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

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Message-ID: <20090930153053.GM3071@balbir.in.ibm.com>
Date:	Wed, 30 Sep 2009 21:00:53 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Pavel Emelyanov <xemul@...nvz.org>
Cc:	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>,
	Herbert Poetzl <herbert@...hfloor.at>,
	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

* 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.

-- 
	Balbir
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