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https://issues.apache.org/jira/browse/YARN-2592?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14146790#comment-14146790
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Carlo Curino commented on YARN-2592:
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I hear you, and I agree we will need to cope with non-cheap preemption for a
while, and even long term not everyone will be nicely preemptable (our work on
YARN-1051 is for example designed to allow people to get very guaranteed and
protected resources when needed).
However, the compromise you propose means that the over-capacity "zone" is
weirdly policed... on one side we expect the "giving" of containers to respect
a notion of fairness (proportional to your rightful capacity), which is in
turns not enforce by preemption. I find this inconsistent.
Moreover, as I was saying, I think this will only spare containers in a rather
narrow band (when imbalance happened among over capacity queues, and no
under-capacity queues are requesting resources yet, and we are above the
dead-zone, and tasks run longer than 2x the grace period). Is this a large
enough use case to require special-casing?
If this is important in practice and an adoption show-stopper I am fine with
compromises, but we should make sure this is the case.
A way to do this is to enable preemption but run it in "observe-only" mode,
where the policy logs what he would like to do without actually doing it... We
can see whether on a real cluster we are often/ever in the scenario you are
trying to address.
> Preemption can kill containers to fulfil need of already over-capacity queue.
> -----------------------------------------------------------------------------
>
> Key: YARN-2592
> URL: https://issues.apache.org/jira/browse/YARN-2592
> Project: Hadoop YARN
> Issue Type: Bug
> Affects Versions: 3.0.0, 2.5.1
> Reporter: Eric Payne
>
> There are scenarios in which one over-capacity queue can cause preemption of
> another over-capacity queue. However, since killing containers may lose work,
> it doesn't make sense to me to kill containers to feed an already
> over-capacity queue.
> Consider the following:
> {code}
> root has A,B,C, total capacity = 90
> A.guaranteed = 30, A.pending = 5, A.current = 40
> B.guaranteed = 30, B.pending = 0, B.current = 50
> C.guaranteed = 30, C.pending = 0, C.current = 0
> {code}
> In this case, the queue preemption monitor will kill 5 resources from queue B
> so that queue A can pick them up, even though queue A is already over its
> capacity. This could lose any work that those containers in B had already
> done.
> Is there a use case for this behavior? It seems to me that if a queue is
> already over its capacity, it shouldn't destroy the work of other queues. If
> the over-capacity queue needs more resources, that seems to be a problem that
> should be solved by increasing its guarantee.
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