Re: [v8 0/4] cgroup-aware OOM killer

Tim Hockin
2017-09-27T18:11:42Z

On Wed, Sep 27, 2017 at 9:23 AM, Roman Gushchin <guro@fb.com> wrote:
> On Wed, Sep 27, 2017 at 08:35:50AM -0700, Tim Hockin wrote:
>> On Wed, Sep 27, 2017 at 12:43 AM, Michal Hocko <mhocko@kernel.org> wrote:
>> > On Tue 26-09-17 20:37:37, Tim Hockin wrote:
>> > [...]
>> >> I feel like David has offered examples here, and many of us at Google
>> >> have offered examples as long ago as 2013 (if I recall) of cases where
>> >> the proposed heuristic is EXACTLY WRONG.
>> >
>> > I do not think we have discussed anything resembling the current
>> > approach. And I would really appreciate some more examples where
>> > decisions based on leaf nodes would be EXACTLY WRONG.
>> >
>> >> We need OOM behavior to kill in a deterministic order configured by
>> >> policy.
>> >
>> > And nobody is objecting to this usecase. I think we can build a priority
>> > policy on top of leaf-based decision as well. The main point we are
>> > trying to sort out here is a reasonable semantic that would work for
>> > most workloads. Sibling based selection will simply not work on those
>> > that have to use deeper hierarchies for organizational purposes. I
>> > haven't heard a counter argument for that example yet.
>>
>
> Hi, Tim!
>
>> We have a priority-based, multi-user cluster. That cluster runs a
>> variety of work, including critical things like search and gmail, as
>> well as non-critical things like batch work. We try to offer our
>> users an SLA around how often they will be killed by factors outside
>> themselves, but we also want to get higher utilization. We know for a
>> fact (data, lots of data) that most jobs have spare memory capacity,
>> set aside for spikes or simply because accurate sizing is hard. We
>> can sell "guaranteed" resources to critical jobs, with a high SLA. We
>> can sell "best effort" resources to non-critical jobs with a low SLA.
>> We achieve much better overall utilization this way.
>
> This is well understood.
>
>>
>> I need to represent the priority of these tasks in a way that gives me
>> a very strong promise that, in case of system OOM, the non-critical
>> jobs will be chosen before the critical jobs. Regardless of size.
>> Regardless of how many non-critical jobs have to die. I'd rather kill
>> *all* of the non-critical jobs than a single critical job. Size of
>> the process or cgroup is simply not a factor, and honestly given 2
>> options of equal priority I'd say age matters more than size.
>>
>> So concretely I have 2 first-level cgroups, one for "guaranteed" and
>> one for "best effort" classes. I always want to kill from "best
>> effort", even if that means killing 100 small cgroups, before touching
>> "guaranteed".
>>
>> I apologize if this is not as thorough as the rest of the thread - I
>> am somewhat out of touch with the guts of it all these days. I just
>> feel compelled to indicate that, as a historical user (via Google
>> systems) and current user (via Kubernetes), some of the assertions
>> being made here do not ring true for our very real use cases. I
>> desperately want cgroup-aware OOM handing, but it has to be
>> policy-based or it is just not useful to us.
>
> A policy-based approach was suggested by Michal at a very beginning of
> this discussion. Although nobody had any strong objections against it,
> we've agreed that this is out of scope of this patchset.
>
> The idea of this patchset is to introduce an ability to select a memcg
> as an OOM victim with the following optional killing of all belonging tasks.
> I believe, it's absolutely mandatory for _any_ further development
> of the OOM killer, which wants to deal with memory cgroups as OOM entities.
>
> If you think that it makes impossible to support some use cases in the future,
> let's discuss it. Otherwise, I'd prefer to finish this part of the work,
> and proceed to the following improvements on top of it.
>
> Thank you!

I am 100% in favor of killing whole groups. We want that too. I just
needed to express disagreement with statements that size-based
decisions could not produce bad results. They can and do.