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https://issues.apache.org/jira/browse/YARN-6960?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16116837#comment-16116837
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Daniel Templeton commented on YARN-6960:
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I'd be curious to understand the use case where you're running into this issue.
My main concern which that fix is that an app that's entering an inactive queue
will not be able to preempt its way into running. In your example, assume we
kill the jobs in root.a and root.b, so that the apps in root.c and root.d share
the cluster 50/50. Now we submit a new app to root.a. Since all we have is an
AM until the AM can run and request other containers, root.a's fair share will
remain 0, and the app in root.a will never be able to preempt the apps in
root.c or root.d.
> definition of active queue allows idle long-running apps to distort fair
> shares
> -------------------------------------------------------------------------------
>
> Key: YARN-6960
> URL: https://issues.apache.org/jira/browse/YARN-6960
> Project: Hadoop YARN
> Issue Type: Bug
> Components: fairscheduler
> Affects Versions: 2.8.1, 3.0.0-alpha4
> Reporter: Steven Rand
> Assignee: Steven Rand
>
> YARN-2026 introduced the notion of only considering active queues when
> computing the fair share of each queue. The definition of an active queue is
> a queue with at least one runnable app:
> {code}
> public boolean isActive() {
> return getNumRunnableApps() > 0;
> }
> {code}
> One case that this definition of activity doesn't account for is that of
> long-running applications that scale dynamically. Such an application might
> request many containers when jobs are running, but scale down to very few
> containers, or only the AM container, when no jobs are running.
> Even when such an application has scaled down to a negligible amount of
> demand and utilization, the queue that it's in is still considered to be
> active, which defeats the purpose of YARN-2026. For example, consider this
> scenario:
> 1. We have queues {{root.a}}, {{root.b}}, {{root.c}}, and {{root.d}}, all of
> which have the same weight.
> 2. Queues {{root.a}} and {{root.b}} contain long-running applications that
> currently have only one container each (the AM).
> 3. An application in queue {{root.c}} starts, and uses the whole cluster
> except for the small amount in use by {{root.a}} and {{root.b}}. An
> application in {{root.d}} starts, and has a high enough demand to be able to
> use half of the cluster. Because all four queues are active, the app in
> {{root.d}} can only preempt the app in {{root.c}} up to roughly 25% of the
> cluster's resources, while the app in {{root.c}} keeps about 75%.
> Ideally in this example, the app in {{root.d}} would be able to preempt the
> app in {{root.c}} up to 50% of the cluster, which would be possible if the
> idle apps in {{root.a}} and {{root.b}} didn't cause those queues to be
> considered active.
> One way to address this is to update the definition of an active queue to be
> a queue containing 1 or more non-AM containers. This way if all apps in a
> queue scale down to only the AM, other queues' fair shares aren't affected.
> The benefit of this approach is that it's quite simple. The downside is that
> it doesn't account for apps that are idle and using almost no resources, but
> still have at least one non-AM container.
> There are a couple of other options that seem plausible to me, but they're
> much more complicated, and it seems to me that this proposal makes good
> progress while adding minimal extra complexity.
> Does this seem like a reasonable change? I'm certainly open to better ideas
> as well.
> Thanks,
> Steve
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