From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261863AbVGLR6q (ORCPT ); Tue, 12 Jul 2005 13:58:46 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261867AbVGLR4y (ORCPT ); Tue, 12 Jul 2005 13:56:54 -0400 Received: from e31.co.us.ibm.com ([32.97.110.129]:3297 "EHLO e31.co.us.ibm.com") by vger.kernel.org with ESMTP id S261892AbVGLR4Y (ORCPT ); Tue, 12 Jul 2005 13:56:24 -0400 Subject: Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt From: john stultz To: George Anzinger Cc: "Martin J. Bligh" , Lee Revell , Diego Calleja , azarah@nosferatu.za.org, Andrew Morton , cw@f00f.org, lkml , torvalds@osdl.org, christoph@lameter.org In-Reply-To: <42D310ED.2000407@mvista.com> References: <200506231828.j5NISlCe020350@hera.kernel.org> <20050708214908.GA31225@taniwha.stupidest.org> <20050708145953.0b2d8030.akpm@osdl.org> <1120928891.17184.10.camel@lycan.lan> <1120932991.6488.64.camel@mindpipe> <20050709203920.394e970d.diegocg@gmail.com> <1120934466.6488.77.camel@mindpipe> <176640000.1121107087@flay> <42D310ED.2000407@mvista.com> Content-Type: text/plain Date: Tue, 12 Jul 2005 10:56:15 -0700 Message-Id: <1121190975.7673.83.camel@cog.beaverton.ibm.com> Mime-Version: 1.0 X-Mailer: Evolution 2.0.4 (2.0.4-4) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2005-07-11 at 17:38 -0700, George Anzinger wrote: > Martin J. Bligh wrote: > >>>Lots of people have switched from 2.4 to 2.6 (100 Hz to 1000 Hz) with no impact in > >>>stability, AFAIK. (I only remember some weird warning about HZ with debian woody's > >>>ps). > >>> > >> > >>Yes, that's called "progress" so no one complained. Going back is > >>called a "regression". People don't like those as much. > > > > > > That's a very subjective viewpoint. Realize that this is a balancing > > act between latency and overhead ... and you're firmly only looking > > at one side of the argument, instead of taking a compromise in the > > middle ... > > > > If I start arguing for 100HZ on the grounds that it's much more efficient, > > will that make 250/300 look much better to you? ;-) > > I would like to interject an addition data point, and I will NOT be subjective. > The nature of the PIT is that it can _hit_ some frequencies better than > others. We have had complaints about repeating timers not keeping good time. > These are not jitter issues, but drift issues. The standard says we may not > return early from a timer so any timer will either be on time or late. The > amount of lateness depends very much on the HZ value. Here is what the values > are for the standard CLOCK_TICK_RATE: > > HZ TICK RATE jiffie(ns) second(ns) error (ppbillion) > 100 1193180 10000000 1000000000 0 > 200 1193180 5000098 1000019600 19600 > 250 1193180 4000250 1000062500 62500 > 500 1193180 1999703 1001851203 1851203 > 1000 1193180 999848 1000847848 847848 > > The jiffie values here are exactly what the kernel uses and are based on the > best one can do with the PIT hardware. > > I am not suggesting any given default HZ, but rather an argumentation of the > help text that goes with it. For those who want timers to repeat at one second > (or multiples there of) this is useful info. > > For you enjoyment I have attached the program used to print this. It allows you > to try additional values... If I recall, 1001 was a decent choice and is relatively close the the expected frequency. Also I think the error is positive instead of negative, so it avoids the "jiffies are shorter then I expected!" issues. >>From your program's output: HZ TICK RATE jiffie(ns) second(ns) error (ppbillion) 1001 1193180 999013 1000012013 12013 thanks -john