Jitter 40.000 and "unexpected realtime delay on task 0 with 57000" good enought?

Hello,

I am new to linux cnc and I bought before installation a minicomputer (and sherline mill) and installed
Debian 10 Buster PREEMPT-RT ISO. I didn't know anything about jitter .... my computer is

ASRock J4005B-ITX,
Intel Celeron Dual-Core J4005, 2.0GHz (Turbo: 2.7GHz),
Intel HD600 Graphics
with parallel port

I have disabled every processor energy saving option in the bios. This droped the Max Jitter only a little bit (-4.000)
I have read "If the Max Jitter is more like 30-50 microseconds, you can still get good results..."
but the displayed error unsettles me.

Thanks for any advice. If someone sees something that I could safely turn of in the bios would be great also.



Jitter starts around 25.000 and then gets higher after a few minutes.

Thank you, Best Regards
Roman

Are you intending to use parallel BOB to control the drivers/motors or a mesa card?
Those measurements might not be great for a parallel port/BOB based system.

As for getting more consistency what settings do you have on the linux boot options?

I will need the parallel port. Boot option ... I found this
BOOT_IMAGE=/boot/vmlinuz-4.19.0-11-rt-amd64 root=UUID=56a0a235-95ea-4827-8f16-5d4340212beb ro quiet
under /proc/cmdline. I have installed the official linuxcnc image and attached the dmesg output.

Share memory from auto to whatever, 16 or 32 or 64MB...
If left in auto it will mess up latency s it will resize the memory reserved for video when it needs it.

On the share memory for video also try different sizes. I have found better prerformance with a smaller value on some systems.

Given you are dual core you might need to look into cpu isolation. I would also do an explicit check on the MHz of the cpu when running your latency tests. Just to make sure that even though you have disabled speed stepping in the bios that you are in fact running at max MHz.

On the max MHz front I have read comments about the fact that the Turbo mode can actually hurt latency not help it so look for you natural top speed (2GHz).

I used this command (might be easier to wrap in a shell script) to check on MHz of cpus I have tested.

watch "grep -E '^model name|^cpu MHz' /proc/cpuinfo"


These are the kernerl boot flags I have used when tuning a dual core system which might be of use:

intel_idle.max_cstate=0 processor.max_cstate=0 isolcpus=1 noirqbalance

you may also need to consider idle=poll if you are not seeing max cpu speed OR use a CPU hog script to drive the CPU to max speed.

Cheers - J.

I set the shared memory to 64MB (it is the lowest possible setting).
Both two cores run at 1996.8 Mhz. But max jitter is only bit lower ~ 39.000