[jira] [Created] (MAPREDUCE-6043) Reducer-preemption does not kick in

["Maysam Yabandeh (JIRA)" <ji...@apache.org>]

Maysam Yabandeh created MAPREDUCE-6043:
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             Summary: 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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