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I edited my original post to include changing the display protocol from Wayland to X11. Comments by rodw and cornholio were based on the terrible jitter that Wayland induces as shown above.

Note that Kunena has the worst forum editor I've ever used, and it was quite a job to edit those posts and restore the formatting. I wonder if forum posting would increase with forum software that was more user friendly.

Looks a lot better.

One thing that was strange when running a basic config I got a real time error. hmmmmm

ATM the best image I've come across is Joco's rpi5 image rod has on his googledrive, that has a 6.6.54 kernel. It's my choice of the pre-built images.

I installed mate desktop, with 2 cores isolated, watching youtube latency is about 17-18us on my pi.

The good news is switching to X11 is trivial.

You can check the session type:    wayland

Launch Rasp-config command line tool in a terminal:
Arrow down to "Advanced Options", and enter.
Arrow down to "Wayland                 Switch between X and Wayland backends", and enter.
If not on the line that says "W1 X11     Openbox window manager with X11 backend", arrow to it and enter.
The popup should say "Openbox on X11 is active" and have "Ok" highlighted. Enter.
It will return to the main screen. Use left-righ arrows to select "Finish". Enter.
Popup asks if you would like to reboot now, with "Yes" highlighted. Enter.

After reboot:    x11

Now here's the interesting apples to apples comparison. The only difference between these two pre-jitter-optimization baseline latency runs is Wayland vs X11.

Wayland:
 

X11:
 

 

Kernel as built following instructions

All my Pi's are 4GB.

Along with my Rpi5 I also have a RPi400, basically the same as a RPi4 with an slightly upped clock speed. I can say I've never come across this issue (even when rod was polishing his iamge utility).
I don't know if this will actually help, but you could try comparing the config.txt files on the Linuxcnc image & the Rasp OS image and see if there is anything that sticks out. This is just a guess.

It might pay to monitor the 3v3 line when booting, both versions, and see if there is anything untoward. Again just another guess.

But yeah it does seem like a confusing issue.

First of all your opening post was great. I'm actaully giving it a go now, just wish I had a nvme drive hooked up to the PCIe bus.
There's a couple of points regarding Linux kernels.

Sometimes the latest kernel may not be the optimum for your hardware.
The kernel you are using is a "release candidate" whilst usable for all intents is still going through some work.
The 6.1.y branch works fine on the RPi4, I think the kernel in the Linuxcnc image maybe 6.1.54
The RPi platforms with regards to Linuxcnc are a little different to the x86 platforms. Your Rpi4 & Rpi5 hardware is what it is. Whereas you may need a specific kernel dpending which x86 motherboard you buy.
The Rpi branch that appears to be active is the 6.6.y branch, so one could assume this is the branch that is receiving the most "bug fixies".
Simple things like isolating cores 2,3 will see latency drop. There doesn't seem to be as many options/tricks for latency on the RPi platforms as the x86 platforms.
I would chose to use a kernel that is a couple of version behind the the latest and greatest if it more tried in the real world and has better latency.
In my discussions with royka we talked about the fact that a kernel that is a little older is sometimes the better option.
Due to the issues with wayland I would recommend the use of the image form the Download page, Xorg is used. Plus it's easier to deploy

Mind tho I'm not saying this is the only way to do it.
What I would suggest is trying some of the earlier kernels to see how performance differs.

One thing regarding the 6.6.y branch.
The latest version is 6.6.70 which I couldn't find any real time patches for.
The previous commit is 6.6.65 for which there are rt patches, but for some reason the overlay's aren't in the source. Which is a pain when building an image using Rod's image builder.

It seems, ATM, the deeper you dive it can seem a little confusing.

Sorry for the multiple posts but this editor is PITA.

In a Debian environment tasksel allows you to install a number of desktops. There is a (X) reason why we use XFCE in the linuxcnc images. It uses X windows not wayland

I can guarantee it's not bricked. I loaded the raspberry Pi OS onto it after that with no problem and was able to access it via SSH to download data from the Pi library. I tried 2.93 as well and got a similar result only this time there were some penguins at the top of the screen instead of skulls  

Just updated the bootloader and tried again. Got a similar result but now the ACT LED is flashing consistently instead of barely visible. 

I'm surprised nobody else is experiencing this. Am I the only 8G user to try these? Do I need to have some additional board attached for it to boot properly?

[Edit, after hearing it might be bricked, I tried the same card on a 4gig Pi4B that I've been using perfectly fine for years with the same result] 

It would be interesting to see the latency values after switching to X. This can be done via the config utility for the Rpi, the actual binary name escapes me ATM.

Thanks for the heads up regarding wayland and axis.

If you're using a Rasp OS image you'll want to swap from wayland to X if you are looking to run axis, the "compatibility layer" is "not quite optimum".

Kernel built whilst using Rod's imager, running over a ssh session.

gene_weber wrote:

I will probably create a separate thread about jitter optimization.

This is badly needed based on the reported latency which is unusable

I have a 4b that is bricked like this.
Try updating the firmware using the Pi Imager. That may work. Otherwise I have no solution

Create the base .config file for the Raspberry Pi 4 64-bit:
NOTE: For a Raspberry Pi 5 64-bit the lines should be:

Make / launch the menuconfig utility:
Install the required package:
Under General Setup, select: Fully Preemptible Kernel (Real-Time)Exit and Save.


Change Local version name. vi .config and change the following line:To:

Build the kernel and log the output (Note this took over 2 hr 10 min):
NOTE: During the compilation process the temperature of the ARM processor increased from ~30°C to as high as 43°C. I have a heat-sink and cooling fan on my Pi. If you don't, you may want to keep an eye on the temp (/usr/bin/vcgencmd measure_temp). The maximum rating on the Pi is 85°C.


Install the kernel modules onto the boot media and log the output:
create a backup image of the current kernel, install the fresh kernel image, overlays, README, and unmount the partitions:

Reboot the Raspberry Pi to run the new kernel:
Check kernel after reboot:    Linux raspberrypi 6.13.0-rc7-rt3-v8_full_preempt+ #1 SMP PREEMPT_RT Tue Jan 14 12:49:46 EST 2025 aarch64 GNU/Linux


From the LinuxCNC downloads page , download LinuxCNC Uspace 2.9.3 arm64 .

Install the Gdebi installer:Run the Gdebi installer and install linuxcnc-uspace_2.9.3_arm64.deb

EDITED 1/16/25: Changed image to X11 pre-optimization latency.
Ran a baseline latency test before making any jitter optimizations.    Note: Using POSIX realtime


I checked the real-time attributes of the latency-histogram process (the process ID for this run was 10527):    pid 10527's current scheduling policy: SCHED_OTHER
    pid 10527's current scheduling priority: 0

I had thought the policy would be SCHED_FIFO, or SCHED_RR. But even after I added CPU isolation it remains SCHED_OTHER.

I will probably create a separate thread about jitter optimization.