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https://issues.apache.org/jira/browse/MAPREDUCE-2973?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13101367#comment-13101367
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Jonathon Hare commented on MAPREDUCE-2973:
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Noted, but regarding the {{JvmManager}}, is there any good reason that the
creation of a new JVM should be preferred over the reuse of an existing idle
one (for the same job) when JVM reuse is enabled?
> Logic for determining whether to create a new JVM can interfere with
> Capacity-Scheduler and JVM reuse
> -----------------------------------------------------------------------------------------------------
>
> Key: MAPREDUCE-2973
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-2973
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: tasktracker
> Affects Versions: 0.20.2
> Environment: N/A
> Reporter: Jonathon Hare
> Priority: Minor
> Attachments: hadoop-patch.txt
>
> Original Estimate: 0h
> Remaining Estimate: 0h
>
> We use the capacity scheduler to enable jobs with large memory requirements
> to be run on our cluster. The individual tasks have a large initial overhead
> when they load cached data. Using the JVM reuse option
> ({{mapred.job.reuse.jvm.num.tasks}}) and by caching data in a static variable
> we can reduce the overhead.
> The current {{JvmManager}} implementation will prefer creating new JVMs to
> reusing existing ones if the number of already created JVMs is less than the
> maximum. In the extreme case where the capacity scheduler is used to limit
> the number of tasks on a node to 1, but the number of [map|reduce] tasks per
> node is set to say 16, then 16 JVMs will be created before one of them is
> reused. Obviously, if the amount of cached data in the memory of each JVM is
> large, then node can rapidly run out of memory! What should really happen in
> this case is that the first created JVM should be reused, and others should
> not be spawned.
> To work-around this problem on our cluster, we have modified the logic in the
> {{reapJVM()}} method in {{JvmManager}} to prefer to reuse an existing JVM
> (idle & belonging to the same job) over starting a new JVM, or killing an
> existing idle JVM.
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