Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755150AbYCJGaL (ORCPT ); Mon, 10 Mar 2008 02:30:11 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752165AbYCJG37 (ORCPT ); Mon, 10 Mar 2008 02:29:59 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:59887 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751165AbYCJG36 (ORCPT ); Mon, 10 Mar 2008 02:29:58 -0400 Date: Mon, 10 Mar 2008 07:29:45 +0100 From: Ingo Molnar To: Alexander van Heukelum Cc: Alexander van Heukelum , Thomas Gleixner , "H. Peter Anvin" , LKML Subject: Re: [PATCH] x86: Change x86 to use generic find_next_bit Message-ID: <20080310062944.GA31542@elte.hu> References: <20080309200103.GA895@mailshack.com> <20080309201016.GA28454@elte.hu> <1205097195.13205.1241421773@webmail.messagingengine.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1205097195.13205.1241421773@webmail.messagingengine.com> User-Agent: Mutt/1.5.17 (2007-11-01) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0002] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1051 Lines: 28 * Alexander van Heukelum wrote: > > ok, that's rather convincing. > > > > the generic version in lib/find_next_bit.c is open-coded C which gcc > > can optimize pretty nicely. > > > > the hand-coded assembly versions in arch/x86/lib/bitops_32.c mostly > > use the special x86 'bit search forward' (BSF) instruction - which i > > know from the days when the scheduler relied on it has some > > non-trivial setup costs. So especially when there's _small_ bitmasks > > involved, it's more expensive. > > Hi, > > BSF is fine, it doesn't need any special setup. [...] under "setup costs" i mean cycles spent by the CPU itself - the instruction itself is simple (of course) and needs no setup. If you look at BSF performance you'll see that it has nontrivial overhead. Ingo -- 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/