Date: Fri, 18 Mar 2011 18:20:29 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Jung-uk Kim <jkim@FreeBSD.org> Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org, Peter Jeremy <peterjeremy@acm.org> Subject: Re: svn commit: r219700 - head/sys/x86/x86 Message-ID: <20110318172852.L1182@besplex.bde.org> In-Reply-To: <201103171706.12993.jkim@FreeBSD.org> References: <201103161644.p2GGi8ug098283@svn.freebsd.org> <20110317200156.GB65858@server.vk2pj.dyndns.org> <201103171706.12993.jkim@FreeBSD.org>
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On Thu, 17 Mar 2011, Jung-uk Kim wrote: > On Thursday 17 March 2011 04:01 pm, Peter Jeremy wrote: >> On 2011-Mar-16 16:44:08 +0000, Jung-uk Kim <jkim@FreeBSD.org> wrote: >>> Log: >>> Revert r219676. >> >> Thanks also. For extra kudos, how about adding a similar function >> to allow the HPET frequency to be over-ridden. That is currently >> the only timecounter that does not allow the user to compensate for >> incorrect hardware frequencies. > > I really hate the idea of adjusting timecounter frequency from > userland. I guess "use ntpd(8)" is not a good answer for some > people. :-( I use ntpd, and also adjust the timecounter frequency so that: - when the timecounter is the TSC, ntpd has less work to do. It syncs faster and its adjustments are smaller, giving less jitter. - there is only 1 place to change when the TSC frequency changes. Instead of changing the driftfile and restarting ntpd to resync after every frequency change, I set the TSC as accurately as possible (normally to within 1 ppm for the current frequency setting), so that ntpd can resync automatically just as easily as it can handle small termperature-related drifts. If you use a sloppy DELAY()-based calibration or a nominal frequency, then you can easily have an error of 50 ppm or more. ntpd will take hours to resync with that. By resync, I mean that the error is usually less than a millisecond and has low variance. - the TSC it is as accurate as possible for other purposes, irrespective of whether it is used for the timecounter. Brucehelp
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