Puzzling clock offset spikes - update 2

Paul Theodoropoulos paul at anastrophe.com
Sun Jul 8 02:33:36 UTC 2018

On 7/5/2018 10:22 AM, Paul Theodoropoulos wrote:
>>> That said, latency and jitter were terrible when I first got my raspi
>>> 1 a couple of years ago; I've never tried it on the 3, so perhaps
>>> wifi on the raspi 3 might not suffer those issues; I will have to
>>> look into that. It'd be nice to be able to untether the little devil
>>> anyway, so I could easily relocate it elsewhere in the house where
>>> ambient temps might be smoother. I will check it out!
First step, fired up wifi on Raspi, unplugged ethernet. No change to the 
repeating double-spikes in offset every 9 - 11 minutes. Latency on wifi 
not bad at all on the 5GHz channel, about 1ms pi to pi on ethernet, now 
1.5 to 2ms over wifi - livable, and consistent, unlike the old days of 
the Pi 1 plus wifi usb adapter.

Since no change, step two: powered down the USB bus -
echo 0x0 > /sys/devices/platform/soc/3f980000.usb/buspower

et voila! repeating double spikes gone, and overall error intervals 
across nearly all metrics much tighter. Still variable, still assorted 
spike-like glitches from time to time, but I think I'm probably at the 
limit of what's attainable on a $35 SBC with peripherals. Overall, I 
think it's pretty darned good, all things considered.

Temperature remains the biggest factor in terms of frequency offset - 
expected - but I'm still refining things.

Oh - and previously on ethernet, my Raspi was burning about 2.4 to 2.7 
watts continuously. On wifi with USB bus powered down - about 1.25 
watts. Running temps dropped from average 58 deg C to about 44 deg C.

Overall, I'm quite pleased.

Link to my ntpviz graphs - https://ntpsec.anastrophe.com/1hr/index.html 
- pay no attention to graphs other than 1hr and 3hr - experimentation 
maketh the graph repugnant. :)

Paul Theodoropoulos

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