About unexpected realtime delay ...

Hi Guys,

After tweaking LinuxCNC on my ASROCK-J3455-ITX board, I got really good latency not exceeding 5000nS. However, the minimum BASE_PERIOD I can set without getting unexpected realtime dalay is 39500nS. That gives me confident 25.3kHz pulse rate. If I go less than 39500nS then as close I approach ~32000nS as sooner after starting LinuxCNC I get the error. I also noticed, that I needed to increase BASE_PERIOD after I added second parallel port. With one parallel port BASE_PERIOD was set to 37500nS and no error was seen.

My question is "Is that CPU speed limits BASE_PERIOD in my case?"

CPU J3455 1.5GHz.

Maksym

I don't think the speed of that CPU should be a limiting factor.
(in the ancient past Linuxcnc had been made to work on PCs as slow as a 500Mhz P3.) With today's OS and Linuxcnc, I would not try to set up a system with anything much slower than a 1Ghz Pentium D with 1G of ram and 2 would be preferable.

How much memory does your PC have? What OS and Kernel version are you running? How long did you run the latency test, and what other tasks did you try to run while testing it?

It has 8G dual channel RAM installed, but only 4 are visible because of 32bit kernel.

myLinuxCNC 3.4-9-rtai-686-pae #1 SMP PREEMPT Debian 3.4.55-4linuxcnc i686 GNU/Linux

LinuxCNC 2.8 from master branch.

I tried everything, many glxgears, mozilla, youtube .... until responce to UI became very slow. In that state I left latency test for approx. 1 hour.

That certainly is enough memory (half or a fourth of that would have probably sufficed.)

I think it is OK to heavy load test it like that for a worst case scenario, but I think it is also very important to test it under more normal loads, and maybe even more so under light loads (when power saving features are most likely to kick in.)

For testing I like to run 2 or 3 GLXgears, open and close a few windows, brows the internet a bit, and copy some files to and from the network, plus let it set idle for several hours.

I'm not too up on the intricacies of setting up a Preem-RT pc. I've only just started messing with one install of it myself, and it isn't even finished. It will run the machine, but I still need to mount and button up all the boards and wiring to make it look neat. I did find that using isolcpus does seem to help latency on this machine

Latest findings. "unexpected realtime delay" appears only if I add function write for second parallel port (it is actually primary integrated port)

addf parport.1.write base-thread

If this function commented then LinuxCNC works with BASE_PERIOD=25000 with no errors

integrated parallel ports on newer motherboards very often use the LPC interface
which is fairly slow compared to the PCI/PCIE bus used by add-in cards. It may take
several usec for the port write access to the on board port