[ntp:questions] Performance estimation

David Taylor david-taylor at blueyonder.co.uk.invalid
Tue Jun 16 13:13:12 UTC 2020

On 16/06/2020 14:04, Miroslav Lichvar wrote:
> On 2020-06-16, David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
>> The clock on a Raspberry Pi ranges from 700 to 1500 MHz, so clock
>> resolution is in the nanosecond range.
> For best timekeeping performance, you would want to set the CPU
> frequency to a fixed value.
>> I would also like to see whether the characteristics of the GPS and its
>> location make a measurable difference to the RPi's timekeeping.  For
>> example: is it better to have a GPS with 3 service capability at a
>> location where the signal is poor, or is it masked by the RPi's
>> performance?  All this with kernel-mode PPS.
> The interrupt latency of the PPS timestamping is probably much larger
> than any errors related to GPS, so I'd say it doesn't matter.

Oh, sorry, that's the CPU performance range between the models, from 
earliest to latest.  Having said that, some of the [later?] models do 
operate a varying CPU frequency.  Perhaps that's why the offset varies 
so little on my very earliest RPi:


I'm inclined to agree with you about interrupt latency, not to mention 
the Ethernet being over USB in the earlier models!  Still nice to make 
measurements to see what can be achieved, and the principles are more 
widely applicable.

Good lockdown learning!

Web: http://www.satsignal.eu

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