[time-nuts] Raspberry Pi tweaks and custom kernel, was RE: PPS for NTP Server - How Close Is "Good Enough"?

David J Taylor david-taylor at blueyonder.co.uk
Mon Jun 15 06:23:32 UTC 2015


Just to be clear, your peer delay numbers for the Pi show and improvement 
from 0.5 ms down to 0.3 ms with some tweaks. A “non-usb” ethernet setup can 
get those delays down much further. In the context of the original question 
“tweaking the USB drivers” this is what was being addressed.


Thanks, Bob.

With the “non-usb” Ethernet BBB the mean delay was 0.173 ms compared with 
the RPi 0.352 ms.  Values of mean delay from other LAN systems ranged from 
0.167 ms (Win-7/64) to 0.209 ms (Win-8.1), so perhaps with that particular 
client PC (Linux, Intel Atom) the values all within the noise of being "as 
good as can be achieved".  If that's true, the extra delay introduced by the 
RPi's USB/Ethernet is some 0.18 ms.

NTP will measure and compensate for that delay, which helps explain why the 
resulting mean offset is just -39 microseconds with the RPi (-19 
microseconds with the BBB, -1 to +24 microseconds with other GPS-synced 

I don't think either system is good enough as a "microsecond" level server, 
but either is fine for "tenth millisecond" level.

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Email: david-taylor at blueyonder.co.uk 

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