Real-time audio in NixOS

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.

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.

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

As an alternative to nix-channels or cloning the project you can instead use it as a flake in a pure system.

{
  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 = [
          ## Other modules will also be here
          inputs.musnix.nixosModules.musnix
          ./configuration.nix ## Configuration file from regular /etc/nixos config
        ];
        specialArgs = { inherit inputs; }; ## Inherit inputs to configuration.nix so you can call inputs.inputname
       };
      };
  }

The above code snippet is a full user-side flake.nix, and it properly adds the repo's git repo to your config and inputs.musnix.nixosModules.musnix automatically adds a musnix modules.

The flake.nix, from the repo, automatically gets "imported" (not an actual import but close enough in function for non-flake users) and it adds musnix packages and modules via adding inputs.musnix.nixosModules.musnix to the modulessection.

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"
    

  • Set the max_user_freq of /sys/class/rtc/rtc0 to 2048.

  • Set the max-user-freq of /proc/sys/dev/hpet to 2048.

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

musnix.kernel.optimize

• NOTE: Enabling this option will rebuild your kernel.
• Description: If enabled, this option will configure the kernel to be preemptible, to use the deadline I/O scheduler, and to use the High Precision Event Timer (HPET).
• Type: boolean
• Default value: false

musnix.kernel.realtime

• NOTE: Enabling this option will rebuild your kernel.
• Description: If enabled, this option will apply the CONFIG_PREEMPT_RT patch to the kernel.
• Type: boolean
• Default value: false

musnix.kernel.packages

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

• Type: package

• Default value: `pkgs.linuxPackages_5_4_rt, which is the major version that NixPkgs unstable currently uses.

• Available packages:

  • pkgs.linuxPackages_3_18_rt
  • pkgs.linuxPackages_4_1_rt
  • pkgs.linuxPackages_4_4_rt
  • pkgs.linuxPackages_4_9_rt
  • pkgs.linuxPackages_4_11_rt
  • pkgs.linuxPackages_4_13_rt
  • pkgs.linuxPackages_4_14_rt
  • pkgs.linuxPackages_4_16_rt
  • pkgs.linuxPackages_4_18_rt
  • pkgs.linuxPackages_4_19_rt
  • pkgs.linuxPackages_5_0_rt
  • pkgs.linuxPackages_5_4_rt
  • pkgs.linuxPackages_5_6_rt

  or:

  • pkgs.linuxPackages_latest_rt (currently pkgs.linuxPackages_5_6_rt)

musnix.kernel.latencytop

• NOTE: Enabling this option will rebuild your kernel.
• NOTE: This option is only intended to be used for diagnostic purposes, and may cause other issues.
• Description: If enabled, this option will configure the kernel to use a latency tracking infrastructure that is used by the "latencytop" userspace tool.
• Type: boolean
• Default value: false

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

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.enable = true, otherwise false

• IRC: #musnix on freenode

• 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