From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752608AbZFRMNt (ORCPT ); Thu, 18 Jun 2009 08:13:49 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751084AbZFRMNm (ORCPT ); Thu, 18 Jun 2009 08:13:42 -0400 Received: from mx2.redhat.com ([66.187.237.31]:36379 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750945AbZFRMNl (ORCPT ); Thu, 18 Jun 2009 08:13:41 -0400 Date: Thu, 18 Jun 2009 14:13:20 +0200 From: Miroslav Lichvar To: Ingo Molnar Cc: John Stultz , Thomas Gleixner , Linus Torvalds , Andrew Morton , LKML Subject: Re: [GIT pull] ntp updates for 2.6.31 Message-ID: <20090618121320.GA13025@localhost> References: <1f1b08da0906151316s7d25f8ceraa1bc967a8abe172@mail.gmail.com> <1f1b08da0906151641u4cd964e6vf1a61afe50cc1d90@mail.gmail.com> <20090616090647.GD13771@elte.hu> <20090616125248.GA23541@localhost> <1245253102.6067.94.camel@jstultz-laptop> <20090617172325.GA32332@localhost> <20090617172601.GA3493@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090617172601.GA3493@elte.hu> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 17, 2009 at 07:26:01PM +0200, Ingo Molnar wrote: > * Miroslav Lichvar wrote: > > > Still, I'd really like to see the original behavior restored. Most > > of the users complaining about slow convergence are probably just > > hitting the calibration problem, which needs to be fixed by other > > means than making PLL faster. Also, users of other systems seem to > > be happy with their slow convergence. At least that's the > > impression I have from NTP lists. > > Wouldnt the goal be to calibrate as fast as possible? (Without any > bad oscillation) Not really. It depends on how noisy is the input signal. On an idle LAN the jitter is just few microseconds, but over internet it easily reaches miliseconds. Over a certain point faster PLL will just make things worse. PLL is mainly about handling the signal noise, frequency adjusting is secondary. When the noise is very low or the update interval is long enough, the frequency variations caused by temperature changes will dominate the signal noise and this is where FLL should kick in. The PLL/FLL switching is controlled by update interval. Ideally it would be adaptive, but NTP is not that sophisticated. By default, FLL is enabled when the interval is longer than 2048 seconds. This is of course not the optimal value for all systems. Unfortunately in kernel it can be configured only to 2048 or 256 and NTP never uses the shorter one. The NTP daemon has its own loop which can be used instead and it allows to use arbitrary values though. -- Miroslav Lichvar