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

["Vinod Kumar Vavilapalli (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/MAPREDUCE-4892?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Vinod Kumar Vavilapalli updated MAPREDUCE-4892:
-----------------------------------------------

    Status: Open  (was: Patch Available)

The latest code changes the and the test fix look good. Cancelling patch and resubmitting to Jenkins just to be sure before committing.
                
> 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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