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Date: Thu, 13 Oct 2011 05:47:11 +0000 (UTC)
From: "Todd Lipcon (Commented) (JIRA)" <jira@apache.org>
To: mapreduce-issues@hadoop.apache.org
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Subject: [jira] [Commented] (MAPREDUCE-2905) CapBasedLoadManager incorrectly
 allows assignment when assignMultiple is true (was: assignmultiple per job)
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    [ https://issues.apache.org/jira/browse/MAPREDUCE-2905?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13126358#comment-13126358 ] 

Todd Lipcon commented on MAPREDUCE-2905:
----------------------------------------

The point isn't to test that this code didn't introduce a regression - the point is to add a regression test for this bug so that _future_ code doesn't regress _this_ fix :)
                
> CapBasedLoadManager incorrectly allows assignment when assignMultiple is true (was: assignmultiple per job)
> -----------------------------------------------------------------------------------------------------------
>
>                 Key: MAPREDUCE-2905
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-2905
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: contrib/fair-share
>    Affects Versions: 0.20.2
>            Reporter: Jeff Bean
>         Attachments: MR-2905.patch, MR-2905.patch.2
>
>
> We encountered a situation where in the same cluster, large jobs benefit from mapred.fairscheduler.assignmultiple, but small jobs with small numbers of mappers do not: the mappers all clump to fully occupy just a few nodes, which causes those nodes to saturate and bottleneck. The desired behavior is to spread the job across more nodes so that a relatively small job doesn't saturate any node in the cluster.
> Testing has shown that setting mapred.fairscheduler.assignmultiple to false gives the desired behavior for small jobs, but is unnecessary for large jobs. However, since this is a cluster-wide setting, we can't properly tune.
> It'd be nice if jobs can set a param similar to mapred.fairscheduler.assignmultiple on submission to better control the task distribution of a particular job.

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