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From "Hadoop QA (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-4892) CombineFileInputFormat node input split can be skewed on small clusters
Date Thu, 20 Dec 2012 23:59:13 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-4892?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13537523#comment-13537523
] 

Hadoop QA commented on MAPREDUCE-4892:
--------------------------------------

{color:green}+1 overall{color}.  Here are the results of testing the latest attachment 
  http://issues.apache.org/jira/secure/attachment/12562000/MAPREDUCE-4892.1.patch
  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-core
hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-jobclient.

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

Test results: https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/3152//testReport/
Console output: https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/3152//console

This message is automatically generated.
                
> CombineFileInputFormat node input split can be skewed on small clusters
> -----------------------------------------------------------------------
>
>                 Key: MAPREDUCE-4892
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-4892
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>            Reporter: Bikas Saha
>            Assignee: Bikas Saha
>             Fix For: 3.0.0
>
>         Attachments: MAPREDUCE-4892.1.patch
>
>
> The CombineFileInputFormat split generation logic tries to group blocks by node in order
to create splits. It iterates through the nodes and creates splits on them until there aren't
enough blocks left on a node that can be grouped into a valid split. If the first few nodes
have a lot of blocks on them then they can end up getting a disproportionately large share
of the total number of splits created. This can result in poor locality of maps. This problem
is likely to happen on small clusters where its easier to create a skew in the distribution
of blocks on nodes.

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