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https://issues.apache.org/jira/browse/MAPREDUCE-20?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Harsh J resolved MAPREDUCE-20.
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Resolution: Duplicate
HADOOP-2399 possibly had fixed this. This has gone stale, and has not been
reported again recently.
> If a mapper of a map/reduce job with combiner has to spill the map output,
> the performance degrades significantly
> -----------------------------------------------------------------------------------------------------------------
>
> Key: MAPREDUCE-20
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-20
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> 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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