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https://issues.apache.org/jira/browse/MAPREDUCE-5844?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14036519#comment-14036519
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Sangjin Lee commented on MAPREDUCE-5844:
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I agree regarding synchronization. The new member variables Maysam introduced
are essentially renames of the existing ones, and the only new method that
accesses these variables is setMapResourceRequest, which is synchronized. We
looked at all code paths that access and/or mutate them, and they are all
synchronized.
The ones that are mentioned in the findbugs warning as unsynchronized are in
fact synchronized, explicitly or implicitly.
> 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, MAPREDUCE-5844.patch, MAPREDUCE-5844.patch,
> 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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