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From "Owen O'Malley (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3940) Reduce often attempts in memory merge with no work
Date Fri, 05 Sep 2008 16:00:49 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12628660#action_12628660
] 

Owen O'Malley commented on HADOOP-3940:
---------------------------------------

There was a suggestion on the list to include this in 0.18.1. That sounds right to me. Thoughts?

> Reduce often attempts in memory merge with no work
> --------------------------------------------------
>
>                 Key: HADOOP-3940
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3940
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: mapred
>    Affects Versions: 0.18.0
>            Reporter: Chris Douglas
>            Assignee: Chris Douglas
>            Priority: Minor
>             Fix For: 0.19.0
>
>         Attachments: 3940-0.patch, 3940-1.patch
>
>
> ReduceTask.ReduceCopier.ShuffleRamManager initializes numRequiredMapOutputs to 0, so
one of the predicates in ShuffleRamManager::waitForDataToMerge, {{numPendingRequests <
numRequiredMapOutputs}}, is false until the first map output is fetched and false again after
the last map output is fetched. The InMemFSMergeThread thread will loop busily in this state.

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