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Posted to common-dev@hadoop.apache.org by "Runping Qi (JIRA)" <ji...@apache.org> on 2008/03/05 06:05:40 UTC

[jira] Created: (HADOOP-2940) If a mapper of a map/reduce job with combiner has to spill the map output, the performance degrades significantly

If a mapper of a map/reduce job with combiner has to spill the map output, the performance degrades significantly
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                 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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[jira] Commented: (HADOOP-2940) If a mapper of a map/reduce job with combiner has to spill the map output, the performance degrades significantly

Posted by "Owen O'Malley (JIRA)" <ji...@apache.org>.
    [ https://issues.apache.org/jira/browse/HADOOP-2940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12575440#action_12575440 ] 

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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