From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030740AbWKOR2V (ORCPT ); Wed, 15 Nov 2006 12:28:21 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1030745AbWKOR2V (ORCPT ); Wed, 15 Nov 2006 12:28:21 -0500 Received: from gw.goop.org ([64.81.55.164]:3805 "EHLO mail.goop.org") by vger.kernel.org with ESMTP id S1030743AbWKOR2Q (ORCPT ); Wed, 15 Nov 2006 12:28:16 -0500 Message-ID: <455B4E2F.7040408@goop.org> Date: Wed, 15 Nov 2006 09:28:15 -0800 From: Jeremy Fitzhardinge User-Agent: Thunderbird 1.5.0.8 (X11/20061107) MIME-Version: 1.0 To: Ingo Molnar CC: Andi Kleen , Eric Dumazet , akpm@osdl.org, Arjan van de Ven , linux-kernel@vger.kernel.org Subject: Re: [PATCH] i386-pda UP optimization References: <1158046540.2992.5.camel@laptopd505.fenrus.org> <1158047806.2992.7.camel@laptopd505.fenrus.org> <200611151227.04777.dada1@cosmosbay.com> <200611151232.31937.ak@suse.de> <20061115172003.GA20403@elte.hu> In-Reply-To: <20061115172003.GA20403@elte.hu> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Ingo Molnar wrote: > Eric's test shows a 5% slowdown. That's far from cheap. > It seems like an absurdly large difference. PDA references aren't all that common in the kernel; for the %gs prefix on PDA accesses to be causing a 5% overall difference in a test like this means that the prefixes would have to be costing hundreds or thousands of cycles, which seems absurd. Particularly since Eric's patch doesn't touch head.S, so the %gs save/restore is still being executed. Are we sure this isn't a cache layout issue? Eric, did you try evicting your executable from pagecache between runs to see if you get variation depending on what physical pages it gets put into? (Making several copies of the executable should have the same effect.) J