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Date: Tue, 24 Aug 2010 15:07:19 -0400 (EDT)
From: "Matei Zaharia (JIRA)" <jira@apache.org>
To: mapreduce-issues@hadoop.apache.org
Subject: [jira] Commented: (MAPREDUCE-2027) fair share scheduler causes
 unnecessary preemption under some conditions
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    [ https://issues.apache.org/jira/browse/MAPREDUCE-2027?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12902020#action_12902020 ] 

Matei Zaharia commented on MAPREDUCE-2027:
------------------------------------------

Actually, looking at trunk, the issue seems to be there too. The methods you need to fix are neededSpeculativeMaps and neededSpeculativeReduces in DefaultTaskSelector.

> fair share scheduler causes unnecessary preemption under some conditions
> ------------------------------------------------------------------------
>
>                 Key: MAPREDUCE-2027
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-2027
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: contrib/fair-share
>            Reporter: Joydeep Sen Sarma
>
> When the cluster hits speculative cap - JobInProgress does not speculate any more. however FairShare Scheduler continues to believe that tasks need to be speculated.
> Under such conditions, FS Scheduler will start preempting repeatedly (because from it's perspective - certain jobs are not getting the slots they deserve (for speculative tasks)).
> There may be other reasons for excessive preemption as well - this one stood out clearly.

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