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https://issues.apache.org/jira/browse/MAPREDUCE-6513?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14961757#comment-14961757
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Sunil G commented on MAPREDUCE-6513:
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Hi [~rohithsharma]
Yes. {{getNumHangingRequests}} looks like a correct metric. Just to add a 
thought to this discussion,  already placed existing reducer requests must be 
served by RM and AM has to reject all those requests. After this part only, 
newly placed map requests can be served. So as we discussed earlier, could we 
also spin out a discussion on resetting already placed reducer requests for a 
faster solution.

> MR job got hanged forever when one NM unstable for some time
> ------------------------------------------------------------
>
>                 Key: MAPREDUCE-6513
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-6513
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: applicationmaster, resourcemanager
>    Affects Versions: 2.7.0
>            Reporter: Bob
>            Assignee: Varun Saxena
>            Priority: Critical
>
> when job is in-progress which is having more tasks,one node became unstable 
> due to some OS issue.After the node became unstable, the map on this node 
> status changed to KILLED state. 
> Currently maps which were running on unstable node are rescheduled, and all 
> are in scheduled state and wait for RM assign container.Seen ask requests for 
> map till Node is good (all those failed), there are no ask request after 
> this. But AM keeps on preempting the reducers (it's recycling).
> Finally reducers are waiting for complete mappers and mappers did n't get 
> container..
> My Question Is:
> ============
> why map requests did not sent AM ,once after node recovery.?



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