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From "Sandy Ryza (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-2026) Fair scheduler : Fair share for inactive queues causes unfair allocation in some scenarios
Date Wed, 28 May 2014 20:19:02 GMT

    [ https://issues.apache.org/jira/browse/YARN-2026?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14011523#comment-14011523

Sandy Ryza commented on YARN-2026:

The nice thing about fair share currently is that it's interpretable as an amount of resources
that, as long as you stay under, you won't get preempted.   Changing it to depend on the running
apps in the cluster severely complicates this.  It used to be that each app and queue's fair
share was min'd with its resource usage+demand, which is sort of a continuous analog to what
you're suggesting, but we moved to the current definition when we added multi-resource scheduling.

I'm wondering if the right way to solve this problem is to allow preemption to be triggered
at higher levels in the queue hierarchy.  I.e. suppose we have the following situation:
* root has two children - parentA and parentB
* each of root's children has two children - childA1, childA2, childB1, and childB2
* the parent queues' minShares are each set to half of the cluster resources
* the child queue' minShares are each set to a quarter of the cluster resources 
* childA1 has a third of the cluster resources
* childB1 and childB2 each have a third of the cluster resources

Even though childA1 is above its fair/minShare, We would see that parentA is below its minShare,
so we would preempt resources on its behalf.  Once we have YARN-596 in, these resources would
end up coming from parentB, and end up going to childA1.

> 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
> 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
> 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.
> 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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