Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755502AbZFEIxb (ORCPT ); Fri, 5 Jun 2009 04:53:31 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751937AbZFEIxV (ORCPT ); Fri, 5 Jun 2009 04:53:21 -0400 Received: from smtp-out.google.com ([216.239.33.17]:60377 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751433AbZFEIxU (ORCPT ); Fri, 5 Jun 2009 04:53:20 -0400 DomainKey-Signature: a=rsa-sha1; s=beta; d=google.com; c=nofws; q=dns; h=mime-version:in-reply-to:references:date:message-id:subject:from:to: cc:content-type:content-transfer-encoding:x-system-of-record; b=J/vBWJYr2RRk/64vgfyWMGxn+UpMUa+k11gvHscYjU4fnPS3hfl0kHYXClzY2HeMd Qoig7U7gIi2vDKZ8EMSnw== MIME-Version: 1.0 In-Reply-To: <20090604053649.GA3701@in.ibm.com> References: <20090604053649.GA3701@in.ibm.com> Date: Fri, 5 Jun 2009 01:53:15 -0700 Message-ID: <6599ad830906050153i1afd104fqe70f681317349142@mail.gmail.com> Subject: Re: [RFC] CPU hard limits From: Paul Menage To: bharata@linux.vnet.ibm.com Cc: linux-kernel@vger.kernel.org, Dhaval Giani , 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 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1381 Lines: 35 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 ...) Paul -- 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/