[ntp:questions] Allan deviation survey

Dave Hart davehart at gmail.com
Thu Sep 16 06:17:35 UTC 2010


On Thu, Sep 16, 2010 at 1:23 AM, David L. Mills <mills at udel.edu> wrote:
> Miroslav,
>
> The fastest machine I can find on campus has precision -22, or about 230 ns.
> Then, I peeked at time.nist.gov, which is actually three machines behind a
> load leveler. It reports to be an i386 running FreeBSD 61. Are you ready for
> this? It reports precision -29 or 1.9 ns! I'm rather suspiciousabout that
> number.

I think this can be attributed to some code that used to be in ntpd
which, on FreeBSD only, used for precision an OS estimate of the clock
resolution in place of the measured latency to read the clock used on
every other platform.  That FreeBSD exception was removed from ntpd
years ago, but apparently after the version in use by NIST.

Dave Hart



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