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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