[ntp:questions] Allan deviation survey

David L. Mills mills at udel.edu
Thu Sep 16 14:47:47 UTC 2010


I'm glad it's gone, as the code was never intended to measure 
resolution. It is intended to measure precision, defined in the 
specification as the time to read the system clock. This turns out  to 
be really important for a client to read two or more sources on the same 
fast Ethernet. The intent is to avoid non-intersecting correctness 


Dave Hart wrote:

>On Thu, Sep 16, 2010 at 1:23 AM, David L. Mills <mills at udel.edu> wrote:
>>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
>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

More information about the questions mailing list