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https://issues.apache.org/jira/browse/MAPREDUCE-1859?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12877863#action_12877863
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Owen O'Malley commented on MAPREDUCE-1859:
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Putting in per-job limits on number of tasks is relatively fraught with errors
and only works if you have exactly one job that *ever* pulls from the database.
In the Capacity scheduler, you can set a max size for a given queue. That is at
least closer to the right semantics so that you can say that any job using the
database needs to use the DB queue.
> maxConcurrentMapTask & maxConcurrentReduceTask per job
> ------------------------------------------------------
>
> Key: MAPREDUCE-1859
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-1859
> Project: Hadoop Map/Reduce
> Issue Type: New Feature
> Components: job submission
> Affects Versions: 0.20.2
> Reporter: Johannes Zillmann
>
> It would be valuable if one could specify the max number of map/reduce slots
> which should be used for a given job. An example would be an map-reduce job
> importing from a database where you don't want 50 map tasks querying one db
> at a time but also you don't want to shrink the overall map task count.
> Also this is probably already possible through Fair/Capacity-Scheduler or an
> own Extension i think it would be a good addition for the default
> TaskScheduler since this seems to be more then a rare used feature.
> This would have the benefit in situations where you don't have
> control/ownership over the cluster as well.
> And its more job-centric whereas the existing scheduler extensions seems to
> be more job-type-centric.
> Implementing this feature should be relatively straightforward. Adding
> something like jobConf.setMaxConcurrentMapTask(int) and respecting this
> configuration in JobQueueTaskScheduler.
> Not sure if this feature would be harmonical with the existing
> Fair/Capacity-Schedulers.
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