Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750761AbVJJMnG (ORCPT ); Mon, 10 Oct 2005 08:43:06 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750765AbVJJMnG (ORCPT ); Mon, 10 Oct 2005 08:43:06 -0400 Received: from scrub.xs4all.nl ([194.109.195.176]:13793 "EHLO scrub.xs4all.nl") by vger.kernel.org with ESMTP id S1750761AbVJJMnF (ORCPT ); Mon, 10 Oct 2005 08:43:05 -0400 Date: Mon, 10 Oct 2005 14:42:43 +0200 (CEST) From: Roman Zippel X-X-Sender: roman@scrub.home To: Ingo Molnar cc: tglx@linutronix.de, linux-kernel@vger.kernel.org, Andrew Morton , george@mvista.com, johnstul@us.ibm.com, paulmck@us.ibm.com, Christoph Hellwig , oleg@tv-sign.ru, tim.bird@am.sony.com Subject: Re: [PATCH] ktimers subsystem 2.6.14-rc2-kt5 In-Reply-To: <20051001112233.GA18462@elte.hu> Message-ID: References: <20050928224419.1.patchmail@tglx.tec.linutronix.de> <20051001112233.GA18462@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 Content-Length: 2039 Lines: 44 Hi, On Sat, 1 Oct 2005, Ingo Molnar wrote: > > Do you have any numbers (besides maybe microbenchmarks) that show a > > real advantage by using per cpu data? What kind of usage do you expect > > here? > > it has countless advantages, and these days we basically only design > per-CPU data structures within the kernel, unless some limitation (such > as API or hw property) forces us to do otherwise. So i turn around the > question: what would be your reason for _not_ doing this clean per-CPU > design for SMP systems? Did I say I'm against it? No, I was just hoping someone put some more thought into it than just "all the other kids are doing it". I was just curious how well it really scales compared to the simple version, e.g. what happens if most timer end up on a single cpu or what happens if we want to start the timer on a different cpu. Is this so wrong that you have to go into attack mode? :( > > The other thing is that this assumes, that all time sources are > > programmable per cpu, otherwise it will be more complicated for a time > > source to run the timers for every cpu, I don't know how safe that > > assumption is. Changing the array of structures into an array of > > pointers to the structures would allow to switch between percpu bases > > and a single base. > > yeah, and that's an assumption that simplifies things on SMP > significantly. PIT on SMP systems for HRT is so gross that it's not > funny. If anyone wants to revive that notion, please do a separate patch > and make the case convincing enough ... Why do use "PIT on SMP" as an extreme example to reject the general concept completely? This doesn't explain, why first such a (simple) SMP design shouldn't exist and why secondly my suggestion is such a big problem. bye, Roman - 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/