[ntp:questions] Allan deviation survey

Miroslav Lichvar mlichvar at redhat.com
Mon Sep 13 09:24:13 UTC 2010

On Fri, Sep 10, 2010 at 10:10:08PM +0000, David L. Mills wrote:
> A previous message implied that, once the Allan characteristic was
> determined, it would show chrony to be better than ntpd. Be advised
> the default time constant (at 64 s poll interval) was specifically
> chosen to match trace 1 on the graph mentioned above.

Wasn't that rather for 16s poll interval? From simulations is seems
that the phase noise would have to be 10-30 times higher (or the
frequency noise lower, but that's unrealistic) to ntpd perform
well at 64s poll interval.

> In other
> words, it is in fact optimum for that characteristic and chrony can
> do no better.

Well, it does better. With phase noise and random-walk frequency
corresponding to the trace 1 from your graph chrony is about 5 times
better than ntpd. With 30 times higher phase noise the difference is
only in order of tens of percent, but it's still better.

Miroslav Lichvar

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