Configuration Files in Spack

Spack has many configuration files. Here is a quick list of them, in case you want to skip directly to specific docs:

YAML Format

Spack configuration files are written in YAML. We chose YAML because it’s human readable, but also versatile in that it supports dictionaries, lists, and nested sections. For more details on the format, see yaml.org and libyaml. Here is an example config.yaml file:

config:
  install_tree: $spack/opt/spack
  module_roots:
    lmod:   $spack/share/spack/lmod
  build_stage:
    - $tempdir
    - /nfs/tmp2/$user

Each spack configuration files is nested under a top-level section corresponding to its name. So, config.yaml starts with config:, and mirrors.yaml starts with mirrors:, etc.

Configuration Scopes

Spack pulls configuration data from files in several directories. There are four configuration scopes. From lowest to highest:

  1. defaults: Stored in $(prefix)/etc/spack/defaults/. These are the “factory” settings. Users should generally not modify the settings here, but should override them in other configuration scopes. The defaults here will change from version to version of Spack.
  2. system: Stored in /etc/spack. These are settings for this machine, or for all machines on which this file system is mounted. The site scope can be used for settings idiosyncratic to a particular machine, such as the locations of compilers or external packages. These settings are presumably controlled by someone with root access on the machine.
  3. site: Stored in $(prefix)/etc/spack/. Settings here affect only this instance of Spack, and they override defaults. The site scope can can be used for per-project settings (one spack instance per project) or for site-wide settings on a multi-user machine (e.g., for a common spack instance).
  4. user: Stored in the home directory: ~/.spack/. These settings affect all instances of Spack and take the highest precedence.

Each configuration directory may contain several configuration files, such as config.yaml, compilers.yaml, or mirrors.yaml. When configurations conflict, settings from higher-precedence scopes override lower-precedence settings.

Commands that modify scopes (e.g., spack compilers, spack repo, etc.) take a --scope=<name> parameter that you can use to control which scope is modified. By default they modify the highest-precedence scope.

Platform-specific scopes

For each scope above, there can also be platform-specific settings. For example, on Blue Gene/Q machines, Spack needs to know the location of cross-compilers for the compute nodes. This configuration is in etc/spack/defaults/bgq/compilers.yaml. It will take precedence over settings in the defaults scope, but can still be overridden by settings in system, system/bgq, site, site/bgq, user, or user/bgq. So, the full scope precedence is:

  1. defaults
  2. defaults/<platform>
  3. system
  4. system/<platform>
  5. site
  6. site/<platform>
  7. user
  8. user/<platform>

You can get the name to use for <platform> by running spack arch --platform. The system config scope has a <platform> section for sites at which /etc is mounted on multiple heterogeneous machines.

Scope precedence

When spack queries for configuration parameters, it searches in higher-precedence scopes first. So, settings in a higher-precedence file can override those with the same key in a lower-precedence one. For list-valued settings, Spack prepends higher-precedence settings to lower-precedence settings. Completely ignoring higher-level configuration options is supported with the :: notation for keys (see Overriding entire sections below).

Simple keys

Let’s look at an example of overriding a single key in a Spack file. If your configurations look like this:

defaults scope:

config:
  install_tree: $spack/opt/spack
  module_roots:
    lmod:   $spack/share/spack/lmod
  build_stage:
    - $tempdir
    - /nfs/tmp2/$user

site scope:

config:
  install_tree: /some/other/directory

Spack will only override install_tree in the config section, and will take the site preferences for other settings. You can see the final, combined configuration with the spack config get <configtype> command:

$ spack config get config
config:
  install_tree: /some/other/directory
  module_roots:
    lmod:   $spack/share/spack/lmod
  build_stage:
    - $tempdir
    - /nfs/tmp2/$user
$ _

Overriding entire sections

Above, the site config.yaml only overrides specific settings in the default config.yaml. Sometimes, it is useful to completely override lower-precedence settings. To do this, you can use two colons at the end of a key in a configuration file. For example, if the site config.yaml above looks like this:

config::
  install_tree: /some/other/directory

Spack will ignore all lower-precedence configuration under the config:: section:

$ spack config get config
config:
  install_tree: /some/other/directory

List-valued settings

Let’s revisit the config.yaml example one more time. The build_stage setting’s value is an ordered list of directories:

defaults

build_stage:
  - $tempdir
  - /nfs/tmp2/$user

Suppose the user configuration adds its own list of build_stage paths:

user

build_stage:
  - /lustre-scratch/$user
  - ~/mystage

Spack will first look at the paths in the site config.yaml, then the paths in the user’s ~/.spack/config.yaml. The list in the higher-precedence scope is prepended to the defaults. spack config get config shows the result:

$ spack config get config
config:
  install_tree: /some/other/directory
  module_roots:
    lmod:   $spack/share/spack/lmod
  build_stage:
    - /lustre-scratch/$user
    - ~/mystage
    - $tempdir
    - /nfs/tmp2/$user
$ _

As in Overriding entire sections, the higher-precedence scope can completely override the lower-precedence scope using ::. So if the user config looked like this:

user

build_stage::
  - /lustre-scratch/$user
  - ~/mystage

The merged configuration would look like this:

$ spack config get config
config:
  install_tree: /some/other/directory
  module_roots:
    lmod:   $spack/share/spack/lmod
  build_stage:
    - /lustre-scratch/$user
    - ~/mystage
$ _