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https://issues.apache.org/jira/browse/MAPREDUCE-6043?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14105665#comment-14105665
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Maysam Yabandeh commented on MAPREDUCE-6043:
--------------------------------------------
After a preemption, the task id is added to
{code}
assignedRequests.preemptionWaitingReduces
{code}
And it is later removed once the preemption is successfully finished. In the
second scenario we observed, although the preemption was successfully finished
its report was not received through RM and hence the variable was not decreased
and future preemptions did not kick in.
> Reducer-preemption does not kick in
> -----------------------------------
>
> Key: MAPREDUCE-6043
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-6043
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Reporter: Maysam Yabandeh
>
> We have seen various cases that reducer-preemption does not kick in and the
> scheduled mappers wait behind running reducers forever. Each time there seems
> to be a different scenario. So far we have tracked down two of such cases and
> the common element between them is that the variables in RMContainerAllocator
> go out of sync since they only get updated when completed container is
> reported by RM. However there are many corner cases that such report is not
> received from RM and yet the MapReduce app moves forward. Perhaps one
> possible fix would be to update such variables also after exceptional cases.
> The logic for triggering preemption is at
> RMContainerAllocator::preemptReducesIfNeeded
> The preemption is triggered if the following is true:
> {code}
> headroom + am * |m| + pr * |r| < mapResourceRequest
> {code}
> where am: number of assigned mappers, |m| is mapper size, pr is number of
> reducers being preempted, and |r| is the reducer size. Each of these
> variables going out of sync will cause the preemption not to kick in. In the
> following comment, we explain two of such cases.
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