From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2992524AbXDRUMl (ORCPT ); Wed, 18 Apr 2007 16:12:41 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S2992513AbXDRUMl (ORCPT ); Wed, 18 Apr 2007 16:12:41 -0400 Received: from smtp.osdl.org ([65.172.181.24]:44963 "EHLO smtp.osdl.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2992506AbXDRUMj (ORCPT ); Wed, 18 Apr 2007 16:12:39 -0400 Date: Wed, 18 Apr 2007 13:11:12 -0700 (PDT) From: Linus Torvalds To: Davide Libenzi cc: Ingo Molnar , Matt Mackall , Nick Piggin , William Lee Irwin III , Peter Williams , Mike Galbraith , Con Kolivas , ck list , Bill Huey , Linux Kernel Mailing List , Andrew Morton , Arjan van de Ven , Thomas Gleixner Subject: Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS] In-Reply-To: Message-ID: References: <20070417062621.GL2986@holomorphy.com> <20070417070155.GF1057@wotan.suse.de> <20070417213954.GE11166@waste.org> <20070418031511.GA18452@wotan.suse.de> <20070418043831.GR11115@waste.org> <20070418050024.GF18452@wotan.suse.de> <20070418055525.GS11115@waste.org> <20070418152355.GU11115@waste.org> <20070418174945.GA7930@elte.hu> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 18 Apr 2007, Davide Libenzi wrote: > > "Perhaps on the rare occasion pursuing the right course demands an act of > unfairness, unfairness itself can be the right course?" I don't think that's the right issue. It's just that "fairness" != "equal". Do you think it "fair" to pay everybody the same regardless of how good a job they do? I don't think anybody really believes that. Equating "fair" and "equal" is simply a very fundamental mistake. They're not the same thing. Never have been, and never will. Now, there's no question that "equal" is much easier to implement, if only because it's a lot easier to agree what it means. "Equal parts" is somethign everybody can agree on. "Fair parts" automatically involves a balancing act, and people will invariably count things differently and thus disagree about what is "fair" and what is not. I don't think we can ever get a "perfect" setup for that reason, but I think we can get something that at least gets reasonably close, at least for the obvious cases. So my suggested test-case of running one process as one user and two processes as another one has a fairly "obviously correct" solution if you have just one CPU's, and you can probably be pretty fair in practice on two CPU's (there's an obvious theoretical solution, whether you can get there with a practical algorithm is another thing). On three or more CPU's, you obviously wouldn't even *want* to be fair, since you can very naturally just give a CPU to each.. Linus