Daniel Templeton commented on YARN-6956:

That behavior was intentional to prevent the case where we preempt 8 1GB 
containers on 8 different nodes to make space for 1 8GB container.

> preemption may only consider resource requests for one node
> -----------------------------------------------------------
>                 Key: YARN-6956
>                 URL: https://issues.apache.org/jira/browse/YARN-6956
>             Project: Hadoop YARN
>          Issue Type: Bug
>          Components: fairscheduler
>    Affects Versions: 2.9.0, 3.0.0-beta1
>         Environment: CDH 5.11.0
>            Reporter: Steven Rand
> I'm observing the following series of events on a CDH 5.11.0 cluster, which 
> seem to be possible after YARN-6163:
> 1. An application is considered to be starved, so {{FSPreemptionThread}} 
> calls {{identifyContainersToPreempt}}, and that calls 
> {{FSAppAttempt#getStarvedResourceRequests}} to get a list of 
> {{ResourceRequest}} instances that are enough to address the app's starvation.
> 2. The first {{ResourceRequest}} that {{getStarvedResourceRequests}} sees is 
> enough to address the app's starvation, so we break out of the loop over 
> {{appSchedulingInfo.getAllResourceRequests()}} after only one iteration: 
> https://github.com/apache/hadoop/blob/trunk/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/FSAppAttempt.java#L1180.
>  We return only this one {{ResourceRequest}} back to the 
> {{identifyContainersToPreempt}} method.
> 3. It turns out that this particular {{ResourceRequest}} happens to have a 
> value for {{getResourceName}} that identifies a specific node in the cluster. 
> This causes preemption to only consider containers on that node, and not the 
> rest of the cluster.
> [~kasha], does that make sense? I'm happy to submit a patch if I'm 
> understanding the problem correctly.

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