From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261434AbVGLP1y (ORCPT ); Tue, 12 Jul 2005 11:27:54 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261453AbVGLP1x (ORCPT ); Tue, 12 Jul 2005 11:27:53 -0400 Received: from dvhart.com ([64.146.134.43]:23734 "EHLO localhost.localdomain") by vger.kernel.org with ESMTP id S261434AbVGLP00 (ORCPT ); Tue, 12 Jul 2005 11:26:26 -0400 Date: Tue, 12 Jul 2005 08:26:27 -0700 From: "Martin J. Bligh" Reply-To: "Martin J. Bligh" To: Vojtech Pavlik , Con Kolivas Cc: linux-kernel@vger.kernel.org, George Anzinger , Lee Revell , Diego Calleja , azarah@nosferatu.za.org, akpm@osdl.org, cw@f00f.org, torvalds@osdl.org, nacc@us.ibm.com, Darren Hart , John Stultz Subject: Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt Message-ID: <18080000.1121181987@[10.10.2.4]> In-Reply-To: <20050712133018.GA8467@ucw.cz> References: <200506231828.j5NISlCe020350@hera.kernel.org> <42D310ED.2000407@mvista.com> <20050712121008.GA7804@ucw.cz> <200507122239.03559.kernel@kolivas.org> <20050712133018.GA8467@ucw.cz> X-Mailer: Mulberry/2.2.1 (Linux/x86) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org >> > 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. Does positive vs negative error make a difference to the timer subsystem? Nish was telling me they had to add 1 extra tick to timer inaccuracies because of the errors ... does changing the polarity of the error affect that (seems like it would ... but I got lost by now ;-))? M.