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https://issues.apache.org/jira/browse/HADOOP-2771?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12616327#action_12616327
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Christian Kunz commented on HADOOP-2771:
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Still an issue in 0.17.1:

Sample job with 100,000 maps:

9,000  reduces: map executes on average in 1hr
18,000 reduces: map executes on average in 1:40 hr

> 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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