From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Subject: Re: [RFD] cgroup: about multiple hierarchies
Date: Fri, 24 Feb 2012 12:33:16 +0100
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To: Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Kay Sievers <kay.sievers-tD+1rO4QERM@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, Lennart Poettering <lennart-mdGvqq1h2p+GdvJs77BJ7Q@public.gmane.org>, Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>

On Thu, 2012-02-23 at 16:38 -0500, Vivek Goyal wrote:
> > > Again, it does not mean I am advocating flat hiearchy. I am just wondering
> > > in case of fully nested hierarchies (task at same level as groups), how
> > > does one explain it to a layman user who understands things in terms of
> > > % of resources. 
> > 
> > If your complete control is % based then I would assume its a % of a %.
> > Simple enough.
> 
> But % of % will vary dynamically and not be static. So if root has got
> 100% of resources and we want 25% of that for a group, then hierarchy
> might look as follows.
> 
>                                 root
>                                 / | \
>                                T1 T2 g1
> 
> T1, T2 are tasks and g1 is the group needing 25% of root's resources. Now
> number of tasks running in parallel to g1 will determine its effective %
> and tasks come and go. So the only way to do this would be that move T1
> and T2 in a child group under root and make sure new tasks don't show up
> in root. 

Which is exactly that the scheduler stuff does.. so tough luck for the
sysad who can't grasp it.

> Otherwise creating a group under root does not ensure that you get minimum
> % of resource. It just makes sure that you can't get more than 25% of
> % resources when things are tight. 

You never said anything about minimum resource guarantees in the initial
problem statement.