Re: [RFD] cgroup: about multiple hierarchies

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Feb 28, 2012 at 10:53:59PM +0100, Peter Zijlstra wrote:
> On Tue, 2012-02-28 at 16:35 -0500, Vivek Goyal wrote:
> > Yes this is how scheduler does to handle hierarchy. Treat task and group
> > at same level. 
> 
> ...
> 
> > Whether it is a good thing or bad thing, I don't know. 
> 
> That's IMO what the cgroupfs interface provides for, if you do anything
> different there's this shadow group that contains the tasks for which
> you then have to provide extra parameter control.
> 
> Furthermore, by treating tasks and groups at the same level you can
> create the extra group, but you can't do the reverse. So its the more
> versatile solution as well.

Agreed that it is more versatile. And one can move all the tasks to a
new group to achieve what a shadow group will do.

The only thing is what is a good default. If we are thinking of dividing
resources in terms of % and writing a user space tool, then in default
model we just don't know what's the %. May be it is dynamically varying
% and should be shown accordingly.

Or if idea of minimum % proportional bandwidth is more natural, then 
we shall have to change userspace and things like systemd to not run
any task in /. Then a user space tool can go through cgroup hierarchy
and calculate minimum % share of a group and display it.

> 
> > I think previous
> > design was allocating a group for every user. I guess, in that case we
> > will have fixed % share of each user (until and unless users are created/
> > removed). 
> 
> Not even, it depended on if the user had anything runnable or not. It
> was very much like the current cgroup stuff if you create a cgroup for
> each user and stick the tasks in.
> 
> The cpu-cgroup stuff is purely runnable based, so every wakeup/sleep
> changes the entire weight distribution, yay! :-)

:-). That's fine. If a group is not using its bandwidth because there is
no runnable task, then other groups get more cpu. I thought that's the 
proportional definition.

Thanks
Vivek