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https://issues.apache.org/jira/browse/MAPREDUCE-5844?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14033298#comment-14033298
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Hadoop QA commented on MAPREDUCE-5844:
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{color:green}+1 overall{color}. Here are the results of testing the latest
attachment
http://issues.apache.org/jira/secure/attachment/12650692/MAPREDUCE-5844.patch
against trunk revision .
{color:green}+1 @author{color}. The patch does not contain any @author
tags.
{color:green}+1 tests included{color}. The patch appears to include 1 new
or modified test files.
{color:green}+1 javac{color}. The applied patch does not increase the
total number of javac compiler warnings.
{color:green}+1 javadoc{color}. There were no new javadoc warning messages.
{color:green}+1 eclipse:eclipse{color}. The patch built with
eclipse:eclipse.
{color:green}+1 findbugs{color}. The patch does not introduce any new
Findbugs (version 1.3.9) warnings.
{color:green}+1 release audit{color}. The applied patch does not increase
the total number of release audit warnings.
{color:green}+1 core tests{color}. The patch passed unit tests in
hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-app
hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core.
{color:green}+1 contrib tests{color}. The patch passed contrib unit tests.
Test results:
https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/4663//testReport/
Console output:
https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/4663//console
This message is automatically generated.
> 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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