Steven Rand created YARN-7290:
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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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