[ntp:questions] Loop Frequency and Offset

Dave Hart hart at ntp.org
Tue Sep 27 14:41:52 UTC 2011

2011/9/27 Miguel Gonçalves <mail at miguelgoncalves.com>:
> 2011/9/27 Dave Hart <hart at ntp.org>
>> I think you're simply confused.  The FreeBSD sysctl you modified isn't
>> adjusting the TSC frequency.  It's adjusting FreeBSD's estimate of the
>> TSC frequency.  I stand by my understanding that a positive frequency
>> compensation reported by ntpd means the clock's uncompensated rate is
>> slow.
> Dear Dave,
> As you hold an ntp.org mail account I will trust you. But, why did the
> frequency adjustment in loopstats reduced drastically when I changed the
> sysctl variable? I assumed NTP did not see the sysctl variable.

I don't suggest you take my word for it, but reason it yourself.  Your
system clock is implemented on top of the TSC.  Your earlier emails
seemed to indicate you believed your sysctl adjustment was resulting
in your TSC rate being changed to match, however, the knob you turned
instead adjusted the system's estimate of how many beats of TSC equate
to 1 SI second, while the TSC's actual rate is unchanged.  Decreasing
the TSC frequency estimate sysctl means FreeBSD will run its UTC clock
faster, as fewer TSC beats are required from the fixed-rate TSC for
each SI second advanced.  As I would expect, speeding up a system
clock which is running slower than UTC to close to UTC's rate meant
ntpd needed a less-positive frequency compensation (drift).

> Anyway, can I let the variable stay as is and let NTP do the adjustment?


Dave Hart

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