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https://issues.apache.org/jira/browse/MAPREDUCE-4892?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Bikas Saha updated MAPREDUCE-4892:
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Status: Patch Available (was: Open)
> 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.alt.patch, 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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