Xianyin Xin commented on YARN-4120:

Hi [~kasha], [~asuresh], [~ashwinshankar77], now both the preemption logic and 
resource allocation logic uses {{comparator}} to sort the {{Schedulables}}. I 
think we have to introduce a different comparator to separate {{usage}} and 
{{usage - preemption}}, just as the patch in YARN-4134. There're also some 
discussion on changing {{Comparator.compare()}} in YARN-3453. I think for a 
collection of comparables, we can use different comparators to compare 
different attributes for different purpose. Any thoughts?

> FSAppAttempt.getResourceUsage() should not take preemptedResource into account
> ------------------------------------------------------------------------------
>                 Key: YARN-4120
>                 URL: https://issues.apache.org/jira/browse/YARN-4120
>             Project: Hadoop YARN
>          Issue Type: Bug
>          Components: fairscheduler
>            Reporter: Xianyin Xin
> When compute resource usage for Schedulables, the following code is envolved,
> {{FSAppAttempt.getResourceUsage}},
> {code}
> public Resource getResourceUsage() {
>   return Resources.subtract(getCurrentConsumption(), getPreemptedResources());
> }
> {code}
> and this value is aggregated to FSLeafQueues and FSParentQueues. In my 
> opinion, taking {{preemptedResource}} into account here is not reasonable, 
> there are two main reasons,
> # it is something in future, i.e., even though these resources are marked as 
> preempted, it is currently used by app, and these resources will be 
> subtracted from {{currentCosumption}} once the preemption is finished. it's 
> not reasonable to make arrange for it ahead of time. 
> # there's another problem here, consider following case,
> {code}
>             root
>            /    \
>       queue1   queue2
>       /    \
> queue1.3, queue1.4
> {code}
> suppose queue1.3 need resource and it can preempt resources from queue1.4, 
> the preemption happens in the interior of queue1. But when compute resource 
> usage of queue1, {{queue1.resourceUsage = it's_current_resource_usage - 
> preemption}} according to the current code, which is unfair to queue2 when 
> doing resource allocating.

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