From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261731AbTJHQX3 (ORCPT ); Wed, 8 Oct 2003 12:23:29 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261733AbTJHQX3 (ORCPT ); Wed, 8 Oct 2003 12:23:29 -0400 Received: from pentafluge.infradead.org ([213.86.99.235]:62938 "EHLO pentafluge.infradead.org") by vger.kernel.org with ESMTP id S261731AbTJHQX1 (ORCPT ); Wed, 8 Oct 2003 12:23:27 -0400 Subject: Re: Time precision, adjtime(x) vs. gettimeofday From: Benjamin Herrenschmidt To: Gabriel Paubert Cc: linuxppc-dev list , Linux Kernel list In-Reply-To: <20031008154846.GA29868@iram.es> References: <1065619951.25818.15.camel@gaston> <20031008154846.GA29868@iram.es> Content-Type: text/plain Message-Id: <1065630178.26943.54.camel@gaston> Mime-Version: 1.0 X-Mailer: Ximian Evolution 1.4.5 Date: Wed, 08 Oct 2003 18:22:58 +0200 Content-Transfer-Encoding: 7bit X-SA-Exim-Mail-From: benh@kernel.crashing.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-Pentafluge-Mail-From: Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org > Well, it it affects gettimeofday which has a precision of 1 part in > 10000 (100 ppm), it means that our boot time timebase calibration was > not very good to start with, on my set of running VME machines I have > the following (values in ppm): > > ../.. Boot time calibration can't be perfect... I depends very much on the quality of what your are calibrating against, and the bus path to it. On most pmacs, I'm calibrating either against a VIA timer which isn't _that_ good or on OF value (which are themselves calibrated, I think, against the KeyLargo timer). On all cases, those will drift some way from what the NTP server will give, either a lot or not, it will. So we may end up adjusting our kernel rate and thus opening a window for the problem. Ben.