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From "Andrew Wang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-7290) Method canContainerBePreempted can return true when it shouldn't
Date Fri, 08 Dec 2017 19:01:00 GMT

    [ https://issues.apache.org/jira/browse/YARN-7290?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16284035#comment-16284035

Andrew Wang commented on YARN-7290:

I pulled this into branch-3.0.0 as well, thanks folks.

> Method 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
>            Assignee: Steven Rand
>             Fix For: 3.0.0, 3.1.0
>         Attachments: YARN-7290-failing-test.patch, YARN-7290.001.patch, YARN-7290.002.patch,
YARN-7290.003.patch, YARN-7290.004.patch, YARN-7290.005.patch
> 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
> 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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