[Thumbgps-devel] performance testing sure gps board via usb serial converter
Ron Frazier (NTP)
timekeepingntplist at c3energy.com
Tue Mar 27 00:59:12 PDT 2012
I thought you'd like to see this. I'm definitely getting performance
from my GPS connecting through a Prolific based serial - USB converter
that is within the specs that you proposed for your project - ie + / -
1ms offsets from GPS time. I posted this on the NTP Questions list
tonight. Some parameters apply only to Windows and some apply only to
Linux. The testing reported was done on Windows. Season to taste for
I've been doing some performance testing with the Sure GPS board
connected via serial - USB converter, with PPS to DCD. I'm running
Win7. Performance is limited by the USB interface, but I've observed
some interesting trends. I have email periodically checking for
messages in the background, and I have 5 browser windows open with about
Thanks to Alby for showing me how to use flag1 on the 20 driver to turn
on pps without using the 22 driver separately. I don't know if there
are any performance differences between the two.
Thanks to Dave Hart for letting me know that flag3 to turn on kernel
discipline doesn't apply to Windows and could cause problems.
All these parameters and permutations are confusing, particularly if
experimenting with Windows and Linux.
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
Does that apply to Linux by the way?
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
OK. It's 3:34 AM and I have to get some sleep. I really have to get
UN-obsessed with this. I really do like to fully understand the
technology I'm using, but GPS + NTPD has turned out to be a real
challenge. Signing off for now and, if there are typos, to bad.
Hopefully, someone will find this useful.
(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such. I don't always see new messages very quickly. If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)
timekeepingdude AT c3energy.com
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