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From "Harsh J (Resolved) (JIRA)" <j...@apache.org>
Subject [jira] [Resolved] (MAPREDUCE-20) If a mapper of a map/reduce job with combiner has to spill the map output, the performance degrades significantly
Date Sat, 31 Dec 2011 08:52:30 GMT

     [ https://issues.apache.org/jira/browse/MAPREDUCE-20?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Harsh J resolved MAPREDUCE-20.
------------------------------

    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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