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Posted to mapreduce-issues@hadoop.apache.org by "Chris Nauroth (JIRA)" <ji...@apache.org> on 2013/01/03 23:46:14 UTC
[jira] [Commented] (MAPREDUCE-4892) CombineFileInputFormat node
input split can be skewed on small clusters
[ https://issues.apache.org/jira/browse/MAPREDUCE-4892?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13543377#comment-13543377 ]
Chris Nauroth commented on MAPREDUCE-4892:
------------------------------------------
+1 for the patch
I applied the patch locally and ran {{TestCombineFileInputFormat}}. Code looks good. I can't think of any other edge cases that this patch doesn't handle.
> 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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