linux-kernel - Re: [RFC] CPU hard limits

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Message-ID: <20090605092455.GG11755@balbir.in.ibm.com>
Date:	Fri, 5 Jun 2009 17:24:55 +0800
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
Cc:	Avi Kivity <avi@...hat.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>,
	Pavel Emelyanov <xemul@...nvz.org>, kvm@...r.kernel.org,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Herbert Poetzl <herbert@...hfloor.at>
Subject: Re: [RFC] CPU hard limits

* Bharata B Rao <bharata@...ux.vnet.ibm.com> [2009-06-05 11:01:59]:

> On Fri, Jun 05, 2009 at 01:27:55PM +0800, Balbir Singh wrote:
> > * Avi Kivity <avi@...hat.com> [2009-06-05 08:21:43]:
> > 
> > > Balbir Singh wrote:
> > >>> But then there is no other way to make a *guarantee*, guarantees come
> > >>> at a cost of idling resources, no? Can you show me any other
> > >>> combination that will provide the guarantee and without idling the
> > >>> system for the specified guarantees?
> > >>>     
> > >>
> > >> OK, I see part of your concern, but I think we could do some
> > >> optimizations during design. For example if all groups have reached
> > >> their hard-limit and the system is idle, should we do start a new hard
> > >> limit interval and restart, so that idleness can be removed. Would
> > >> that be an acceptable design point?
> > >
> > > I think so.  Given guarantees G1..Gn (0 <= Gi <= 1; sum(Gi) <= 1), and a  
> > > cpu hog running in each group, how would the algorithm divide resources?
> > >
> > 
> > As per the matrix calculation, but as soon as we reach an idle point,
> > we redistribute the b/w and start a new quantum so to speak, where all
> > groups are charged up to their hard limits.
> 
> But could there be client models where you are required to strictly
> adhere to the limit within the bandwidth and not provide more (by advancing
> the bandwidth period) in the presence of idle cycles ?
>

Good point, I think so, so I think there is should be a good default
and configurable for the other case. 

-- 
	Balbir
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