Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754499AbYCCOQH (ORCPT ); Mon, 3 Mar 2008 09:16:07 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752448AbYCCOPy (ORCPT ); Mon, 3 Mar 2008 09:15:54 -0500 Received: from smtp25.orange.fr ([193.252.22.23]:28675 "EHLO smtp25.orange.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752097AbYCCOPx convert rfc822-to-8bit (ORCPT ); Mon, 3 Mar 2008 09:15:53 -0500 X-ME-UUID: 20080303141550287.463B01C000B5@mwinf2557.orange.fr Message-ID: <47CC080E.9010203@cosmosbay.com> Date: Mon, 03 Mar 2008 15:15:42 +0100 From: Eric Dumazet User-Agent: Thunderbird 1.5.0.14 (Windows/20071210) MIME-Version: 1.0 To: Pekka Enberg Cc: Nick Piggin , netdev@vger.kernel.org, Linux Kernel Mailing List , yanmin_zhang@linux.intel.com, David Miller , Christoph Lameter Subject: Re: [rfc][patch 3/3] use SLAB_ALIGN_SMP References: <20080303093449.GA15091@wotan.suse.de> <20080303093624.GC15091@wotan.suse.de> <47CBCAB0.2040604@cosmosbay.com> <20080303124142.GB13138@wotan.suse.de> <47CBF683.10201@cosmosbay.com> <20080303134622.GD13138@wotan.suse.de> <84144f020803030553s35a40dd8yf88585ccd5a599fd@mail.gmail.com> In-Reply-To: <84144f020803030553s35a40dd8yf88585ccd5a599fd@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1548 Lines: 41 Pekka Enberg a ?crit : > Hi, > > On Mon, Mar 3, 2008 at 3:46 PM, Nick Piggin wrote: > >> > Maybe we need to use three flags to separate the meanings ? >> > >> > SLAB_HINT_SMP_ALIGN >> > SLAB_HINT_HWCACHE_ALIGN >> > SLAB_HWCACHE_ALIGN /* strong requirement that two objects dont share a >> > cache line */ >> >> Possibly, but I'm beginning to prefer that strong requirements should >> request the explicit alignment (they can even use cache_line_size() after >> Pekka's patch to make it generic). I don't like how the name implies >> that you get a guarantee, however I guess in practice people are using it >> more as a hint (or because they vaguely hope it makes their code run >> faster :)) >> > > At least historically SLAB_HWCACHE_ALIGN has been just a hint, > although slab tries very hard to satisfy it (see the comments in > mm/slab.c). Why do we need stronger guarantees than that, btw? > > This reminds me a previous attempt of removing SLAB_HWCACHE_ALIGN http://kernel.org/pub/linux/kernel/people/christoph/patch-archive/2007/2.6.21-rc6/remove_hwcache_align At that time Christoph didnt took into account the CONFIG_SMP thing (false sharing avoidance), but also that L1_CACHE_SIZE is a compile constant, that can differs with cache_line_size() -- 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/