From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dhaval Giani Subject: Re: [RFC] CPU hard limits Date: Fri, 5 Jun 2009 15:18:11 +0530 Message-ID: <20090605094811.GD4601@linux.vnet.ibm.com> References: <20090604053649.GA3701@in.ibm.com> <6599ad830906050153i1afd104fqe70f681317349142@mail.gmail.com> <20090605092733.GA27486@in.ibm.com> <6599ad830906050232n11aa30d8xfcda0a279a482f32@mail.gmail.com> Reply-To: Dhaval Giani Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: bharata@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, Balbir Singh , Vaidyanathan Srinivasan , Gautham R Shenoy , Srivatsa Vaddagiri , Ingo Molnar , Peter Zijlstra , Pavel Emelyanov , Avi Kivity , kvm@vger.kernel.org, Linux Containers , Herbert Poetzl To: Paul Menage Return-path: Received: from e28smtp09.in.ibm.com ([59.145.155.9]:47045 "EHLO e28smtp09.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750951AbZFEJsX (ORCPT ); Fri, 5 Jun 2009 05:48:23 -0400 Content-Disposition: inline In-Reply-To: <6599ad830906050232n11aa30d8xfcda0a279a482f32@mail.gmail.com> Sender: kvm-owner@vger.kernel.org List-ID: On Fri, Jun 05, 2009 at 02:32:51AM -0700, Paul Menage wrote: > On Fri, Jun 5, 2009 at 2:27 AM, Bharata B Rao wrote: > >> > >> Suppose 10 cgroups each want 10% of the machine's CPU. We can just > >> give each cgroup an equal share, and they're guaranteed 10% if they > >> try to use it; if they don't use it, other cgroups can get access to > >> the idle cycles. > > > > 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 al 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%. How would you be able to do that with any other mechanism? Thanks, -- regards, Dhaval