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Posted to mapreduce-issues@hadoop.apache.org by "Bikas Saha (JIRA)" <ji...@apache.org> on 2012/12/19 20:59:12 UTC

[jira] [Created] (MAPREDUCE-4892) CombineFileInputFormat node input split can be skewed on small clusters

Bikas Saha created MAPREDUCE-4892:
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             Summary: 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


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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