Raspberry Pi 4

Have you tried futzing with isolcpus?
EDIT: I had to set isolcpus=1,2,3 to get decent latency on my RPI3/LinuxCNC test machine:

No.

Well now I have. And it made all the difference in the world.
The following is with wlan0 disabled. Leaving wlan0 up and surfing at the same time
has a huge impact on the latency numbers.Surfing brings the max latency numbers
up a factor 5, around +- 100 us.




I can try some more tomorrow, it's getting late here.

I have tried some, but not yet been able to get a step going. I use the 7i76E mesa card via the ethernet port.
It is possible to get it going to the point where gmoccapy is displayed and reacts to Machine On/Off.
The 7i76E displays all the inputs and outputs as it scrolls by during startup.
But when I try to jog, there is a core dump by rtapi_app. No steps are taken.

I have noticed that "iptables -N hm2-eth-rules-output2 fails with "iptables: Operation not supported"
Obviously iptables is gone, say hello to nftables. OrThat helps for those messages.
Still, when I jog the tiniest distance I getand then there is no reaction.

I am unable to find that line in dmesg. I have never got any messages from linuxcnc in the system log file.
It is the rtapi_app that crashes. I have looked at it with strace and the last system command is "accept"
then crash. Intended to attach to rtapi_app with gdb, it should take me to the failing part upon crash.
But not succesful