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

Bob Camp kb8tq at n1k.org
Sat Jun 13 14:14:06 EDT 2015


Hi

At best, simply because of the way that USB interface works, you can go from the ~1/2 ms range into the ~1/4 ms range. Rewriting the low level drivers may be required.  This compares to an equivalent lag on a device with a built in ethernet of < 0.001 ms. The easy thing to do is to simply kill off everything else that is running on the device. That goes double for anything running on USB (keyboards / mice etc). 

Since you can get boards with built in (integrated in the MCU)  ethernet for < $50, it’s not clear how much tweaking time this is worth. You don’t get any real use out of the Pi’s fancy graphics in this case. The board really is not optimized for doing this sort of thing. Also - check the threads on Pi heat sinking and boards burning out. If you plan to use this full time -  invest the money in cooling now.  

Bob

> On Jun 13, 2015, at 2:06 AM, David J Taylor <david-taylor at blueyonder.co.uk> wrote:
> 
> Hi Max!  Thanks for the information, I was wondering if you had documented what you did to your Raspberry Pi so that it might be reproducible to someone like me (a newcomer time-nut and intermediate Linux user) ... you had said:
> []
> Thanks so much for your assistance!  Sorry if these questions have been posted before, but I am very curious about your setup as it nearly matches mine!
> 
>   -Randal "r3" of CubeCentral
> =======================================
> 
> I would also like to know how to best tweak the Raspberry Pi 2 for best
> performance as an NTP server. Although I have been using Linux-based
> firmware in my routers for several years now, I have never actually
> worked In Linux before.
> 
> I have gone through several tutorials on compiling a custom kernel, only
> one or two have actually ended in a compilation, but then I couldn't
> figure out where the kernel and modules were and get them onto the Pi. I
> have tried this under Ubuntu x64, Debian x64, and Mint 32 bit. So far
> Mint has been the best, I successfully compiled and I found the kernel,
> but cannot figure out where the modules are. Anybody suggest a really
> nice tutorial for learning this Linux stuff? My experience so far is
> really leading me to appreciate Windows.
> 
> Thanks
> Ed
> =======================================
> 
> Randal, Ed,
> 
> Just in case you missed it, I am also a beginner to Linux, so I documented my steps to get NTP working on the Raspberry Pi on my Web site.  There's a quick-start guide here:
> 
> http://www.satsignal.eu/ntp/Raspberry-Pi-quickstart.html
> 
> and a more blog-like set of detailed notes here:
> 
> http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html
> 
> I hope they may be of some help.  Typically I'm seeing well under 5 microsecond offsets reported by the RPi themselves:
> 
> http://www.satsignal.eu/mrtg/performance_ntp.php
> 
> although with different GPS devices, antennas, and system loads the performance does vary.  The polled USB is, perhaps, the main limitation to the device as a server on the LAN, although my own tests have shown RMS offsets reported by a remote client on a quiet LAN of 39 microseconds and a jitter of 38 microseconds.  Likely on a busier LAN the network itself might be the limiting factor.
> 
> I am no longer convinced that there is a significant gain to to be had by recompiling the kernel, now that PPS support for interrupts is included in the current Raspbian kernels.  I'm willing to be convinced otherwise, though.  Recompiling is a long and painful process, and cross-compiling presents further problems!
> 
> Cheers,
> David
> -- 
> SatSignal Software - Quality software written to your requirements
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk 
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