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From "Nathan Marz (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5160) Hadoop reduce scheduler sometimes leaves machines idle
Date Fri, 06 Feb 2009 19:03:59 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5160?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12671241#action_12671241
] 

Nathan Marz commented on HADOOP-5160:
-------------------------------------

Reducing the number of reduce slots to 1 per box is impractical. This job that requires the
1 reducer per machine configuration runs as part of a much, much larger workflow, where every
other job benefits from having multiple reducers on each machine.

> Hadoop reduce scheduler sometimes leaves machines idle
> ------------------------------------------------------
>
>                 Key: HADOOP-5160
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5160
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: mapred
>            Reporter: Nathan Marz
>
> I have a MapReduce application with number of reducers equal to the number of machines
in the cluster (and with speculative execution turned off). However, Hadoop schedules multiple
reduces to run on single machines and leaves other machines idle. This causes contention and
seriously slows down the job. Hadoop should employ the simple heuristic of utilizing as many
machines as possible when scheduling reduces.

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