer can fail to make progress during shuffle if too many reducers complete consecutively
Date Thu, 18 Oct 2012 22:32:03 GMT
Jason Lowe created MAPREDUCE-4733:
-------------------------------------

             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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