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

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Message-ID: <20090605095931.GE4601@linux.vnet.ibm.com>
Date:	Fri, 5 Jun 2009 15:29:31 +0530
From:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
To:	Paul Menage <menage@...gle.com>
Cc:	bharata@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	Balbir Singh <balbir@...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>,
	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Herbert Poetzl <herbert@...hfloor.at>
Subject: Re: [RFC] CPU hard limits

On Fri, Jun 05, 2009 at 02:51:18AM -0700, Paul Menage wrote:
> On Fri, Jun 5, 2009 at 2:48 AM, Dhaval Giani<dhaval@...ux.vnet.ibm.com> wrote:
> >> > Now if 11th group with same shares comes in, then each group will now
> >> > get 9% of CPU and that 10% guarantee breaks.
> >>
> >> So you're trying to guarantee 11 cgroups that they can each get 10% of
> >> the CPU? That's called over-committing, and while there's nothing
> >> wrong with doing that if you're confident that they'll not all need
> >> their 10% at the same time, there's no way to *guarantee* them all
> >> 10%. You can guarantee them all 9% and hope the extra 1% is spare for
> >> those that need it (over-committing), or you can guarantee 10 of them
> >> 10% and give the last one 0 shares.
> >>
> >> How would you propose to guarantee 11 cgroups each 10% of the CPU
> >> using hard limits?
> >>
> >
> > You cannot guarantee 10% to 11 groups on any system (unless I am missing
> > something). The sum of guarantees cannot exceed 100%.
> 
> That's exactly my point. I was trying to counter Bharata's statement, which was:
> 
> > > Now if 11th group with same shares comes in, then each group will now
> > > get 9% of CPU and that 10% guarantee breaks.
> 
> which seemed to be implying that this was a drawback of using shares
> to implement guarantees.
> 

OK :). Glad to see I did not get it wrong.

I think we are focusing on the wrong use case here. Guarantees is just a
useful side-effect we get by using hard limits. I think the more
important use case is where the provider wants to limit the amount of
time a user gets (such as in a cloud).

Maybe we should direct our attention in solving that problem? :)

thanks,
-- 
regards,
Dhaval
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