Ashwin Shankar commented on YARN-2214:

Thanks Karthik !

> FairScheduler: preemptContainerPreCheck() in FSParentQueue delays convergence 
> towards fairness
> ----------------------------------------------------------------------------------------------
>                 Key: YARN-2214
>                 URL: https://issues.apache.org/jira/browse/YARN-2214
>             Project: Hadoop YARN
>          Issue Type: Improvement
>          Components: scheduler
>    Affects Versions: 2.5.0
>            Reporter: Ashwin Shankar
>            Assignee: Ashwin Shankar
>             Fix For: 2.6.0
>         Attachments: YARN-2214-v1.txt, YARN-2214-v2.txt
> preemptContainerPreCheck() in FSParentQueue rejects preemption requests if 
> the parent queue is below fair share. This can cause a delay in converging 
> towards fairness when the starved leaf queue and the queue above fairshare 
> belong under a non-root parent queue(ie their least common ancestor is a 
> parent queue which is not root).
> Here is an example :
> root.parent has fair share = 80% and usage = 80%
> root.parent.child1 has fair share =40% usage = 80%
> root.parent.child2 has fair share=40% usage=0%
> Now a job is submitted to child2 and the demand is 40%.
> Preemption will kick in and try to reclaim all the 40% from child1.
> When it preempts the first container from child1,the usage of root.parent 
> will become <80%, which is less than root.parent's fair share,causing 
> preemption to stop.So only one container gets preempted in this round 
> although the need is a lot more. child2 would eventually get to half its fair 
> share but only after multiple rounds of preemption.
> Solution is to remove preemptContainerPreCheck() in FSParentQueue and keep it 
> only in FSLeafQueue(which is already there).

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