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https://issues.apache.org/jira/browse/MAPREDUCE-4502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13437688#comment-13437688
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Tsuyoshi OZAWA commented on MAPREDUCE-4502:
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I'm working on refactoring ApplicationMaster of MapReduce to come true re-using 
containers(MAPREDUCE-3902) with Siddharth.
                
> 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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