[jira] [Created] (YARN-7290) canContainerBePreempted can return true when it shouldn't

["Steven Rand (JIRA)" <ji...@apache.org>]

Steven Rand created YARN-7290:
---------------------------------

             Summary: canContainerBePreempted can return true when it shouldn't
                 Key: YARN-7290
                 URL: https://issues.apache.org/jira/browse/YARN-7290
             Project: Hadoop YARN
          Issue Type: Bug
          Components: fairscheduler
    Affects Versions: 3.0.0-beta1
            Reporter: Steven Rand


In FSAppAttempt#canContainerBePreempted, we make sure that preempting the given container would not put the app below its fair share:

{code}
    // Check if the app's allocation will be over its fairshare even
    // after preempting this container
    Resource usageAfterPreemption = Resources.clone(getResourceUsage());

    // Subtract resources of containers already queued for preemption
    synchronized (preemptionVariablesLock) {
      Resources.subtractFrom(usageAfterPreemption, resourcesToBePreempted);
    }

    // Subtract this container's allocation to compute usage after preemption
    Resources.subtractFrom(
        usageAfterPreemption, container.getAllocatedResource());
    return !isUsageBelowShare(usageAfterPreemption, getFairShare());
{code}

However, this only considers one container in isolation, and fails to consider containers for the same app that we already added to {{preemptableContainers}} in FSPreemptionThread#identifyContainersToPreemptOnNode. Therefore we can have a case where we preempt multiple containers from the same app, none of which by itself puts the app below fair share, but which cumulatively do so.

I've attached a patch with a test to show this behavior. The flow is:

1. Initially greedyApp runs in {{root.preemptable.child-1}} and is allocated all the resources (8g and 8vcores)
2. Then starvingApp runs in {{root.preemptable.child-2}} and requests 2 containers, each of which is 3g and 3vcores in size. At this point both greedyApp and starvingApp have a fair share of 4g (with DRF not in use).
3. For the first container requested by starvedApp, we (correctly) preempt 3 containers from greedyApp, each of which is 1g and 1vcore.
4. For the second container requested by starvedApp, we again (this time incorrectly) preempt 3 containers from greedyApp. This puts greedyApp below its fair share, but happens anyway because all six times that we call {{return !isUsageBelowShare(usageAfterPreemption, getFairShare());}}, the value of {{usageAfterPreemption}} is 7g and 7vcores (confirmed using debugger).

So in addition to accounting for {{resourcesToBePreempted}}, we also need to account for containers that we're already planning on preempting in FSPreemptionThread#identifyContainersToPreemptOnNode. 



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