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https://issues.apache.org/jira/browse/MAPREDUCE-2168?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12926996#action_12926996
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Allen Wittenauer commented on MAPREDUCE-2168:
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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 outputs.
> 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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