[ntp:questions] Isolated Network Drift Problem

Calvin Webster cwebster at ec.rr.com
Wed Nov 26 13:28:39 UTC 2008

On Wed, 2008-11-26 at 00:52 +0000, Unruh wrote:
> >Okay, so I'll just be using the difference in seconds between one
> >invocation of "date" and the next.
> >How do I translate these seconds into a usable value for "ntptime -f"?
> 1sec/day=11.47 PPM. scale for other values. 
> (Ie, convert the time difference to seconds per day, and multiply by 11.5)
> If the computer time is fast you need to slow down the computer clock.
> >> >Ex:
> >> >[root at fluid root]# cat /etc/ntp/drift
> >> >20.196
> >> 
> >> >##?? Is this a good source to measure the drift?
> >> 
> >> >##>> Correct for the drift:
> >> 
> >> >ntptime -f 20.196
> >> 
> >> No way you will get that kind of accuracy using this procedure.
> >> You will be lucky with a few PPM
> >I wasn't aware that I had illustrated any level of accuracy. As I said,
> >I'm shooting for about 1 sec per day.
> 20.196 is a number which seems to be accurate to .001 PPM.:-)

Oh, I see what you're saying. All I did was to pull the number
from /etc/ntp/drift (ntpd's drift file) and paste it into the ntptime
command. Doesn't the "-f" argument specify frequency offset and the
drift file contains the frequency offset calculated to that point?

> >I'll look at "chrony" but I want whatever NTP implementation we use to
> >be as maintenance free as possible once it's in operation. I don't want
> >someone to have to manually update anything every day or week.
> Well, that daily or weekly input of the time is to keep your system on
> time. It will drift. And if you want to correct that drift youhave to give
> it more data. 

1 minute every couple of months would be fine.

> >That's what I'm asking. Is there a kernel parameter to set frequency
> >offset? If not, how do I make the calculated offset persist across
> >reboots? Aside from a kernel parameter entry in /etc/sysctl.conf the
> >only way I can think of is to add the full comand "ntptime -f (offset in
> >PPM)" to "/etc/rc.local" or "/etc/rc.sysinit".
> You could do that. 
> Note that the recent linux kernels have apparently totally messed up the tsc hardware
> clock. The drift rate changes by 50PPM between successive reboots, so you
> old drift file is useless on the new boot. You need to make sure that the
> machine use the acpi_pm (?) or hpet as their timing source, not the tsc. 

Thanks for that info I'll keep that in mind. Well, in my notes
anyway. :-)


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