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From "Vinod K V (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5160) Hadoop reduce scheduler sometimes leaves machines idle
Date Wed, 04 Feb 2009 12:21:59 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5160?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12670315#action_12670315
] 

Vinod K V commented on HADOOP-5160:
-----------------------------------

Nathan, what scheduler are you using? Assuming it is the default one, the default scheduler
does have load balancing code to distribute map *and* reduce tasks evenly on all the TaskTtrackers
depending on each TT's slots. Can you give more information about your observation - things
like hadoop version, size of your cluster, more info about your job etc.?

> 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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