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