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From "Naganarasimha G R (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-6025) Few issues in synchronization in CapacityScheduler & AbstractYarnScheduler
Date Tue, 27 Dec 2016 00:58:58 GMT

    [ https://issues.apache.org/jira/browse/YARN-6025?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15779295#comment-15779295

Naganarasimha G R commented on YARN-6025:

My bad. i wrongly read the code as CS holding its private lock (different from the AbstractYarnScheduler)
because of which i had given earlier few comments. Now i realize many things clearly. Thanks
 to [~sunilg] (for offline chat) and [~wangda] for clarifying it.
But just one more doubt. 

FairScheduler holds write lock while doing {{super.nodeUpdate(nm)}} and CS holds read lock,
should it not be that we should hold a lock inside {{AbstractYarnScheduler.nodeUpdate(RMNode)}}
as AbstractYarnScheduler.nodeUpdate is mainly sending out the events for the container finished,
updating the health status. In fact all the AYS.nodeUpdate operations could be done even without
any lock so multiple nodes could update these states in parallel, but may be read lock is
required so that write lock will wait for all readlock operations to finish and then do operations
under it. 

For the first point {{"removing the syncronization on nodeUpdate and extending and keeping
it in FifoScheduler"}} will update the patch.

> Few issues in synchronization in CapacityScheduler & AbstractYarnScheduler
> --------------------------------------------------------------------------
>                 Key: YARN-6025
>                 URL: https://issues.apache.org/jira/browse/YARN-6025
>             Project: Hadoop YARN
>          Issue Type: Sub-task
>          Components: capacity scheduler, scheduler
>            Reporter: Naganarasimha G R
>            Assignee: Naganarasimha G R
>         Attachments: YARN-6025.01.patch
> YARN-3139 does optimization on the locks by introducing ReentrantReadWriteLock to remove
synchronized but seems to have some issues.
> # CapacityScheduler
> #* nodeUpdate(RMNode) need not be synchronized, as its the only one to be in the class
> #* setLastNodeUpdateTime in nodeUpdate needs to be updated with readLock ? then getLastNodeUpdateTime
is done without any lock and more over its volatile.
> #* getUserGroupMappingPlacementRule need not be public as its held called within and
not used in test and further is called from initScheduler and reinitialize where both are
holding write locks so i presume getting read locks are of no use.
> # AbstractYarnScheduler
> #* recoverContainersOnNode is synchronized as well as holds write lock on the complete
method so i presume we do not require synchronized here.
> #* nodeUpdate method too is synchronized but if i see the updates done inside i do not
see any place where node update from two different nodes will have any issues (except for
schedulerHealth which is taken care internally with concurrentHashMap), And even if require
we could better use write lock. (also depends on the decision of next point)
> #* readLock is only used in containerLaunchedOnNode which i am not completely sure whether
its required to have a read lock here, suppose we do not require then whether there is any
use of read write locks in AbstractYarnScheduler as in general there is performance overhead
in using readwrite lock over synchronized blocks on frequently accessed code path.

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