[ntp:questions] performance testing sure gps board via usb serialconverter

David J Taylor david-taylor at blueyonder.co.uk.invalid
Tue Mar 27 09:42:04 UTC 2012


> All these parameters and permutations are confusing, particularly if 
> experimenting with Windows and Linux.

Agreed!

> Thanks to David Taylor for encouraging me to experiment with 
> interpolation, using the environment variable 
> NTPD_USE_INTERP_DANGEROUS=1 to turn on and NTPD_USE_SYSTEM_CLOCK=1 to 
> turn off.
>
> Does that apply to Linux by the way?

No.

> In the past, I thought realtime priority for the NTPD process was 
> causing problems.  So, I've been experimenting with both priority and 
> interpolation.  Realtime is the default priority.
>
> The most accurate time source I have is my GPS.  Internet, in my case, 
> doesn't even come close.  So, I'm testing min and max offsets of my 
> computer's clock to the GPS polling every 8 seconds.  The tests weren't 
> too scientific nor too long, but I still saw some interesting results.
>
> 1) Interpolation ON , Above Normal Priority, + 1.00 / - 0.75 ms, Total 
> Range 1.75 ms
> 2) Interpolation ON , Realtime     Priority, + 0.99 / - 0.67 ms, Total 
> Range 1.66 ms
> 3) Interpolation OFF, Above Normal Priority, + 1.21 / - 1.19 ms, Total 
> Range 2.40 ms
> 4) Interpolation OFF, Realtime     Priority, + 1.13 / - 1.02 ms, Total 
> Range 2.15 ms
>
> Comparing lines 1 and 2, going from Above Normal to Realtime priority 
> with interpolation on reduces range by .09 ms.
>
> Comparing lines 3 and 4, going from Above Normal to Realtime priority 
> with interpolation off reduces range by .25 ms.
>
> Comparing lines 3 and 1, in that order, turning interpolation on at 
> Above Normal priority, reduces range by .65 ms.
>
> Comparing lines 4 and 2, in that order, turning interpolation on at 
> Realtime priority, reduces range by .49 ms.
>
> Conclusion, I'm leaving interpolation on, and I'm leaving the process at 
> Realtime priority.
>
[]
> Hopefully, someone will find this useful.
>
> Sincerely,
>
> Ron

Yes, it's interesting to see the result, Ron.  With the interpolation 
enabled, the errors in timestamping the pseudo-serial data should be very 
much reduced, but there is still the USB ~1 millisecond sampling interval 
left, so interpolation may only reduced the offset by a factor of around 
two at best.  Good to see that you are now getting better results with 
real-time than simply above normal priority - it suggests that things are 
behaving as expected.

Cheers,
David 



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