[ntp:questions] Allan deviation survey

Miroslav Lichvar mlichvar at redhat.com
Wed Sep 15 11:22:03 UTC 2010

On Wed, Sep 15, 2010 at 01:34:04AM +0000, David L. Mills wrote:
> However, the ntpd
> measurements are after the clock filter and before the kernel call,
> while I suspect yours are after the discipline and before the kernel
> call. The two measurements are not comparable. Look at it this way.
> At a poll interval of 16 s, the PLL reduces a given time offset by a
> factor of 256, so in fact a 1-ms offset actually causes a
> 4-microsecond change in the clock phase. If that were the criteria
> to judge performance, ntpdt would look 256 times better than
> advertised. I am not here judging whether chrony is better than ntpd
> or not, just that the performance measurements be comparable and
> honest.

If this is about the clknetsim results, please note that the offset
statistic is made from offsets captured directly from the simulated
clock at one second interval. No data reported by ntpd or chrony, or
values passed in the ntp_adjtime call are ever used in the statistic.

But of course you are right that the offsets reported in ntpd's
loopstats and chrony's tracking files are not the same and to make a
fair comparison outside simulator the raw data from peerstats and
measurements files have to be processed instead.

Miroslav Lichvar

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