From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262176AbULLXth (ORCPT ); Sun, 12 Dec 2004 18:49:37 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262178AbULLXtd (ORCPT ); Sun, 12 Dec 2004 18:49:33 -0500 Received: from gprs214-177.eurotel.cz ([160.218.214.177]:13189 "EHLO amd.ucw.cz") by vger.kernel.org with ESMTP id S262175AbULLXtV (ORCPT ); Sun, 12 Dec 2004 18:49:21 -0500 Date: Mon, 13 Dec 2004 00:42:56 +0100 From: Pavel Machek To: Con Kolivas Cc: Andrea Arcangeli , linux-kernel@vger.kernel.org Subject: Re: dynamic-hz Message-ID: <20041212234256.GK6272@elf.ucw.cz> References: <20041211142317.GF16322@dualathlon.random> <20041212163547.GB6286@elf.ucw.cz> <20041212222312.GN16322@dualathlon.random> <41BCD5F3.80401@kolivas.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <41BCD5F3.80401@kolivas.org> X-Warning: Reading this can be dangerous to your mental health. User-Agent: Mutt/1.5.6+20040722i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Hi! > >The overhead is a single l1 cacheline in the paths manipulating HZ > >(rather than having an immediate value hardcoded in the asm, it reads it > >from a memory location not in the icache). Plus there are some > >conversion routines in the USER_HZ usages. It's not a measurable > >difference. > > Just being devils advocate here... > > I had variable Hz in my tree for a while and found there was one > solitary purpose to setting Hz to 100; to silence cheap capacitors. > > The rest of my users that were setting Hz to 100 for so-called > performance gains were doing so under the false impression that cpu > usage was lower simply because of the woefully inaccurate cpu usage > calcuation at 100Hz. > > The performance benefit, if any, is often lost in noise during > benchmarks and when there, is less than 1%. So I was wondering if you > had some specific advantage in mind for this patch? Is there some > arch-specific advantage? I can certainly envision disadvantages to lower Hz. Actually, I measured about 1W power savings with HZ=100. That's about as much as spindown of disk saves... Pavel -- People were complaining that M$ turns users into beta-testers... ...jr ghea gurz vagb qrirybcref, naq gurl frrz gb yvxr vg gung jnl!