linux-kernel - Re: [RFD] cgroup: about multiple hierarchies

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Message-ID: <20120228223109.GL9920@redhat.com>
Date:	Tue, 28 Feb 2012 17:31:09 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tejun Heo <tj@...nel.org>, Li Zefan <lizf@...fujitsu.com>,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kay Sievers <kay.sievers@...y.org>,
	Lennart Poettering <lennart@...ttering.net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@...radead.org>
Subject: Re: [RFD] cgroup: about multiple hierarchies

On Tue, Feb 28, 2012 at 11:00:21PM +0100, Peter Zijlstra wrote:
> On Tue, 2012-02-28 at 16:54 -0500, Vivek Goyal wrote:
> > On Tue, Feb 28, 2012 at 10:43:54PM +0100, Peter Zijlstra wrote:
> > > On Tue, 2012-02-28 at 16:35 -0500, Vivek Goyal wrote:
> > > > For
> > > > cpu controller, it is priority at the group level no fixed minimum/maximum
> > > > % shares. And that's a limitation of treating task and group at same level.
> > > 
> > > Depends on what you mean by min/max %, you can do it on the group level
> > > by using bandwidth caps (for max) or inverted (max on everybody else,
> > > for min).
> > 
> > I was referring to using pure proportional controller. max bandwidth is
> > new and I am looking for a quick documentation file which describes
> > what are the knobs and how to use it. Did not find any in
> > Documentation/cgroups/. Is there any documentation available?
> 
> Its written in C, its at kernel/sched/fair.c ;-)

/me does not know enough of scheduler code to parse it fast. But now I found
Documentation/scheduler/sched-bwc.txt to explain what max bandwidth
control is.

> 
> > I am assuming that max are being specified for groups in some absolute
> > quantity. That is fine. It will not still be max %, as again for % you
> > need fixed number of entities at any level and that's not the case with
> > tasks.
> > 
> > Minimum for one group (max for everyone else) will also only work if 
> > task and groups are not at same level.
> 
> I'm really not seeing this.

Assume I have following hierarchy.

				 root
			       /  |  \
			   Tasks  G1  G2

Assume I have given upper limit of 20% to G1. (period=50ms, quota=10ms), 
then it still does not gurantee the minimum bandwidth for G2. If I 
upper limit G2 also, then I am assuming it just gives minimum gurantee
for any tasks in root. (Right now I am assuming just 1 cpu in the system
for simplicity).

So to get a minimum % bandwdith gurantee for G2, I shall have to move
"Tasks" in a children group. Now both upper limit/proportional weight 
on G3 will help to determine minimum % share of G2.

				 root
			       /  |  \
			      G3  G1  G2
			      |
			     Tasks

Please correct me if I have understood the whole thing wrong.

Thanks
Vivek
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