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https://issues.apache.org/jira/browse/MAPREDUCE-5844?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Maysam Yabandeh updated MAPREDUCE-5844:
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Attachment: MAPREDUCE-5844.patch
Thanks [~kasha] for the comments. I am attaching a new patch that has them
applied.
I was thinking about a proper name for setReduceResourceReqt. On one hand, by
changing it to setReduceResourceRequest it becomes more readable. On the other
hand, by using setReduceResourceReqt we adhere to the java standard for naming
getters and setter (here reduceResourceReqt). I am more inclined towards the
latter and I was wondering if you are ok with that.
> Reducer Preemption is too aggressive
> ------------------------------------
>
> Key: MAPREDUCE-5844
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-5844
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Reporter: Maysam Yabandeh
> Assignee: Maysam Yabandeh
> Attachments: MAPREDUCE-5844.patch, MAPREDUCE-5844.patch,
> MAPREDUCE-5844.patch
>
>
> We observed cases where the reducer preemption makes the job finish much
> later, and the preemption does not seem to be necessary since after
> preemption both the preempted reducer and the mapper are assigned
> immediately--meaning that there was already enough space for the mapper.
> 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.
> The original idea apparently was that if headroom is not big enough for the
> new mapper requests, reducers should be preempted. This would work if the job
> is alone in the cluster. Once we have queues, the headroom calculation
> becomes more complicated and it would require a separate headroom calculation
> per queue/job.
> So, as a result headroom variable is kind of given up currently: *headroom is
> always set to 0* What this implies to the speculation is that speculation
> becomes very aggressive, not considering whether there is enough space for
> the mappers or not.
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