[ntp:questions] Which version of Linux works best?

Miroslav Lichvar mlichvar at redhat.com
Thu Mar 11 12:40:36 UTC 2010


On Wed, Mar 10, 2010 at 03:25:45PM -0000, David J Taylor wrote:
> - and one for Bill, how much better might chrony be than official NTP?

In my experience, chrony is about 3-20 times better than NTP when using
the same poll interval. The more stable is temperature and CPU load the
smaller is the difference.

> Does it have a preferred Linux, or even freeBSD?

The BSD drivers in chrony are using only adjtime() call for clock
corrections (similar to the NTP daemon mode), so for optimal results
running chrony on Linux might be necessary.

There were some issues related to the kernel tickless feature which
affected chrony's initial synchronization. They were fixed only very
recently, so if you need to get below 1 us in less than one minute,
the latest kernel (2.6.33) or kernel compiled without CONFIG_NO_HZ
is recommended.

As for PPS source, LinuxPPS patch can be applied to kernel (hopefully
it will be merged into mainline soon), or PPS samples from gpsd can be
used instead, versions 2.90 and later works best. In my tests the gpsd
source has about two times worse dispersion and there is a small shift
when compared to the kernel source. There is also a difference in
resolution (nanoseconds vs microseconds).

> The system would /not/ be in a temperature controlled environment.

I did a NTP vs chrony comparison last June with GPS 18x LVC in an
office environment, clock drift was moving in about 0.8ppm range. Here
are distributions of PPS samples received from gpsd:

http://fedorapeople.org/~mlichvar/chrony/chrony_vs_ntp.png

With recent chrony, NTP and kernel versions the results might be
different though.

-- 
Miroslav Lichvar




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