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https://issues.apache.org/jira/browse/HADOOP-2940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12575440#action_12575440
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Owen O'Malley commented on HADOOP-2940:
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I suspect this is caused by HADOOP-2399. Can you try it?
> If a mapper of a map/reduce job with combiner has to spill the map output,
> the performance degrades significantly
> -----------------------------------------------------------------------------------------------------------------
>
> Key: HADOOP-2940
> URL: https://issues.apache.org/jira/browse/HADOOP-2940
> Project: Hadoop Core
> Issue Type: Bug
> Components: mapred
> Affects Versions: 0.16.0
> Reporter: Runping Qi
>
> I have a map/reduce job whose reducers combine a group of values into a
> single value.
> The average reduction rate is about 3 to 1. The execution time for the job
> with the reducer as its combiner ,
> is twice of that for the case without using combiner. This is completely
> counter-intuitive.
> When I looked at the job execution more carefully, I noticed that this longer
> execution time for the
> job was mainly due to a few mappers that generated spills. The final merge of
> the spills seems
> took a much longer time with combiner than without combiner.
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