5Stack

Appearance

Low-Latency Kernel on Ubuntu ​

A low-latency kernel lets the CPU switch to a game server's work sooner, which matters for Counter-Strike: hit registration and movement depend on every tick being processed on time, with as little jitter as possible.

It helps by:

  • Minimizing processing delays in the operating system
  • Providing more predictable response times
  • Reducing system jitter and inconsistencies

WARNING

  • Not every node needs it. Low latency trades some throughput for faster response.
  • Keep a way back: the steps below only add a boot option, so removing it restores the old behaviour.

Ubuntu no longer builds a separate low-latency kernel (it was dropped in 25.04). Canonical's replacement is the standard generic kernel booted with preempt=full, which makes the kernel fully preemptible. This also works on 24.04.

  1. Add the boot option and update GRUB:

    bash
    echo 'GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT preempt=full"' \
  | sudo tee /etc/default/grub.d/99-5stack-low-latency.cfg
sudo update-grub

    Files in /etc/default/grub.d/ are read after /etc/default/grub, so this adds to the boot options you already have instead of replacing them.

  2. Reboot:

    bash
    sudo reboot

  3. Verify that preempt=full is active:

    bash
    cat /proc/cmdline

    The output should include preempt=full.

Within a minute the node shows Low Latency on the Game Server Nodes page.

To undo it, delete /etc/default/grub.d/99-5stack-low-latency.cfg, run sudo update-grub and reboot.

Legacy: the linux-lowlatency kernel ​

Older guides, including earlier versions of this one, installed the linux-lowlatency kernel. Ubuntu no longer builds it, so 5stack shows a warning on nodes that still run it. Move those nodes to the generic kernel:

  1. Install the generic kernel and add the boot option from the section above:

    bash
    sudo apt update
sudo apt install linux-generic
echo 'GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT preempt=full"' \
  | sudo tee /etc/default/grub.d/99-5stack-low-latency.cfg
sudo update-grub

  2. Reboot, then check that you are on the generic kernel:

    bash
    uname -r

    The output should end in -generic. If it still shows lowlatency, choose the generic kernel for the next boot first. One way to do this is GrubReboot, for example grub-reboot '1>4', where the numbers depend on your GRUB menu.

  3. Once uname -r shows the generic kernel and cat /proc/cmdline includes preempt=full, remove the low-latency packages:

    bash
    LOWLAT_PACKAGES=$(dpkg --list | grep lowlatency | awk '{print $2}')
sudo apt purge -y $LOWLAT_PACKAGES
sudo update-grub

The warning on the node clears on its next status update.

INFO

For even better low-latency performance, set your CPU Governor to "performance" mode and make sure the setting persists after reboot.