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https://issues.apache.org/jira/browse/MAPREDUCE-5844?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14011249#comment-14011249
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Maysam Yabandeh commented on MAPREDUCE-5844:
--------------------------------------------
Thanks [~wangda]. I guess there is no unanimity about the default value of the
threshold, as some also suggested to have its default equal to zero. We are
planning to have it enabled in our config settings anyway, but I let the
community decide on the default value in the source code.
I think using the timestamp of the last allocated mapper is interesting since
we would keep only one timestamp versus one per mapper request. The challenge
however would be that it does not clarify how recent is each map request. We
can of course have another single timestamp for earliest received map request
but maintaining it after one of the many inflight map requests get allocated
would add a bit of more complexity to the patch and its logic. I figured having
the logic in the patch as simple as possible would justify a new timestamp
field per container request.
> 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
>
>
> 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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