Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756228AbZFAXWq (ORCPT ); Mon, 1 Jun 2009 19:22:46 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753052AbZFAXWj (ORCPT ); Mon, 1 Jun 2009 19:22:39 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:56112 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752716AbZFAXWi (ORCPT ); Mon, 1 Jun 2009 19:22:38 -0400 Date: Tue, 2 Jun 2009 01:22:23 +0200 From: Ingo Molnar To: John Stultz Cc: mingo@redhat.com, hpa@zytor.com, riel@redhat.com, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, tglx@linutronix.de, linux-tip-commits@vger.kernel.org, Miroslav Lichvar Subject: Re: [tip:timers/ntp] ntp: adjust SHIFT_PLL to improve NTP convergence Message-ID: <20090601232223.GH749@elte.hu> References: <200905051956.n45JuVo9025575@imap1.linux-foundation.org> <1243542817.29511.523.camel@jstultz-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1243542817.29511.523.camel@jstultz-laptop> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2350 Lines: 55 * John Stultz wrote: > On Wed, 2009-05-06 at 09:46 +0000, tip-bot for john stultz wrote: > > Commit-ID: 22cfbbfd9f67b67fe073010f51cb71d3632387d5 > > Gitweb: http://git.kernel.org/tip/22cfbbfd9f67b67fe073010f51cb71d3632387d5 > > Author: john stultz > > AuthorDate: Wed, 6 May 2009 11:43:57 +0200 > > Committer: Ingo Molnar > > CommitDate: Wed, 6 May 2009 11:44:02 +0200 > > > > ntp: adjust SHIFT_PLL to improve NTP convergence > > > > The conversion to the ntpv4 reference model > > f19923937321244e7dc334767eb4b67e0e3d5c74 ("ntp: convert to the NTP4 > > reference model") in 2.6.19 added nanosecond resolution the adjtimex > > interface, but also changed the "stiffness" of the frequency adjustments, > > causing NTP convergence time to greatly increase. > > > > SHIFT_PLL, which reduces the stiffness of the freq adjustments, was > > designed to be inversely linked to HZ, and the reference value of 4 was > > designed for Unix systems using HZ=100. However Linux's clock steering > > code mostly independent of HZ. > > > > So this patch reduces the SHIFT_PLL value from 4 to 2, which causes NTPd > > behavior to match kernels prior to 2.6.19, greatly reducing convergence > > times, and improving close synchronization through environmental thermal > > changes. > > > > The patch also changes some l's to L's in nearby code to avoid misreading > > 50l as 501. > > > > [ Impact: tweak NTP algorithm for faster convergence ] > > Hey Ingo, > > So I've been speaking with Miroslav (cc'ed) who maintains > the RH ntpd packages, and he's concerned that this patch takes us > out of NTP's expected behavior, which may cause problems when > dealing with non-linux systems using NTP. I might be missing something here - but Linux converging faster seems like a genuinely good thing. What non-Linux problem could there be? Linux's convergence is really Linux's private issue. (since things like the PIT calibration are random noise for which there can be no external expecation about our convergence speed anyway.) Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/