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https://issues.apache.org/jira/browse/HADOOP-2771?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12619154#action_12619154
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Chris Douglas commented on HADOOP-2771:
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Does this job run with a combiner? That would also add time spent in each spill.
> changing the number of reduces dramatically changes the time of the map time
> ----------------------------------------------------------------------------
>
> Key: HADOOP-2771
> URL: https://issues.apache.org/jira/browse/HADOOP-2771
> Project: Hadoop Core
> Issue Type: Bug
> Components: mapred
> Affects Versions: 0.17.1
> Reporter: Owen O'Malley
>
> By changing the number of reduces, the time for an individual map changes
> radically. By running the same program and data with different numbers of
> reduces (2500, 7500, 25000) the times for each map changed radically (0:50,
> 1:20, 5h).
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