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From "Daniel Templeton (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-6960) definition of active queue allows idle long-running apps to distort fair shares
Date Mon, 07 Aug 2017 16:41:00 GMT

    [ https://issues.apache.org/jira/browse/YARN-6960?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16116837#comment-16116837

Daniel Templeton commented on YARN-6960:

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
> {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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