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Date: Tue, 17 Apr 2012 08:13:52 -0700
From: Tejun Heo <tj@kernel.org>
To: Glauber Costa <glommer@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
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Subject: Re: Cgroup in a single hierarchy (Was: Re: [RFD] Merge task
 counter into memcg)
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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