From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965033AbXAMGqW (ORCPT ); Sat, 13 Jan 2007 01:46:22 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S964958AbXAMGqW (ORCPT ); Sat, 13 Jan 2007 01:46:22 -0500 Received: from mail.suse.de ([195.135.220.2]:57124 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965033AbXAMGqW (ORCPT ); Sat, 13 Jan 2007 01:46:22 -0500 Date: Sat, 13 Jan 2007 07:46:18 +0100 From: Nick Piggin To: Alan Cc: Andrew Morton , Ingo Molnar , Linux Kernel Mailing List Subject: Re: [patch] sched: avoid div in rebalance_tick Message-ID: <20070113064618.GA30425@wotan.suse.de> References: <20070112060213.GB28611@wotan.suse.de> <20070112095940.0795a998@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070112095940.0795a998@localhost.localdomain> User-Agent: Mutt/1.5.9i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 12, 2007 at 09:59:40AM +0000, Alan wrote: > On Fri, 12 Jan 2007 07:02:13 +0100 > Nick Piggin wrote: > > > Just noticed this while looking at a bug. > > Avoid an expensive integer divide 3 times per CPU per tick. > > Integer divide is cheap on some modern processors, and multibit shift > isn't on all embedded ones. > > How about putting back scale = 1 and using > > scale += scale; > > instead of the shift and getting what ought to be even better results OK, how about this? It only works out to be around 0.01% of my P3's CPU time at 1000HZ, but it also did make the x86 code 16 bytes smaller. -- Avoid expensive integer divide 3 times per CPU per tick. A userspace test of this loop went from 26ns, down to 19ns on a G5; and from 123ns down to 28ns on a P3. (Also avoid a variable bit shift, as suggested by Alan. The effect of this wasn't noticable on the CPUs I tested with). Signed-off-by: Nick Piggin Index: linux-2.6/kernel/sched.c =================================================================== --- linux-2.6.orig/kernel/sched.c +++ linux-2.6/kernel/sched.c @@ -2887,14 +2887,16 @@ static void active_load_balance(struct r static void update_load(struct rq *this_rq) { unsigned long this_load; - int i, scale; + unsigned int i, scale; this_load = this_rq->raw_weighted_load; /* Update our load: */ - for (i = 0, scale = 1; i < 3; i++, scale <<= 1) { + for (i = 0, scale = 1; i < 3; i++, scale += scale) { unsigned long old_load, new_load; + /* scale is effectively 1 << i now, and >> i divides by scale */ + old_load = this_rq->cpu_load[i]; new_load = this_load; /* @@ -2904,7 +2906,7 @@ static void update_load(struct rq *this_ */ if (new_load > old_load) new_load += scale-1; - this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) / scale; + this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i; } }