Jian He commented on YARN-3243:

thanks Wangda !
- ParentQueue#canAssignToThisQueue, 
if (totalUsedCapacityRatio >= maxAvailCapacity) {
  canAssign = false;

instead of comparing with ratio, I think it might be simpler to compare 
resource value

> CapacityScheduler should pass headroom from parent to children to make sure 
> ParentQueue obey its capacity limits.
> -----------------------------------------------------------------------------------------------------------------
>                 Key: YARN-3243
>                 URL: https://issues.apache.org/jira/browse/YARN-3243
>             Project: Hadoop YARN
>          Issue Type: Bug
>          Components: capacityscheduler, resourcemanager
>            Reporter: Wangda Tan
>            Assignee: Wangda Tan
>         Attachments: YARN-3243.1.patch
> Now CapacityScheduler has some issues to make sure ParentQueue always obeys 
> its capacity limits, for example:
> 1) When allocating container of a parent queue, it will only check 
> parentQueue.usage < parentQueue.max. If leaf queue allocated a container.size 
> > (parentQueue.max - parentQueue.usage), parent queue can excess its max 
> resource limit, as following example:
> {code}
>         A  (usage=54, max=55)
>        /     \
>       A1     A2 (usage=1, max=55)
> (usage=53, max=53)
> {code}
> Queue-A2 is able to allocate container since its usage < max, but if we do 
> that, A's usage can excess A.max.
> 2) When doing continous reservation check, parent queue will only tell 
> children "you need unreserve *some* resource, so that I will less than my 
> maximum resource", but it will not tell how many resource need to be 
> unreserved. This may lead to parent queue excesses configured maximum 
> capacity as well.
> With YARN-3099/YARN-3124, now we have {{ResourceUsage}} class in each class, 
> *here is my proposal*:
> - ParentQueue will set its children's ResourceUsage.headroom, which means, 
> *maximum resource its children can allocate*.
> - ParentQueue will set its children's headroom to be (saying parent's name is 
> "qA"): min(qA.headroom, qA.max - qA.used). This will make sure qA's 
> ancestors' capacity will be enforced as well (qA.headroom is set by qA's 
> parent).
> - {{needToUnReserve}} is not necessary, instead, children can get how much 
> resource need to be unreserved to keep its parent's resource limit.
> - More over, with this, YARN-3026 will make a clear boundary between 
> LeafQueue and FiCaSchedulerApp, headroom will consider user-limit, etc.

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