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https://issues.apache.org/jira/browse/YARN-6191?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15866746#comment-15866746
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Jason Lowe commented on YARN-6191:
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This is similar to the FairScheduler problem described in YARN-3054. Since it
always picks on the lowest priority containers, for MapReduce that is always
the maps. Some jobs can get preempted often enough to ever allow it to finish.
> CapacityScheduler preemption by container priority can be problematic for
> MapReduce
> -----------------------------------------------------------------------------------
>
> Key: YARN-6191
> URL: https://issues.apache.org/jira/browse/YARN-6191
> Project: Hadoop YARN
> Issue Type: Bug
> Components: capacityscheduler
> Reporter: Jason Lowe
>
> A MapReduce job with thousands of reducers and just a couple of maps left to
> go was running in a preemptable queue. Periodically other queues would get
> busy and the RM would preempt some resources from the job, but it _always_
> picked the job's map tasks first because they use the lowest priority
> containers. Even though the reducers had a shorter running time, most were
> spared but the maps were always shot. Since the map tasks ran for a longer
> time than the preemption period, the job was in a perpetual preemption loop.
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