Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262677AbUKQXOl (ORCPT ); Wed, 17 Nov 2004 18:14:41 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262577AbUKQXOE (ORCPT ); Wed, 17 Nov 2004 18:14:04 -0500 Received: from e31.co.us.ibm.com ([32.97.110.129]:58051 "EHLO e31.co.us.ibm.com") by vger.kernel.org with ESMTP id S262669AbUKQXJz (ORCPT ); Wed, 17 Nov 2004 18:09:55 -0500 Subject: Re: summary (Re: [patch] prefer TSC over PM Timer) From: john stultz To: george@mvista.com Cc: dean gaudet , Dominik Brodowski , "Pallipadi, Venkatesh" , Alan Cox , linux@brodo.de, lkml In-Reply-To: <419BD0FF.4020602@mvista.com> References: <88056F38E9E48644A0F562A38C64FB60035C613D@scsmsx403.amr.corp.intel.com> <1100705495.32698.38.camel@localhost.localdomain> <419BD0FF.4020602@mvista.com> Content-Type: text/plain Date: Wed, 17 Nov 2004 15:09:43 -0800 Message-Id: <1100732984.3891.9.camel@leatherman> Mime-Version: 1.0 X-Mailer: Evolution 2.0.2 (2.0.2-3) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1534 Lines: 33 On Wed, 2004-11-17 at 14:30 -0800, George Anzinger wrote: > The APIC timer is again on a different "rock" which is not designed for time > keeping and, again, is calibrated at boot up against the "GOLD" standard PIT. > > IMHO, the best time keeping we can get in and x86 box is to: > > a) set up the PIT up to do the 1/HZ ticks (once set up we do not need to touch > it again so the I/O access issues become mute), > > b) select either the TSC (if we think it is stable) or the pm_timer to do the > short term between tick interpolation and also to detect and correct for PIT > interrupt overrun (like we missed a tick or two). We should prefer the TSC here > because of speed and that it is read every gettimeofday() access. My only qualm here is that using the TSC to interpolate between timer ticks allows for time inconsistencies. If the TSC isn't cumulatively accurate, then when used in between ticks it will cause minor inaccuracies and possible inconsistencies. I'd instead prefer picking a single time source, and using NTP to correct for drift or inaccurate calibration. Also breaking time subsystem from requiring regular periodic ticks allows for tickless systems and additional power management savings. But this should be saved for another thread. thanks -john - 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/