[ntp:questions] 500ppm - is it too small?

Harlan Stenn stenn at ntp.org
Sun Aug 16 21:29:05 UTC 2009


>>> In article <o48hm.41192$PH1.17076 at edtnps82>, Unruh <unruh-spam at physics.ubc.ca> writes:

Unruh> So I have 9 clocks.  The rates are -190, 19 , -106, -67,-200 -219,
Unruh> -115 -140 221

Unruh> On reboot, the latter changed from 221 to 215 (Which took ntp about 6
Unruh> hours to recover from)

Unruh> The clock scaling in linux seems to suffered a real problem in the
Unruh> past year or two, so that the rate from one reboot to the next can
Unruh> change by 50PPM, which then takes ntp a long time to recover from.

Unruh> two years ago those same clocks, running earlier kernels, had rates
Unruh> of 5 -17 45 27 23 100 101 -10 8 -39 39 25.

Unruh> It will not take much more degredation for the clocks to surpass the
Unruh> 500PPM limit. And this is not due to any change in the hardware. It
Unruh> seems to be kernel software and the scaling calibration being
Unruh> performed at bootup.

So it looks like the problem is in the way your kernel is deciding the value
of 'tick' at boot time.

Why is it better to 'fix' this problem in ntpd instead of either fixing the
kernel boot calibration or finding a way to override the kernel calibration
routine's "wrong choice"?

We really would prefer to keep bloat out of ntpd, and this problem, to me,
should really be fixed closer to its source.
-- 
Harlan Stenn <stenn at ntp.org>
http://ntpforum.isc.org  - be a member!




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