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From "Hadoop QA (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-5689) MRAppMaster does not preempt reducer when scheduled Maps cannot be full filled
Date Fri, 03 Jan 2014 01:19:51 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-5689?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13861063#comment-13861063

Hadoop QA commented on MAPREDUCE-5689:

{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 1 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/4297//testReport/
Console output: https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/4297//console

This message is automatically generated.

> MRAppMaster does not preempt reducer when scheduled Maps cannot be full filled
> ------------------------------------------------------------------------------
>                 Key: MAPREDUCE-5689
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-5689
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>    Affects Versions: 3.0.0, 2.2.0
>            Reporter: Lohit Vijayarenu
>            Assignee: Lohit Vijayarenu
>            Priority: Critical
>         Attachments: MAPREDUCE-5689.1.patch, MAPREDUCE-5689.2.patch
> We saw corner case where Jobs running on cluster were hung. Scenario was something like
this. Job was running within a pool which was running at its capacity. All available containers
were occupied by reducers and last 2 mappers. There were few more reducers waiting to be scheduled
in pipeline. 
> At this point two mappers which were running failed and went back to scheduled state.
two available containers were assigned to reducers, now whole pool was full of reducers waiting
on two maps to be complete. 2 maps never got scheduled because pool was full.
> Ideally reducer preemption should have kicked in to make room for Mappers from this code
in RMContaienrAllocator
> {code}
> int completedMaps = getJob().getCompletedMaps();
>     int completedTasks = completedMaps + getJob().getCompletedReduces();
>     if (lastCompletedTasks != completedTasks) {
>       lastCompletedTasks = completedTasks;
>       recalculateReduceSchedule = true;
>     }
>     if (recalculateReduceSchedule) {
>       preemptReducesIfNeeded();
> {code}
> But in this scenario lastCompletedTasks is always completedTasks because maps were never
completed. This would cause job to hang forever. As workaround if we kill few reducers, mappers
would get scheduled and caused job to complete.

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