2.8.2/Buster: should I use isolcpus=1 or cpuset

Hi all

After some quite happy years with 2.7.x, I'm testing some refurbished machines with 2.8.2. IIRC, I have read quite a few times that isolcpus=1 was deprecated and that last linuxcnc versions use cpuset. So I'm trying to use cpuset, yet I'm getting better latencies with isolcpus=

Could you guys give me any pointer about how to use cpuset or should I continue to use isolcpus=?

Cheers

It turns out that the word "deprecated" is in kernel parameters doc for 4.19

www.kernel.org/doc/html/v4.19/admin-guid...rnel-parameters.html

You are very probably right about cpuset not mentioned here, but I got it indirectly from this helpful andypugh's answer

So, going to SabbyX's github , one gets info about both isolcpu's and cpuset

Yet some more clarity would be quite helpful, he talks about insmod:

...to see what the latency could really be using isolcpus/make sure linuxcnc isn't getting disturbed by other system tasks

In the cpuset initscript example, again the code seems quite clear to me, however having also the commands to start each part of linuxcnc in each cpuset would be also helpful

I'm going to test, among others, a 4-core computer and, since this is a more powerful machine, isolcpus is deprecated by kernel developers so probably missing in the future, and cpuset is said to be "better than isolcpus", I'm trying cpuset (at this moment in an Athlon X2 dual core machine). Yet I'm getting better results with isolcpus, so I guess I'm getting something wrong about launching linuxcnc via cpuset (and probably so even via isolcpus) .

So my questions are about:

1- How to launch linuxcnc to be sure it gets one or more CPUs exclusively for himself.
2- In the example, SabbyX seems to put the GUI (axis, whatever) in the second core, which wouldn't be a real-time one. ¿Am I getting it right? What command should I use to do that?

(edited twice, first for typos then for tag mess)
Cheers

Well, I'm fighting my way through it right now. Should I be able to get somewhere I'll report about it. Would be easier if anyone could confirm if launching linuxcnc via taskset is OK, etc. In absence of confirmation/instrucyions, I'll do it my way...

About editing message tags in source: nope, I tried it some three times before giving up. It got messed anyway

I use isolcpus

I generally run quad cores
so the CPUs numbering starts from "0" zero to 3 in my case.

I tend to use 1,2,3

but I test all configs on latency
set / change
reboot
retest for about 5 mins
set/change
repeat

write the numbers on a notepad and use the lowest number then do a long 2 hour test.

Well, I have to give cpuset a try yet, but I have been comparing 2.8.2-rt vs 2.8.2-rtpreempt and Debian vs Devuan. I guess cpuset will have to wait until next weekend. I have been reading linuxcnc script so I have now a guess on how the different parts of it could be launched.

Test computer is an HPCompaq 5850 MT, Athlon 64 X" dualcore, with an almost useless BIOS.

My boot line has nowatchdog usbcore.nousb acpi_irq_nobalance isolcpus=1 idle=poll in all cases. The usbcore.nousb part for sure seems to be useless after you work your smp-affinity but I maintained it just because the first test had it. nowatchdog has quite good effects once system's load gets up. I need idle=poll because this computer BIOS doesn't allow you to set anything related to C-states.

Devuan 3 Beowulf is Debian Buster without systemd/pulseaudio. I hate systemd's guts and pulseaudio is his closest friend. What I did on Devuan is just installing the basic system utilities, then a very basic Xorg server, with just WindowMaker window manager and xterm, then installed linuxcnc kernel and application.

Both Devuan Beowulf and Debian Buster let me down when doing update-grub if there was more than a system installed, I don't know where the grub.cfgs are gone but I know for sure that I needed to edit the boot line at boot time in grub itself

With 2.8.2-rtpreempt on both Debian and Devuan, I noticed that, even with isolcpus=1 on the boot line, my NIC (driver tg3, and probably others like USB) was sending IRQs to cpu 1. So latency tests were bad, well over 100 usec, reaching even 600-800 usec latency in just a few minutes. I needed to put at work an smp-affinity script along the lines of this post . I just stripped the ubuntu parts and placed it on /etc/rc.local on Devuan so it is executed at boot. On Debian, where systemd has to control the rackets, that's just bad practice. So I just execute it myself before launching latency-histogram with "taskset 1 latency-histogram". Then results are quite good enough to do software stepping, just a little over 25 usec on Debian and a little under 20 usec on Devuan.

2.8.2-rt worked wonders for me. No need to do nothing once you have isolcpus=1 on your grub boot line. Without it, it hangs wonderfully. Latency results are almost the same thing on Debian and Devuan, about 5 usec. I'm still dazzled. I noticed than the system load and memory consumption were better on the light weight Devuan system, let's say about half the RAM and 40% less load.


Debian Buster 2.8.2-rtpreempt with no smp-affinity work

Debian Buster 2.8.2-rtpreempt with retouched smp_affinity

Debian Buster 2.8.2-rt

Devuan Beowulf 2.8.2-rtpreempt

Devuan Beowulf 2.8.2-rt


 

Well, cpuset didn't gave any great results. So far, I got better latencies by using isolcpus=

Hi tatel, I just built my system. I am running Debian 12 with no window manager, just twm and an xterm started with xinit. Without isolcpus and the default BIOS settings I was seeing spikes in jitter to 175.

I fixed the BIOS settings to turn off power management and it dropped down to about 25, but still had an occasional spike.

Then I set isolcpus to 2,3 (quad core machine) and ran latency-histogram again. Rock steady at 5 jitter.

Finally my question: I looked at ps and see that both latency-histogram and glxgears (I ran 6 copies of glxgears) are running on CPU 0. I don't recognize anything on CPU 2 or 3 as being related to latency-histogram. Should I be using taskset to make latency-histogram run on the isolated CPUs? And when I actually run linuxcnc, should I be using taskset to run that on the isolated CPUs?

Thanks,
Dan