From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932340Ab0EXXX1 (ORCPT ); Mon, 24 May 2010 19:23:27 -0400 Received: from terminus.zytor.com ([198.137.202.10]:43442 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932293Ab0EXXXZ (ORCPT ); Mon, 24 May 2010 19:23:25 -0400 Message-ID: <4BFB0990.6030400@zytor.com> Date: Mon, 24 May 2010 16:19:44 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100430 Fedora/3.0.4-2.fc12 Thunderbird/3.0.4 MIME-Version: 1.0 To: Dan Magenheimer CC: john stultz , Brian Bloniarz , Ingo Molnar , Thomas Gleixner , Peter Zijlstra , Andi Kleen , Arjan van de Ven , Venkatesh Pallipadi , chris.mason@oracle.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] x86: Export tsc related information in sysfs References: <4BF58B59.7080901@athenacr.com> <1274727116.2954.5.camel@localhost.localdomain> <4BFADF9D.9050209@zytor.com> <1274733566.2954.73.camel@localhost.localdomain> <3ec7f284-1507-47fb-b5a2-eea29f68c627@default> <4BFAFE17.8060105@zytor.com 1274741362.2954.80.camel@localhost.localdomain> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/24/2010 04:16 PM, Dan Magenheimer wrote: >> From: john stultz [mailto:johnstul@us.ibm.com] >> Also, you don't really need extra accuracy, you just need it to be the >> same from boot to boot. NTP keeps the correction factor persistent from >> boot to boot via the drift file. The boot argument is just trying to >> save the time (possibly hours depending on ntp config) after a reboot >> for NTP to correct for the new error introduced by calibration. > > I was assuming that extra accuracy would decrease the ntp > convergence time by about the same factor (5-6 bits of extra > accuracy would decrease ntp convergence time by 32-64x). > Is that an incorrect assumption? > Yes. >> From: H. Peter Anvin [mailto:hpa@zytor.com] >> A longer sample would make sense if the goal is to freeze it into a >> kernel command line variable, but the real question is how many people >> would actually do that (and how many people would then suffer problems >> because they upgraded their CPU/mobo and got massive failures on post- >> boot.) > > Not sure why upgraded mobo's would fail due to a longer sample? Not due to a longer sample, but a frozen sample. > As more and more systems become dependent on clocksource==tsc > and more and more people assume nanosecond-class measurements > are relatively accurate, I'd expect the accuracy of tsc_khz > to become more important. While desktop users might bristle > at an extra second of boot delay, I'll bet many server > farm administrators would gladly pay that upfront cost > if they know an option exists. Not really. The delta measurements aren't the issue here, but rather walltime convergence. -hpa