linux-kernel - Re: [RFC v2 PATCH 0/8] CFS Hard limits

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Message-ID: <20090930223039.GO484@MAIL.13thfloor.at>
Date:	Thu, 1 Oct 2009 00:30:39 +0200
From:	Herbert Poetzl <herbert@...hfloor.at>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	Pavel Emelyanov <xemul@...nvz.org>, bharata@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Paul Menage <menage@...gle.com>,
	Mike Waychison <mikew@...gle.com>
Subject: Re: [RFC v2 PATCH 0/8] CFS Hard limits - v2

On Wed, Sep 30, 2009 at 09:00:53PM +0530, Balbir Singh wrote:
> * Pavel Emelyanov <xemul@...nvz.org> [2009-09-30 19:10:27]:
> > Balbir Singh wrote:
> > > * Pavel Emelyanov <xemul@...nvz.org> [2009-09-30 17:36:29]:
> > >> Bharata B Rao wrote:
> > >>> Hi,
> > >>>
> > >>> Here is the v2 post of hard limits feature for CFS group scheduler. This
> > >>> RFC post mainly adds runtime borrowing feature and has a new locking scheme
> > >>> to protect CFS runtime related fields.
> > >>>
> > >>> It would be nice to have some comments on this set!
> > >> I have a question I'd like to ask before diving into the code.
> > >> Consider I'm a user, that has a 4CPUs box 2GHz each and I'd like
> > >> to create a container with 2CPUs 1GHz each. Can I achieve this
> > >> after your patches?
> > > 
> > > I don't think the GHz makes any sense, consider CPUs with frequency
> > > scaling. If I can scale from 1.6GHz to say 2.6GHz or 2GHz to 4GHz,
> > > what does it mean for hard limit control? Hard limits define control
> > > over existing bandwidth, anything else would be superficial and hard
> > > hard to get right for both developers and users.
> > 
> > Two numbers for configuring limits make even less sense OTOH ;)
> > By assigning 2GHz for 4GHz CPU I obviously want half of its power ;)
> > Please, see my reply to vatsa@ in this thread.

> But it makes life more difficult for the administrator to think in
> terms of GHz -- no? Specifically with different heterogeneous systems.
> I think it would be chaotic in a data center to configure GHz for
> every partition. Not to say that it makes it even more confusing when
> running on top of KVM. Lets say I create two vCPUs and I specifiy GHz
> outside, do I expect to see it in /proc/cpuinfo?

> I'd like to hear what others think about GHz as well.

for me, using something like GHz or even BogoMips would
not make any sense whatsoever ... I would be able to
understand percentage, but this has some other implications
as it does not handle granularity ....

Linux-VServer uses interval and rate, which is very similar
to your setup, except that we use two pairs to achive
a differentiation for 'busy' and 'idle' cases, i.e. when
the cpu(s) would go idle, we switch from R1/I1 to R2/I2
for all guests, allowing to distribute the excess differently
than the 'active' part ... but only one set is needed for
hard limits ...

best,
Herbert

> -- 
> 	Balbir
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