Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754588AbZFENEV (ORCPT ); Fri, 5 Jun 2009 09:04:21 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752053AbZFENEP (ORCPT ); Fri, 5 Jun 2009 09:04:15 -0400 Received: from mx2.redhat.com ([66.187.237.31]:43310 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751263AbZFENEO (ORCPT ); Fri, 5 Jun 2009 09:04:14 -0400 Message-ID: <4A291753.7090205@redhat.com> Date: Fri, 05 Jun 2009 16:02:11 +0300 From: Avi Kivity User-Agent: Thunderbird 2.0.0.21 (X11/20090320) MIME-Version: 1.0 To: Paul Menage CC: bharata@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, Dhaval Giani , Balbir Singh , Vaidyanathan Srinivasan , Gautham R Shenoy , Srivatsa Vaddagiri , Ingo Molnar , Peter Zijlstra , Pavel Emelyanov , kvm@vger.kernel.org, Linux Containers , Herbert Poetzl Subject: Re: [RFC] CPU hard limits References: <20090604053649.GA3701@in.ibm.com> <6599ad830906050153i1afd104fqe70f681317349142@mail.gmail.com> In-Reply-To: <6599ad830906050153i1afd104fqe70f681317349142@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1785 Lines: 48 Paul Menage wrote: > On Wed, Jun 3, 2009 at 10:36 PM, Bharata B > Rao wrote: > >> - Hard limits can be used to provide guarantees. >> >> > > This claim (and the subsequent long thread it generated on how limits > can provide guarantees) confused me a bit. > > Why do we need limits to provide guarantees when we can already > provide guarantees via shares? > > 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. > > Suppose cgroup A wants a guarantee of 50% and two others, B and C, > want guarantees of 15% each; give A 50 shares and B and C 15 shares > each. In this case, if they all run flat out they'll get 62%/19%/19%, > which is within their SLA. > > That's not to say that hard limits can't be useful in their own right > - e.g. for providing reproducible loadtesting conditions by > controlling how much CPU a service can use during the load test. But I > don't see why using them to implement guarantees is either necessary > or desirable. > > (Unless I'm missing some crucial point ...) > How many shares does a cgroup with a 0% guarantee get? Ideally, the scheduler would hand out cpu time according to weight and demand, then clamp over-demand by a cgroup's limit and boost the share to meet guarantees. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/