Steven Rand created YARN-6960:
---------------------------------
Summary: 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: 3.0.0-alpha4, 2.8.1
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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