Can't eek out that last bit of jitter

Actually I found a mistake in my grub command line.  Now my histogram is better, in that there are only a few excursions to debug now. 
 

It looks like only interrupt that seems to fire at roughly the same frequency is IRQ 20 which is described as

IO-APIC 20-fasteoi pata_pdc2027x[0000:03.05.0]

It is interesting that it fires on CPU1.  I thought I had a command in my grub setup to disable hyperthreading.
 

pata = Parallel ATA or the old hard drive comms, not used since .... long ago
If you are using an normal IDE hard drive, not much you can except maybe changing the IRQ number in BIOS, although some boards might now allow it.
If you are not using IDE, then disabling them in BIOS might just be possible.
Mind you, almost all SATA controllers can emulate PATA, so if your SSD is not set to AHCI, disabling IDE is not possible, but setting the SATA to AHCI in BIOS should remove PATA showing and using an IRQ.

Thank you Tommy,
You were right that that was the PATA setting in the bios.  I changed it to AHCI, and now that interrupt is no longer on the list anymore. 
 
Unfortunately, it doesn't appear that was the cause of my jitter.

Now the only things causing interrupts are
18 - Radeon HD 4250 internal graphics
LOC - (Local timer interrupts)
RES (Rescheduling Interrupts)

Is there anyway to see what causes the RES and LOC timer interrupts to see if the jitter is caused by one of those?

Sometimes 19 also happens, but it doesn't seem to sync up with times that the jitter has variance.

For some reason the nosmt=force setting in the kernal boot parameters doesn't seem to work anymore with the rtai kernel.  Should nosmt still work in the rtai kernel?

I have the following settings in:
ro quiet skew_tick=1 nosmt=force mitigations=auto,nosmt irqaffinity=0-1 kthread_cpus=0-1 rcu_nocb_poll rcu_nocbs=2-7 nohz=on nohz_full=2-7 isolcpus=2-7 intel_pstate=disable idle=poll cpufreq.off=1 intel_idle.max_cstate=0 amd_idle.max_cstate=0 processor.max_cstate=0 nowatchdog nosoftlockup acpi_irq_nobalance noirqbalance

I'm also using the irq affinity script from:  forum.linuxcnc.org/38-general-linuxcnc-q...ead-and-irq-affinity

Should I try a pci graphics card instead of the Radeon HD4250 integrated graphics?  Any recommendations?

webbyguy wrote:

Should I try a pci graphics card instead of the Radeon HD4250 integrated graphics?  Any recommendations?

Yes, Matrox G200 or G450, but those are AGP, so very old.
You can try any older ATI/AMD you can find for 10-30$, but first check the motherboard if it has AGP or PCI-E slot.

I got an R5 340X off of ebay, and I tried it out.  It didn't seem to make any difference in the RTAI or PREEMPT_RT kernels.
 

I feel like I'm missing some option.

I can't figure out why nosmt isn't working for the rtai kernel.

The only interrupts that are firing are RES, LOC and 24 which is the new graphics card.

Last time I had a bit of a succes on preempt_rt with moving the base thread to another cpu (only with 6.13), but perhaps worth a shot.
Change the CPU numbers and run as sudo. In the script CPU1 is the base thread and CPU2 the servo thread.

forum.linuxcnc.org/18-computer/48079-can...cnc?start=390#321927