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From "Ashwin Shankar (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-2026) Fair scheduler : Fair share for inactive queues causes unfair allocation in some scenarios
Date Tue, 08 Jul 2014 19:07:05 GMT

    [ https://issues.apache.org/jira/browse/YARN-2026?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14055326#comment-14055326
] 

Ashwin Shankar commented on YARN-2026:
--------------------------------------

[~sandyr],[~kasha]
Sure, we can incorporate fair share for active queues in FairSharePolicy and DominantResourceFairnessPolicy
rather than creating new ones.

However regarding having two notions of fairness, I have couple of concerns :
1. Confusion/Debugging inconvenience : users looking at the UI would be confused since fair
share preemption would be happening randomly(based on
internal dynamic fair share) rather than when usage is below "half fair share" shown on the
UI. Also it might become a nightmare to debug when we have lots of preemption happening and
we have two notions of fair share.
2. Code becomes kind of complicated - we would call ComputeFairShares twice at each node in
the queue hierarchy,one with active child queues and other with all child queues. OR if you
are thinking of changing ComputeFairShares itself rather than calling it twice, it still would
look kinda messy with
setting dynamic/static fair shares inside it based on active queues.

I hope I'm making sense here. Thoughts ?
Is there a strong reason why we can't just show the dynamic fair share on the UI,which is
the reality ?

> 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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