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From "Hadoop QA (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-4733) Reducer can fail to make progress during shuffle if too many reducers complete consecutively
Date Fri, 19 Oct 2012 19:36:12 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-4733?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13480298#comment-13480298

Hadoop QA commented on MAPREDUCE-4733:

{color:green}+1 overall{color}.  Here are the results of testing the latest attachment 
  against trunk revision .

    {color:green}+1 @author{color}.  The patch does not contain any @author tags.

    {color:green}+1 tests included{color}.  The patch appears to include 5 new or modified
test files.

    {color:green}+1 javac{color}.  The applied patch does not increase the total number of
javac compiler warnings.

    {color:green}+1 javadoc{color}.  The javadoc tool did not generate any warning messages.

    {color:green}+1 eclipse:eclipse{color}.  The patch built with eclipse:eclipse.

    {color:green}+1 findbugs{color}.  The patch does not introduce any new Findbugs (version
1.3.9) warnings.

    {color:green}+1 release audit{color}.  The applied patch does not increase the total number
of release audit warnings.

    {color:green}+1 core tests{color}.  The patch passed unit tests in hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-app

    {color:green}+1 contrib tests{color}.  The patch passed contrib unit tests.

Test results: https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/2948//testReport/
Console output: https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/2948//console

This message is automatically generated.
> 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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