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From "Allen Wittenauer (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-2168) We should implement limits on shuffle connections to TaskTracker per job
Date Mon, 01 Nov 2010 14:51:24 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-2168?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12926996#action_12926996

Allen Wittenauer commented on MAPREDUCE-2168:

If this situation occurs, the system is tuned to have too many tasks per node.  The workload
of the DN and TT must be taken into considering when setting the number of tasks per run.

> We should  implement limits on shuffle connections to TaskTracker per job
> -------------------------------------------------------------------------
>                 Key: MAPREDUCE-2168
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-2168
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>            Reporter: Liyin Liang
> As trailing map tasks will be attacked by all reduces simultaneously, all the worker
threads that for the http server of a TaskTracker may be occupied  by one job's reduce tasks
to fetch map outputs. Then this tasktracker's iowait and load will be very high (100+ in our
cluster, we set tasktracker.http.threads with 100). What's more, other job's reduces have
to wait some time (may be several minutes) to connect to the TaskTracker to fetch there map's
> So I think we should implement limits on shuffle connections:
> 1. limit the worker threads' number maybe percent  occupied  the same job's reduces ;
> 2. limit the worker threads' number serving the same map output simultaneously.
> Thoughts? 
> ps: we are using hadoop 0.19.

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