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

Roman Gushchin
2017-09-20T22:24:03Z

On Tue, Sep 19, 2017 at 01:54:48PM -0700, David Rientjes wrote:
> On Fri, 15 Sep 2017, Roman Gushchin wrote:
>
> > > > > But then you just enforce a structural restriction on your configuration
> > > > > because
> > > > > root
> > > > > / \
> > > > > A D
> > > > > /\
> > > > > B C
> > > > >
> > > > > is a different thing than
> > > > > root
> > > > > / | \
> > > > > B C D
> > > > >
> > > >
> > > > I actually don't have a strong argument against an approach to select
> > > > largest leaf or kill-all-set memcg. I think, in practice there will be
> > > > no much difference.
> > > >
> > > > The only real concern I have is that then we have to do the same with
> > > > oom_priorities (select largest priority tree-wide), and this will limit
> > > > an ability to enforce the priority by parent cgroup.
> > > >
> > >
> > > Yes, oom_priority cannot select the largest priority tree-wide for exactly
> > > that reason. We need the ability to control from which subtree the kill
> > > occurs in ancestor cgroups. If multiple jobs are allocated their own
> > > cgroups and they can own memory.oom_priority for their own subcontainers,
> > > this becomes quite powerful so they can define their own oom priorities.
> > > Otherwise, they can easily override the oom priorities of other cgroups.
> >
> > I believe, it's a solvable problem: we can require CAP_SYS_RESOURCE to set
> > the oom_priority below parent's value, or something like this.
> >
> > But it looks more complex, and I'm not sure there are real examples,
> > when we have to compare memcgs, which are on different levels
> > (or in different subtrees).
> >
>
> It's actually much more complex because in our environment we'd need an
> "activity manager" with CAP_SYS_RESOURCE to control oom priorities of user
> subcontainers when today it need only be concerned with top-level memory
> cgroups. Users can create their own hierarchies with their own oom
> priorities at will, it doesn't alter the selection heuristic for another
> other user running on the same system and gives them full control over the
> selection in their own subtree. We shouldn't need to have a system-wide
> daemon with CAP_SYS_RESOURCE be required to manage subcontainers when
> nothing else requires it. I believe it's also much easier to document:
> oom_priority is considered for all sibling cgroups at each level of the
> hierarchy and the cgroup with the lowest priority value gets iterated.

I do agree actually. System-wide OOM priorities make no sense.

Always compare sibling cgroups, either by priority or size, seems to be
simple, clear and powerful enough for all reasonable use cases. Am I right,
that it's exactly what you've used internally? This is a perfect confirmation,
I believe.

Thanks!