LinuxCNC on Raspberry Pi 5 CPU 3 not running

Hi,
I used this image: rpi-5-debian-bookworm-6.1.61-rt15-arm64-ext4-2023-11-17-1520.img.xz
to set up a Raspberry Pi 5 with LinuxCNC.
So far it worked fine. Thanks a lot to all the people that put this image together!!

Now I discovered, that the third CPU (Number "2") of the Raspberry is not running. When LinuxCNC is started, using "htop", I can see, that the third CPU has no load, it is idleing. In the cmdline.txt there is this entry: isolcpus=2,3 and it seams, that LinuxCNC only uses the fourth (Number "3") CPU.
Is there something that can be done to use all the computing power of the Raspberry Pi 5?

2 cores run the day to day stuff
2 cores run the real-time stuff

Without isolating 2 cores latency takes a hit. It’s a common fix for latency on all platforms.

The third CPU (Number "2") of the Raspberry is not running.
LinuxCNC ("realtime stuff") only uses Core Number 3 (the fourth)

Is there something that can be done to use both Cores (Number 2 and 3 = third and fourth) for LinuxCNC?

Don't worry about it.

Everything will work fine when running a machine.

Have you tried htop when running gcode and actually running a machine ?

You can modify the isolcpu parameter, it's under the /boot folder in a subfolder, mind tho the boot folder has a different structure to the standard RPi one. It's been a while since I've used my RPi5 since helping Rod put the image together (due to the lack of a SPI driver for Mesa hardware).

Long story short latency will suffer without isolation. And depending how cache is shared between the cores there can be a performance hit by isolating the incorrect pair of CPUs (this was from a long forgotten paper). At the end of the day it's not outright performance this is vital with Linuxcnc but latency. Having a responsive interface and more importantly having latency within spec is all that matters. It's not as if you'll using the machine as a desktop whilst running a machine. Remember it's a single use computer, not a desktop.

Play around with isolating CPUs run some latency tests and see how you go. BTW what hardware are you using to control your machine.

Even on a multi core machine with no cpus isolated if the load doesn't require it you will have cores that are more or less idle.

@cornholio:
Thank you for the reply.
"Have you tried htop when running gcode and actually running a machine ?"
No, so far I only did "dry-tests".
I plan to retrofit a EMCO F1 CNC Woodworker. For this machine I bought a Benezan Tripple Beast ( a long time ago...). My plan is to set up LinuxCNC on a Raspberry Pi 5 to control the Tripplebeast.
My last machine was a portal router (DIY) that was (still is) controlled by DC-Servos with UHU3.0-controllers that I build (also a very long time ago)

Regarding the RP5: I hope I can find time to test the setup on the bench next week.
When I have news, I'll post them here.

Than you for your help.

I give up.

LinuxCNC just running a servo-thread only runs  a single Realtime thread. By definition thats gunna run on a single core as you observed. Maybe the base thread runs on a second core but i have never used a base thread.

When you run a ethernet connected device, machine latency is no longer important, its the network latency that limits performance. I think this means we should review how we evaluate PC suitability for LinuxCNC. latancy-histogram gives better results with multiple isolated cores but does that matter at all?