Nixos & linuxcnc

github.com/MarcWeber/nixpkgs/tree/e2/linuxcnc
I got far packaging linuxcnc but on the old i686 (1GB RAM) showing realtime=posix in stepconf the measure jitter like stuff did no longer show up. I patched to allow running as root because I failed/ was to lazy to think about what to setuid in what way.

The kernel is 5.4 is boot.kernelPackages = pkgs.linuxPackages-rt_5_4; (RT_REALTIME with _).

Is there anything trivial I have missed or should I be using an alternative realtime configuration ?

I compiled with --with-realtime=uspace .

Or should I retry with rtai patches eg 5.2 kernel ? www.rtai.org/userfiles/downloads/RTAI/rtai-5.2.tar.bz2

You could try the command-line latency test. That would give some idea if the realtime system is working.

Did it say "POSIX realtime" or "POSIX non-realtime"?

POSIX realtime. But as I said then it looked like the system turned so busy that it was even hard to shut it down.