From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261407AbVGLNaX (ORCPT ); Tue, 12 Jul 2005 09:30:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261419AbVGLNaX (ORCPT ); Tue, 12 Jul 2005 09:30:23 -0400 Received: from styx.suse.cz ([82.119.242.94]:43427 "EHLO mail.suse.cz") by vger.kernel.org with ESMTP id S261407AbVGLNaU (ORCPT ); Tue, 12 Jul 2005 09:30:20 -0400 Date: Tue, 12 Jul 2005 15:30:18 +0200 From: Vojtech Pavlik To: Con Kolivas Cc: linux-kernel@vger.kernel.org, George Anzinger , "Martin J. Bligh" , Lee Revell , Diego Calleja , azarah@nosferatu.za.org, akpm@osdl.org, cw@f00f.org, torvalds@osdl.org, christoph@lameter.org Subject: Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt Message-ID: <20050712133018.GA8467@ucw.cz> References: <200506231828.j5NISlCe020350@hera.kernel.org> <42D310ED.2000407@mvista.com> <20050712121008.GA7804@ucw.cz> <200507122239.03559.kernel@kolivas.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200507122239.03559.kernel@kolivas.org> User-Agent: Mutt/1.5.6i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 12, 2005 at 10:39:00PM +1000, Con Kolivas wrote: > On Tue, 12 Jul 2005 22:10, Vojtech Pavlik wrote: > > The PIT crystal runs at 14.3181818 MHz (CGA dotclock, found on ISA, ...) > > and is divided by 12 to get PIT tick rate > > > > 14.3181818 MHz / 12 = 1193182 Hz > > > > The reality is that the crystal is usually off by 50-100 ppm from the > > standard value, depending on temperature. > > > > HZ ticks/jiffie 1 second error (ppm) > > --------------------------------------------------- > > 100 11932 1.000015238 15.2 > > 200 5966 1.000015238 15.2 > > 250 4773 1.000057143 57.1 > > 300 3977 0.999931429 -68.6 > > 333 3583 0.999964114 -35.9 > > 500 2386 0.999847619 -152.4 > > 1000 1193 0.999847619 -152.4 > > > > Some HZ values indeed fit the tick frequency better than others, up to > > 333 the error is lost in the physical error of the crystal, for 500 and > > 1000, it definitely is larger, and thus noticeable. > > > > Some (less round and nice) values of HZ would fit even better: > > > > HZ ticks/jiffie 1 second error (ppm) > > --------------------------------------------------- > > 82 14551 1.000000152 0.2 > > > Most interesting... Would 838 Hz be a much better choice than 1000 then? > (apart from the ugliness). No, 838 isn't significantly better. 864 and 627 would be better candidates: HZ ticks/jiffie 1 second error (ppm) --------------------------------------------------- 627 1903 0.999999314 -0.7 838 1424 1.000109105 109.1 864 1381 1.000001829 1.8 A good HZ value would make ntpd significantly happier, if the crystal is of reasonable quality. 152ppm (1000Hz) is 13 seconds a day, 0.7 ppm (627Hz) is 22 seconds a year. -- Vojtech Pavlik SuSE Labs, SuSE CR