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SubjectRe: Cgroup in a single hierarchy (Was: Re: [RFD] Merge task counter into memcg)
Hello, Glauber.

On Thu, Apr 12, 2012 at 02:04:24PM -0300, Glauber Costa wrote:
> 1) Right now, the controllers work independently, and that code will
> have to live for at least some years anyway. So leave it there.
>
> 2) But also, insert optimization code that can be enabled/disabled
> when companion cgroups are in the same hierarchy.
>
> 3) After we mount the cgroup, apply those optimization to all of
> them from the cgroup core (the current bind stuff is just way to
> weird for that, IMHO)
>
> 4) Then we start telling userspace people to favor co-mounts as much
> as they can
>
> 5) Pray.

Pretty similar to the plan that I was thinking about.

* Provide both mechanisms from the kernel while implementing new
features / optimizations with the assumption that there's one
hierarchy.

* Make the switch to single hierarchy from userland, probably by
implementing a (policy based) userland thing which takes over the
whole cgroup hierarchy.

* Phase out multiple hierarchy support from kernel slowly.

That said, there are quite a few obstacles including being able to
support most (probably not all) use cases possible under multiple
hierarchies and managing the added complexity over the transition
period. I don't think it's gonna be easy. Needs more thinking.

Thanks.

--
tejun


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