LinuxCNC latency and jitter improvements with PREEMPT_RT kernel parameter tuning

It's so rare to see a well written article written solely to help others.

Very commonly, articles are written by companies who want you to buy something.

Not quite as good as the first though...

Warning: I have a machine with a 12-core Intel i7 (numbered 0 to 11). The numbers shown in this answer will vary depending on the number of cores in each machine. Do not use Ctrl+C/Ctrl+V. You must read the articles cited in the thread. The calculations for the numbers are there.

Ladies and gentlemen, this looks promising!

It seems that POSIX realtime (PREEMPT_RT kernels) is finally working decently.

This is my machine (linuxcnc info):

The file:    /tmp/linuxcnc_info.txt
can be posted to a forum or a web site like:
     pastebin.com
in order to provide information about the linuxcnc
system and configuration.

VIEWER=kate
                Date: sáb 29 nov 2025 19:36:45 CET
            UTC Date: sáb 29 nov 2025 18:36:45 UTC
        this program: /home/xiuser/linuxcnc/scripts/linuxcnc_info
              uptime:  19:36:45 up  1:04,  1 user,  load average: 0,35, 0,50, 0,79
      lsb_release -a: Distributor ID:    Debian
Description:    Debian GNU/Linux 12 (bookworm)
Release:    12
Codename:    bookworm
            linuxcnc: /home/xiuser/linuxcnc/scripts/linuxcnc
                 pwd: /home/xiuser
                USER: xiuser
             LOGNAME: xiuser
                HOME: /home/xiuser
              EDITOR: 
              VISUAL: 
            LANGUAGE: 
                TERM: xterm-256color
           COLORTERM: truecolor
             DISPLAY: :0.0
             DESKTOP: lightdm-xsession
        display size:     3360x1080 pixels (889x285 millimeters)
                PATH: /home/xiuser/linuxcnc/scripts:/home/xiuser/linuxcnc/bin:/home/xiuser/.local/bin:/home/xiuser/linuxcnc/bin:/home/xiuser/linuxcnc/scripts:/home/xiuser/linuxcnc/tcl:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games

uname items:
         nodename -n: HPi7
      kernel-name -s: Linux
      kernel-vers -v: #1 SMP PREEMPT_RT Debian 6.12.57-1~bpo12+1 (2025-11-17)
          machine -m: x86_64
        processor -p: unknown
         platform -i: unknown
      oper system -o: GNU/Linux

/proc items:
             cmdline: BOOT_IMAGE=/boot/vmlinuz-6.12.57+deb12-rt-amd64 root=UUID=6dec6435-2cb8-427a-9697-c8d82ce45bdb ro net.ifnames=0 skew_tick=1 nosmt=force kthread_cpus=0-5 irqaffinity=0-5 rcu_nocb_poll rcu_nocbs=6-11 nohz=on nohz_full=6-11 isolcpus=6-11 intel_pstate=disable amd_pstate=disable idle=poll cpufreq.off=1 cpuidle.off=1 intel_idle.max_cstate=0 amd_idle.max_cstate=0 nowatchdog nosoftlockup i915.force_probe=!4680 xe.force_probe=4680
          model name:  12th Gen Intel(R) Core(TM) i7-12700
               cores:  12
             cpu MHz:  3685.961
             parport: 
              serial: 

Versions:
                 gcc: gcc (Debian 12.2.0-14+deb12u1) 12.2.0
              python: Python 3.11.2
                 git: git version 2.39.5
          git commit: 2cee0486b4
                 tcl: 8.6
                  tk: 8.6
               glade: glade 3.40.0

linuxcnc_var all:

     LINUXCNCVERSION: 2.10.0~pre0
LINUXCNC_AUX_GLADEVCP: /usr/share/linuxcnc/aux_gladevcp
LINUXCNC_AUX_EXAMPLES: /usr/share/linuxcnc/aux_examples
            REALTIME: /home/xiuser/linuxcnc/scripts/realtime
                 RTS: uspace
          HALLIB_DIR: /home/xiuser/linuxcnc/lib/hallib
              PYTHON: /usr/bin/python3.11

This is the latency histogram before tuning:

 

And this is the histogram after the changes...

 

First change: A new kernel that includes the new Intel Xe video driver, which will replace the venerable i915. Xe is in kernels >= 6.12.

kernel-vers -v: #1 SMP PREEMPT_RT Debian 6.12.57-1~bpo12+1 (2025-11-17)

Second change: the GRUB_CMDLINE_LINUX_DEFAULT boot line in Grub. Now cmdline is:

cmdline: BOOT_IMAGE=/boot/vmlinuz-6.12.57+deb12-rt-amd64 root=UUID=6dec6435-2cb8-427a-9697-c8d82ce45bdb ro net.ifnames=0 skew_tick=1 nosmt=force kthread_cpus=0-5 irqaffinity=0-5 rcu_nocb_poll rcu_nocbs=6-11 nohz=on nohz_full=6-11 isolcpus=6-11 intel_pstate=disable amd_pstate=disable idle=poll cpufreq.off=1 cpuidle.off=1 intel_idle.max_cstate=0 amd_idle.max_cstate=0 nowatchdog nosoftlockup i915.force_probe=!4680 xe.force_probe=4680

Note: force_probe => i915 OFF and Xe ON. My video device ID is 4680. Yours may be different. lspci gives the ID for each machine.

Third change: Modifying kernel parameters at runtime:

sudo sysctl kernel.timer_migration=0
sudo sysctl kernel.sched_rt_runtime_us=-1

 

tommylight wrote:

Found another one
blog.loriowar.com/how-to-reduce-jitter-on-linuxcnc

This one at least mentions network latency. In my view this is more important than system latency where you are not using software stepping.