Latency Spikes With No Load

Just like the title says, my latency spikes when my system isn't under load.

I am running a pine64 on armbian with a preempt-rt kernel.

Attached is the Latency Histogram for no base, no load, no base heavy load.

The standard latency test has a max jitter of about 50,000 when under load or when running that base thread.

Any Ideas as how I can fix this? I haven't really seen anyone use a pine before. It a 4 or so year old board so there is probably a reason why.

If this was x86 I would be temped to add "idle=poll" to the kernel command line via grub
Not sure if you can do a similar thing here.

It might also be related to power management, is there any "turbo" mode or clock speed
switching enabled?

It uses Uboot I believe. There is the option to edit something similar to cmdline.txt for the pi. That's how I added the isolcpus.

I'll see if I can add something like idle=poll.

I'll also see if I can force a bit of an overclock to make sure it isn't under underclocking on me. I have a desktop cooler strapped to this thing so heat isn't an issue.

I'll update when I add these changes and test them.

I set the cpu governor to performance and the latency looks alot better when not under load.

Given the latency histogram when the pine was under load, would that be usable for a system with a 7i76e?

Yeah. My result looked just like the graph under load. If it inst good enough, I'll try to overclock the system and see if it improves.

I haven't tried that yet. I'll try it again when the backup is finished.

It seems weird to me that overclocking would make it worse. Would it at least benefit from a memory overclock?

Nothing seems to get it down much lower. I'll just switch my focus to the pi 4 which has been done before.

The 80 or 90 usec seems ok

Do you have a Mesa Ethernet card to try?

Worst case you may need to run at 500Hz servo thread rather than 1 KHz
(This is normally OK unless you have a very high acceleration machine)