Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934816AbZDBC0k (ORCPT ); Wed, 1 Apr 2009 22:26:40 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752909AbZDBC0b (ORCPT ); Wed, 1 Apr 2009 22:26:31 -0400 Received: from smtp.ultrahosting.com ([74.213.174.254]:34971 "EHLO smtp.ultrahosting.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751436AbZDBC0a (ORCPT ); Wed, 1 Apr 2009 22:26:30 -0400 Date: Wed, 1 Apr 2009 22:21:57 -0400 (EDT) From: Christoph Lameter X-X-Sender: cl@qirst.com To: Ingo Molnar cc: Matthew Wilcox , Linus Torvalds , Tejun Heo , Martin Schwidefsky , rusty@rustcorp.com.au, tglx@linutronix.de, x86@kernel.org, linux-kernel@vger.kernel.org, hpa@zytor.com, Paul Mundt , rmk@arm.linux.org.uk, starvik@axis.com, ralf@linux-mips.org, davem@davemloft.net, cooloney@kernel.org, kyle@mcmartin.ca, grundler@parisc-linux.org, takata@linux-m32r.org, benh@kernel.crashing.org, rth@twiddle.net, ink@jurassic.park.msu.ru, heiko.carstens@de.ibm.com, Nick Piggin , Peter Zijlstra Subject: Re: [PATCH UPDATED] percpu: use dynamic percpu allocator as the default percpu allocator In-Reply-To: <20090402021024.GA26446@elte.hu> Message-ID: References: <49D2B209.9060000@kernel.org> <20090401154913.GA31435@elte.hu> <20090401190113.GA734@elte.hu> <20090401223241.GA28168@elte.hu> <20090401225754.GN8014@parisc-linux.org> <20090402021024.GA26446@elte.hu> User-Agent: Alpine 1.10 (DEB 962 2008-03-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1284 Lines: 28 On Thu, 2 Apr 2009, Ingo Molnar wrote: > Note that my argument was different though: that assumptions about > variable correlation are very hard to track and validate, and that > IMHO we should be using __read_mostly generously (we know _that_ > attribute with a rather high likelyhood), and we should group the > remaining variables together, starting at a cacheline aligned > address. But then you decrease the density of accessed to the __read_mostly sections. The cachelines are not hot in the caches anymore which is an average performance reduction. > A sub-sub argument was that perhaps we should not split .data and > .bss variables into separate sections - it doubles the chance of > false cacheline sharing and spreads the cacheline footprint. False cacheline sharing is something normal that comes with the cpu caching schemes. As long as there is no significant impact on performance we are fine with it. Extensive measures to avoid false cacheline sharing on unimportant variables increases the cache footprint of code. -- 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/