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Date:      Mon, 27 Mar 1995 14:44:42 +1000
From:      Bruce Evans <bde@zeta.org.au>
To:        current@FreeBSD.org, starkhome!gene@sbstark.cs.sunysb.edu
Subject:   Re: Kernel time and xntpd
Message-ID:  <199503270444.OAA07627@godzilla.zeta.org.au>

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>I have been running xntpd for awhile now on some of my FreeBSD systems,
>and it seems that xntpd has to perform a step adjustment of about
>-0.14 sec approximately once an hour to stay in sync with a non-FreeBSD
>remote time source.  This is an error of about 40ppm.  On the other hand,
>other FreeBSD systems seem to stay in sync with a FreeBSD master with
>a stability of less than 1ppm.  This would seem to indicate that there
>are still errors in the kernel timekeeping in FreeBSD.  Are any currently
>known?

The 8254 timer frequency is very hardware dependent.  I have two systems
that have a relative drift of 3.6 seconds/hour.  One drifts forwards,
the other backwards.

The frequency should be calibrated.  This is easier if you have an accurate
clock to calibrate it agains (the RTC is said to be less accurate and/or
more unstable).  Try changing TIMER_FREQ to
((int) (NOMINAL_TIMER_FREQ * (1 + 0.14 / 3600))).

Bruce



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