musnix

Real-time audio in NixOS

About

musnix provides a set of simple, high-level configuration options for doing real-time audio work in NixOS, including optimizing the kernel, applying the CONFIG_PREEMPT_RT patch to it, and adjusting various low-level system settings.

Basic Usage

Clone this project.

Add the following to your configuration.nix:

{
  imports =
    [ # ...
      /path/to/musnix/clone
    ];

  musnix.enable = true;
  users.users.johndoe.extraGroups = [ "audio" ];
}

To update musnix, run git pull.

Later sections of this document contain information about the various configuration options.

Using musnix as a channel

As an alternative to the above approach, you can instead add musnix as a channel:

sudo -i nix-channel --add https://github.com/musnix/musnix/archive/master.tar.gz musnix
sudo -i nix-channel --update musnix

Add <musnix> to imports in your configuration.nix:

{
  imports =
    [ # ...
      <musnix>
    ];

  musnix.enable = true;
  users.users.johndoe.extraGroups = [ "audio" ];
}

To update musnix, run:

sudo -i nix-channel --update musnix

Using musnix as a flake

As an alternative to the above approaches, you can also add musnix as a flake:

{
  inputs = {
    nixpkgs = { url = "github:NixOS/nixpkgs/nixos-unstable"; };
    musnix  = { url = "github:musnix/musnix"; };
  };
  outputs = inputs: rec {
    nixosConfigurations = {
      example-config = inputs.nixpkgs.lib.nixosSystem {
        system = "x86_64-linux";
        modules =
          [ # ...
            inputs.musnix.nixosModules.musnix
            ./configuration.nix
          ];
        specialArgs = { inherit inputs; };
      };
    };
  };
# ...

Base Options

musnix.enable

• Description: Enable musnix, a module for real-time audio.

• Type: boolean

• Default value: false

• NOTE: This option must be set to true to use other musnix base options.

• Details: If enabled, this option will do the following:

  • Activate the performance CPU frequency scaling governor.

  • Set vm.swappiness to 10.

  • Set the following udev rules:

    KERNEL=="rtc0", GROUP="audio"
    KERNEL=="hpet", GROUP="audio"
    DEVPATH=="/devices/virtual/misc/cpu_dma_latency", OWNER="root", GROUP="audio", MODE="0660"
    

  • Set the following PAM limits:

    @audio  -       memlock unlimited
    @audio  -       rtprio  99
    @audio  soft    nofile  99999
    @audio  hard    nofile  99999
    

  • Set the following environment variables to default install locations in NixOS:

    • VST_PATH
    • VST3_PATH
    • LXVST_PATH
    • LADSPA_PATH
    • LV2_PATH
    • DSSI_PATH

  • Allow users to install plugins in the following directories:

    • ~/.vst
    • ~/.vst3
    • ~/.lxvst
    • ~/.ladspa
    • ~/.lv2
    • ~/.dssi

musnix.alsaSeq.enable

• Description: If enabled, load ALSA Sequencer kernel modules. Currently, this only loads the snd_seq and snd_rawmidi modules.
• Type: boolean
• Default value: true

musnix.ffado.enable

• Description: If enabled, use the Free FireWire Audio Drivers (FFADO).
• Type: boolean
• Default value: false

musnix.rtcqs.enable

• Description: If enabled, install the rtcqs command-line utulity, which analyzes the system and makes suggestions about what to change to make it more audio-friendly.
• Type: boolean
• Default value: false

musnix.soundcardPciId

• Description: The PCI ID of the primary soundcard. Used to set the PCI latency timer.

  To find the PCI ID of your soundcard:

  lspci | grep -i audio
  

• Type: string

• Default value: ""

• Example value: "00:1b.0"

• NOTE: If you have a USB sound card, this option is not useful.

Kernel Options

NOTE: The following kernel options can be used without setting musnix.enable = true;

musnix.kernel.realtime

• NOTE: Enabling this option will rebuild your kernel.
• Description: If enabled, this option will enable CONFIG_PREEMPT_RT on the kernel.
  • For kernels 6.12 and later, PREEMPT_RT is available natively in the mainline kernel — no external patch is required. Set musnix.kernel.packages to any mainline kernel >= 6.12 and musnix will automatically apply the required configuration.
  • For kernels older than 6.12, set musnix.kernel.packages to one of the RT-patched packages provided by the musnix overlay (see below).
• Type: boolean
• Default value: false

musnix.kernel.packages

• Description: This option allows you to select the kernel used by musnix.

• Type: package

• Default value: pkgs.linuxPackages

  Mainline kernels 6.12+ (recommended) — musnix will automatically enable PREEMPT_RT via kernel configuration overrides; no patched package is needed:

  musnix.kernel.packages = pkgs.linuxPackages;        # default NixOS kernel (if >= 6.12)
  musnix.kernel.packages = pkgs.linuxPackages_6_12;   # specific LTS version
  musnix.kernel.packages = pkgs.linuxPackages_latest; # latest mainline
  

  Pre-6.12 kernels — use an RT-patched package from the musnix overlay:

  • pkgs.linuxPackages_6_1_rt
  • pkgs.linuxPackages_6_6_rt
  • pkgs.linuxPackages_6_8_rt
  • pkgs.linuxPackages_6_9_rt
  • pkgs.linuxPackages_6_11_rt

  or the convenience aliases:

  • pkgs.linuxPackages_rt (currently pkgs.linuxPackages_6_6_rt)
  • pkgs.linuxPackages_latest_rt (currently pkgs.linuxPackages_6_11_rt)

  Note: When a mainline kernel >= 6.12 is selected, musnix overrides it with EXPERT=y, PREEMPT_RT=y, and unsets options that are mutually exclusive with RT (PREEMPT_VOLUNTARY, PREEMPT_FULL, RT_GROUP_SCHED). ignoreConfigErrors = true is set so that version-specific disappearing symbols (e.g. i915 GVT, Rust assertions) do not break the build.

rtirq Options

NOTES:

• The following rtirq options can be used without setting musnix.enable = true;
• musnix.kernel.realtime must be set to true to use these options.

musnix can also install and run the rtirq script as a systemd service.

To see a list of options for using this feature, use the following command:

nixos-option musnix.rtirq

To see a description of one of the listed options (in this case enable):

nixos-option musnix.rtirq.enable

Other Options

musnix.das_watchdog.enable

• Description: If enabled, start the das_watchdog service. This service will ensure that a realtime process won't hang the machine.
• Type: boolean
• Default value: true if musnix.kernel.realtime = true, otherwise false

Communication

• IRC: #musnix on libera.chat

More Information

• http://wiki.linuxaudio.org/wiki/system_configuration
• http://wiki.linuxaudio.org/wiki/system_configuration#rtirq
• http://subversion.ffado.org/wiki/IrqPriorities
• https://wiki.archlinux.org/index.php/Pro_Audio
• https://nixos.org/wiki/Audio_HOWTO