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https://issues.apache.org/jira/browse/YARN-2026?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14058009#comment-14058009
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Sandy Ryza commented on YARN-2026:
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I think Ashwin makes a good point.
I think displaying both is reasonable if we present it in a careful way. For
example, it might make sense to add tooltips that explain the difference.
> Fair scheduler : Fair share for inactive queues causes unfair allocation in
> some scenarios
> ------------------------------------------------------------------------------------------
>
> Key: YARN-2026
> URL: https://issues.apache.org/jira/browse/YARN-2026
> Project: Hadoop YARN
> Issue Type: Bug
> Components: scheduler
> Reporter: Ashwin Shankar
> Assignee: Ashwin Shankar
> Labels: scheduler
> Attachments: YARN-2026-v1.txt, YARN-2026-v2.txt
>
>
> Problem1- While using hierarchical queues in fair scheduler,there are few
> scenarios where we have seen a leaf queue with least fair share can take
> majority of the cluster and starve a sibling parent queue which has greater
> weight/fair share and preemption doesn’t kick in to reclaim resources.
> The root cause seems to be that fair share of a parent queue is distributed
> to all its children irrespective of whether its an active or an inactive(no
> apps running) queue. Preemption based on fair share kicks in only if the
> usage of a queue is less than 50% of its fair share and if it has demands
> greater than that. When there are many queues under a parent queue(with high
> fair share),the child queue’s fair share becomes really low. As a result when
> only few of these child queues have apps running,they reach their *tiny* fair
> share quickly and preemption doesn’t happen even if other leaf
> queues(non-sibling) are hogging the cluster.
> This can be solved by dividing fair share of parent queue only to active
> child queues.
> Here is an example describing the problem and proposed solution:
> root.lowPriorityQueue is a leaf queue with weight 2
> root.HighPriorityQueue is parent queue with weight 8
> root.HighPriorityQueue has 10 child leaf queues :
> root.HighPriorityQueue.childQ(1..10)
> Above config,results in root.HighPriorityQueue having 80% fair share
> and each of its ten child queue would have 8% fair share. Preemption would
> happen only if the child queue is <4% (0.5*8=4).
> Lets say at the moment no apps are running in any of the
> root.HighPriorityQueue.childQ(1..10) and few apps are running in
> root.lowPriorityQueue which is taking up 95% of the cluster.
> Up till this point,the behavior of FS is correct.
> Now,lets say root.HighPriorityQueue.childQ1 got a big job which requires 30%
> of the cluster. It would get only the available 5% in the cluster and
> preemption wouldn't kick in since its above 4%(half fair share).This is bad
> considering childQ1 is under a highPriority parent queue which has *80% fair
> share*.
> Until root.lowPriorityQueue starts relinquishing containers,we would see the
> following allocation on the scheduler page:
> *root.lowPriorityQueue = 95%*
> *root.HighPriorityQueue.childQ1=5%*
> This can be solved by distributing a parent’s fair share only to active
> queues.
> So in the example above,since childQ1 is the only active queue
> under root.HighPriorityQueue, it would get all its parent’s fair share i.e.
> 80%.
> This would cause preemption to reclaim the 30% needed by childQ1 from
> root.lowPriorityQueue after fairSharePreemptionTimeout seconds.
> Problem2 - Also note that similar situation can happen between
> root.HighPriorityQueue.childQ1 and root.HighPriorityQueue.childQ2,if childQ2
> hogs the cluster. childQ2 can take up 95% cluster and childQ1 would be stuck
> at 5%,until childQ2 starts relinquishing containers. We would like each of
> childQ1 and childQ2 to get half of root.HighPriorityQueue fair share ie
> 40%,which would ensure childQ1 gets upto 40% resource if needed through
> preemption.
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