[ntp:questions] PPS kit for 2.6 linux kernels?

ramadog at gmail.com ramadog at gmail.com
Fri Apr 13 22:25:17 UTC 2007

Michael L. Semon wrote:
> 1) Use my current Linux + LinuxPPS and sit and watch as ntpd
> darts +/- 120 us over the course of a few hours, even when idle.
> 2) Reboot to FreeBSD and let the kernel, hardpps, and the NTP kernel
> code make more radical short-term adjustments, and ntpd will dart +/- 10
> us over a few hours.
> 3) Recompile glibc with support with older kernels without breaking my
> system, then fetch Linux, fetch Ulrich's last PPSkit alpha, and
> hope it all works.

If you do try a 2.6.17.X kernel maybe try with linuxpps. It seems
there were changes to the linux clock in 2.6.18 that make convergence
a lot slower. With 2.6.17 or 2.6.16 + linuxpps and default minpoll
maxpoll my time usually stays within +/- 10us. Minpoll 4 maxpoll 4 and
it is down to +/- 3us. For me 2.6.18 and later with default minpoll
maxpoll means +/- 250us. Minpoll 4 maxpoll 4 and I am still looking at
+/- 40us.

My drift changes by less than 0.6ppm during the temperature cycle of a
day. Freebsd or 2.6.17 (and lower) + linuxpps have no trouble
following that change. 2.6.18 and later react too slowly.


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