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https://issues.apache.org/jira/browse/MAPREDUCE-4502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13426365#comment-13426365
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Tsuyoshi OZAWA commented on MAPREDUCE-4502:
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We prototyped the combining the result of maps per node/rack.
the the result of our benchmark is available at mapreduce-dev
ML(http://goo.gl/wWBWQ).
> Multi-level aggregation with combining the result of maps per node/rack
> -----------------------------------------------------------------------
>
> Key: MAPREDUCE-4502
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-4502
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Components: applicationmaster, mrv2
> Reporter: Tsuyoshi OZAWA
>
> The shuffle costs is expensive in Hadoop in spite of the existence of
> combiner, because the scope of combining is limited within only one MapTask.
> To solve this problem, it's a good way to aggregate the result of maps per
> node/rack by launch combiner.
> This JIRA is to implement the multi-level aggregation infrastructure,
> including combining per container(MAPREDUCE-3902 is related), coordinating
> containers by application master without breaking fault tolerance of jobs.
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