Sandy Ryza commented on YARN-2026:

I had a conversation with [~kkambatl] about this, and he convinced me that we 
should turn this on in all cases - i.e. modify FairSharePolicy and 
DominantResourceFairnessPolicy instead of creating additional policies.  Sorry 
to vacillate on this.

Some additional comments on the code:
+    return this.getNumRunnableApps() > 0;

+          || (sched instanceof FSQueue && ((FSQueue) sched).isActive())) {
Instead of using instanceof, can we add an isActive method to Schedulable, and 
always return true for it in AppSchedulable?

+    out.println("       <queue name=\"childA1\" />");
+    out.println("       <queue name=\"childA2\" />");
+    out.println("       <queue name=\"childA3\" />");
+    out.println("       <queue name=\"childA4\" />");
+    out.println("       <queue name=\"childA5\" />");
+    out.println("       <queue name=\"childA6\" />");
+    out.println("       <queue name=\"childA7\" />");
+    out.println("       <queue name=\"childA8\" />");
Do we need this many children?

+    out.println("</queue>");
+    out.println("</allocations>");
Unnecessary newline

+  public void testFairShareActiveOnly_ShareResetsToZeroWhenAppsComplete()
Take out underscore.

+  private void setupCluster(int mem, int vCores) throws IOException {
Give this method a name that's more descriptive of the kind of configuration 
it's setting up.

+  private void setupCluster(int nodeMem) throws IOException {
Can this call the setupCluster that takes two arguments?

To help with the fight against TestFairScheduler becoming a monstrosity, the 
tests should go into a new test file.  TestFairSchedulerPreemption is a good 
example of how to do this.

+    int nodeVcores = 10;
Nit: "nodeVCores"

> 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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