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https://issues.apache.org/jira/browse/YARN-1198?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14157524#comment-14157524
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Craig Welch commented on YARN-1198:
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FYI, it's not possible to call the getAndCalculateHeadroom because nothing can
synchronize on the queue during the allocation call without deadlocking - this
is why it's necessary to break out the headroom they way it is here and store
some items (such as the LeafQueue.User, which comes from the usermanager and
syncs on the queu) to avoid any synchronization on the queue itself during the
final headroom calculation in the allocate/getHeadroom step. It's not a bad
thing to do anyway, to reduce the number of operations (somewhat) in that final
headroom calculation - but it is also why we can't just call the
getAndCalculateHeadroom as such (unchanged) in allocate()
> Capacity Scheduler headroom calculation does not work as expected
> -----------------------------------------------------------------
>
> Key: YARN-1198
> URL: https://issues.apache.org/jira/browse/YARN-1198
> Project: Hadoop YARN
> Issue Type: Bug
> Reporter: Omkar Vinit Joshi
> Assignee: Craig Welch
> Attachments: YARN-1198.1.patch, YARN-1198.10.patch,
> YARN-1198.11-with-1857.patch, YARN-1198.11.patch, YARN-1198.2.patch,
> YARN-1198.3.patch, YARN-1198.4.patch, YARN-1198.5.patch, YARN-1198.6.patch,
> YARN-1198.7.patch, YARN-1198.8.patch, YARN-1198.9.patch
>
>
> Today headroom calculation (for the app) takes place only when
> * New node is added/removed from the cluster
> * New container is getting assigned to the application.
> However there are potentially lot of situations which are not considered for
> this calculation
> * If a container finishes then headroom for that application will change and
> should be notified to the AM accordingly.
> * If a single user has submitted multiple applications (app1 and app2) to the
> same queue then
> ** If app1's container finishes then not only app1's but also app2's AM
> should be notified about the change in headroom.
> ** Similarly if a container is assigned to any applications app1/app2 then
> both AM should be notified about their headroom.
> ** To simplify the whole communication process it is ideal to keep headroom
> per User per LeafQueue so that everyone gets the same picture (apps belonging
> to same user and submitted in same queue).
> * If a new user submits an application to the queue then all applications
> submitted by all users in that queue should be notified of the headroom
> change.
> * Also today headroom is an absolute number ( I think it should be normalized
> but then this is going to be not backward compatible..)
> * Also when admin user refreshes queue headroom has to be updated.
> These all are the potential bugs in headroom calculations
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