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https://issues.apache.org/jira/browse/YARN-2910?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14234023#comment-14234023
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Akira AJISAKA commented on YARN-2910:
-------------------------------------

As [~wilfreds] mentioned, synchronization can cause RM slow down if there are 
many getQueueInfo requests from clients at a time, so I'm thinking 
{{CopyOnWriteArrayList}} might be better.
By the way, we should fix CapacityScheduler also. (YARN-2922)

> FSLeafQueue can throw ConcurrentModificationException
> -----------------------------------------------------
>
>                 Key: YARN-2910
>                 URL: https://issues.apache.org/jira/browse/YARN-2910
>             Project: Hadoop YARN
>          Issue Type: Bug
>          Components: fairscheduler
>    Affects Versions: 2.5.0
>            Reporter: Wilfred Spiegelenburg
>            Assignee: Wilfred Spiegelenburg
>         Attachments: FSLeafQueue_concurrent_exception.txt, YARN-2910.patch
>
>
> The list that maintains the runnable and the non runnable apps are a standard 
> ArrayList but there is no guarantee that it will only be manipulated by one 
> thread in the system. This can lead to the following exception:
> {noformat}
> 2014-11-12 02:29:01,169 ERROR [RMCommunicator Allocator] 
> org.apache.hadoop.mapreduce.v2.app.rm.RMContainerAllocator: ERROR IN 
> CONTACTING RM.
> java.util.ConcurrentModificationException: 
> java.util.ConcurrentModificationException
> at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:859)
> at java.util.ArrayList$Itr.next(ArrayList.java:831)
> at 
> org.apache.hadoop.yarn.server.resourcemanager.scheduler.fair.FSLeafQueue.getResourceUsage(FSLeafQueue.java:147)
> at 
> org.apache.hadoop.yarn.server.resourcemanager.scheduler.fair.FSAppAttempt.getHeadroom(FSAppAttempt.java:180)
> at 
> org.apache.hadoop.yarn.server.resourcemanager.scheduler.fair.FairScheduler.allocate(FairScheduler.java:923)
> at 
> org.apache.hadoop.yarn.server.resourcemanager.ApplicationMasterService.allocate(ApplicationMasterService.java:516)
> {noformat}
> Full stack trace in the attached file.
> We should guard against that by using a thread safe version from 
> java.util.concurrent.CopyOnWriteArrayList



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