Reducing Jitter

Hello

I am currently trying to get LinuxCNC working on a Raspberry Pi. It's about a few weeks that I am trying, so I guess is the moment to ask, but my expertise is still limited: forgive me if I ask some newbie stuff.

So, what is the current status:

First, I installed a fresh Raspian Linux on the machine.

Second, I Cross-compiled an RT kernel following the page here . After that, using `uname` print the correct new kernel with PREEMPT in it. I did some tests with "cyclictest" and results, even if not very good, are still bellow 150us maximum and average 20us

I configured the kernel to start with following options:

After, I cloned LinuxCNC and following this guide

What with LinuxCNC?

Running the test: "latency-histogram" I get terrible results with maximum jitter over 700-1400us

It seem to me that there is no preemption usage, but I am unable to check it.

Also, trying to add CPU affinity to the two latency-histogram threads does not seem to help.

Conclusion and question

I fully understand that the RaspberryPi cheap computer will not provides 5us jitter control, but at the moment it seem the CPU is able of more than what I got.

My question/s are:

How to ensure LinuxCNC is using Preemptive kernel features?
If using Preemptive, why is the jitter so high?
Any hint of how to improve and bound this jitter?

Notes:
LinuxCNC version used: 2.8.0~pre1, from today repository status.

Does "halrun" then "exit" say "Note: using POSIX realtime" ?

I think that there are kernel compilation options that need to be set to allow preempt-rt to work properly too.
wiki.linuxfoundation.org/realtime/docume...ions/preemptrt_setup

Thanks andipugh for your answer, I appreciate it!

I obviously set the CONFIG_PREEMPT_RT_FULL, but I did not had a look to the debug options nor other possibly related configurations. I will have a look deeper.

Halrun return the following line:
Note: Using POSIX realtime

Escain wrote: Note: Using POSIX realtime

OK, that means that it is using preempt-rt.

Off the Stretch preempt-rt ISO, with no tweaks, my PC is managing about 10uS latency, so you seem to be a bit unlucky.

I am using a version of machinekit on RPI3 ( 2 of them ), had to update and switch to software GL so it is more usable, there is nothing i could do to lower latency but i did not try much as it works nicely with the PiDiCNC controlers.