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From "Tsuyoshi OZAWA (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-5221) Reduce side Combiner is not used when using the new API
Date Fri, 07 Jun 2013 08:51:22 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-5221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13677898#comment-13677898
] 

Tsuyoshi OZAWA commented on MAPREDUCE-5221:
-------------------------------------------

bq. 1. There are no codes to test with MiniMRYarnCluster with launching reduce-side combiner.
TestMRAppWithCombiner seems to be enough, but reduce-side combiners are actually not launched
because data size is too small to launch combiners.

I've checked code again, and I've found that this is not true for reducer-side combiner. Reducer-side
combiner is launched every time InMemoryMerger#merge is called - e.g. when ShuffleScheduler#copySucceeded
is called. Therefore, TestMRAppWithCombiner with new MapReduce API is enough to test in this
case. 

And, MAPREDUCE-5294 and MAPREDUCE-5295 are now blockers of this ticket.
                
> Reduce side Combiner is not used when using the new API
> -------------------------------------------------------
>
>                 Key: MAPREDUCE-5221
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-5221
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>    Affects Versions: 2.0.4-alpha
>            Reporter: Siddharth Seth
>            Assignee: Tsuyoshi OZAWA
>         Attachments: MAPREDUCE-5221.1.patch, MAPREDUCE-5221.2.patch, MAPREDUCE-5221.3.patch
>
>
> If a combiner is specified using o.a.h.mapreduce.Job.setCombinerClass - this will silently
ignored on the reduce side since the reduce side usage is only aware of the old api combiner.
> This doesn't fail the job - since the new combiner key does not deprecate the old key.

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