Package Settings (packages.yaml)
Spack allows you to customize how your software is built through the
packages.yaml file. Using it, you can make Spack prefer particular
implementations of virtual dependencies (e.g., MPI or BLAS/LAPACK),
or you can make it prefer to build with particular compilers. You can
also tell Spack to use external software installations already
present on your system.
At a high level, the
packages.yaml file is structured like this:
packages: package1: # settings for package1 package2: # settings for package2 # ... all: # settings that apply to all packages.
So you can either set build preferences specifically for one package, or you can specify that certain settings should apply to all packages. The types of settings you can customize are described in detail below.
Spack’s build defaults are in the default
etc/spack/defaults/packages.yaml file. You can override them in
etc/spack/packages.yaml. For more
details on how this works, see Configuration Scopes.
Spack can be configured to use externally-installed packages rather than building its own packages. This may be desirable if machines ship with system packages, such as a customized MPI that should be used instead of Spack building its own MPI.
External packages are configured through the
Here’s an example of an external configuration:
packages: openmpi: externals: - spec: "email@example.comfirstname.lastname@example.org arch=linux-debian7-x86_64" prefix: /opt/openmpi-1.4.3 - spec: "email@example.comfirstname.lastname@example.org arch=linux-debian7-x86_64+debug" prefix: /opt/openmpi-1.4.3-debug - spec: "email@example.comfirstname.lastname@example.org arch=linux-debian7-x86_64" prefix: /opt/openmpi-1.6.5-intel
This example lists three installations of OpenMPI, one built with GCC,
one built with GCC and debug information, and another built with Intel.
If Spack is asked to build a package that uses one of these MPIs as a
dependency, it will use the pre-installed OpenMPI in
the given directory. Note that the specified path is the top-level
install prefix, not the
packages.yaml can also be used to specify modules to load instead
of the installation prefixes. The following example says that module
CMake/3.7.2 provides cmake version 3.7.2.
cmake: externals: - spec: email@example.com modules: - CMake/3.7.2
packages.yaml begins with a
packages: attribute, followed
by a list of package names. To specify externals, add an
attribute under the package name, which lists externals.
Each external should specify a
spec: string that should be as
well-defined as reasonably possible. If a
package lacks a spec component, such as missing a compiler or
package version, then Spack will guess the missing component based
on its most-favored packages, and it may guess incorrectly.
Each package version and compiler listed in an external should have entries in Spack’s packages and compiler configuration, even though the package and compiler may not ever be built.
Prevent packages from being built from sources
Adding an external spec in
packages.yaml allows Spack to use an external location,
but it does not prevent Spack from building packages from sources. In the above example,
Spack might choose for many valid reasons to start building and linking with the
latest version of OpenMPI rather than continue using the pre-installed OpenMPI versions.
To prevent this, the
packages.yaml configuration also allows packages
to be flagged as non-buildable. The previous example could be modified to
packages: openmpi: externals: - spec: "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64" prefix: /opt/openmpi-1.4.3 - spec: "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64+debug" prefix: /opt/openmpi-1.4.3-debug - spec: "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64" prefix: /opt/openmpi-1.6.5-intel buildable: False
The addition of the
buildable flag tells Spack that it should never build
its own version of OpenMPI from sources, and it will instead always rely on a pre-built
concretizer:reuse is on (see Concretization Settings (concretizer.yaml) for more information on that flag)
pre-built specs include specs already available from a local store, an upstream store, a registered
buildcache or specs marked as externals in
concretizer:reuse is off, only
external specs in
packages.yaml are included in the list of pre-built specs.
If an external module is specified as not buildable, then Spack will load the external module into the build environment which can be used for linking.
buildable does not need to be paired with external packages.
It could also be used alone to forbid packages that may be
buggy or otherwise undesirable.
Non-buildable virtual packages
Virtual packages in Spack can also be specified as not buildable, and external implementations can be provided. In the example above, OpenMPI is configured as not buildable, but Spack will often prefer other MPI implementations over the externally available OpenMPI. Spack can be configured with every MPI provider not buildable individually, but more conveniently:
packages: mpi: buildable: False openmpi: externals: - spec: "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64" prefix: /opt/openmpi-1.4.3 - spec: "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64+debug" prefix: /opt/openmpi-1.4.3-debug - spec: "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64" prefix: /opt/openmpi-1.6.5-intel
Spack can then use any of the listed external implementations of MPI to satisfy a dependency, and will choose depending on the compiler and architecture.
In cases where the concretizer is configured to reuse specs, and other
(available via stores or buildcaches) are not wanted, Spack can be configured to require
specs matching only the available externals:
packages: mpi: buildable: False require: - one_of: [ "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64", "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64+debug", "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64" ] openmpi: externals: - spec: "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64" prefix: /opt/openmpi-1.4.3 - spec: "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64+debug" prefix: /opt/openmpi-1.4.3-debug - spec: "firstname.lastname@example.orgemail@example.com arch=linux-debian7-x86_64" prefix: /opt/openmpi-1.6.5-intel
This configuration prevents any spec using MPI and originating from stores or buildcaches to be reused,
unless it matches the requirements under
packages:mpi:require. For more information on requirements see
Automatically Find External Packages
You can run the spack external find command
to search for system-provided packages and add them to
After running this command your
packages.yaml may include new entries:
packages: cmake: externals: - spec: firstname.lastname@example.org prefix: /usr
Generally this is useful for detecting a small set of commonly-used packages; for now this is generally limited to finding build-only dependencies. Specific limitations include:
Packages are not discoverable by default: For a package to be discoverable with
spack external find, it needs to add special logic. See here for more details.
The logic does not search through module files, it can only detect packages with executables defined in
PATH; you can help Spack locate externals which use module files by loading any associated modules for packages that you want Spack to know about before running
spack external find.
Spack does not overwrite existing entries in the package configuration: If there is an external defined for a spec at any configuration scope, then Spack will not add a new external entry (
spack config blame packagescan help locate all external entries).
Spack can be configured to always use certain compilers, package versions, and variants during concretization through package requirements.
Package requirements are useful when you find yourself repeatedly specifying the same constraints on the command line, and wish that Spack respects these constraints whether you mention them explicitly or not. Another use case is specifying constraints that should apply to all root specs in an environment, without having to repeat the constraint everywhere.
Apart from that, requirements config is more flexible than constraints on the command line, because it can specify constraints on packages when they occur as a dependency. In contrast, on the command line it is not possible to specify constraints on dependencies while also keeping those dependencies optional.
The package requirements configuration is specified in
keyed by package name and expressed using the Spec syntax. In the simplest
case you can specify attributes that you always want the package to have
by providing a single spec string to
packages: libfabric: require: "@1.13.2"
In the above example,
libfabric will always build with version 1.13.2. If you
need to compose multiple configuration scopes
require accepts a list of
packages: libfabric: require: - "@1.13.2" - "%gcc"
In this case
libfabric will always build with version 1.13.2 and using GCC
as a compiler.
For more complex use cases, require accepts also a list of objects. These objects
must have either a
any_of or a
one_of field, containing a list of spec strings,
and they can optionally have a
when and a
packages: openmpi: require: - any_of: ["@4.1.5", "%gcc"] message: "in this example only 4.1.5 can build with other compilers"
any_of is a list of specs. One of those specs must be satisfied
and it is also allowed for the concretized spec to match more than one.
In the above example, that means you could build
If a custom message is provided, and the requirement is not satisfiable, Spack will print the custom error message:
$ spack spec email@example.com%clang ==> Error: in this example only 4.1.5 can build with other compilers
We could express a similar requirement using the
packages: openmpi: require: - any_of: ["%gcc"] when: "@:4.1.4" message: "in this example only 4.1.5 can build with other compilers"
In the example above, if the version turns out to be 4.1.4 or less, we require the compiler to be GCC.
For readability, Spack also allows a
spec key accepting a string when there is only a single
packages: openmpi: require: - spec: "%gcc" when: "@:4.1.4" message: "in this example only 4.1.5 can build with other compilers"
This code snippet and the one before it are semantically equivalent.
Finally, instead of
any_of you can use
one_of which also takes a list of specs. The final
concretized spec must match one and only one of them:
packages: mpich: require: - one_of: ["+cuda", "+rocm"]
In the example above, that means you could build
mpich+rocm but not
one_of, the order of specs indicates a
preference: items that appear earlier in the list are preferred
(note that these preferences can be ignored in favor of others).
When using a conditional requirement, Spack is allowed to actively avoid the triggering
when=... spec) if that leads to a concrete spec with better scores in
the optimization criteria. To check the current optimization criteria and their
priorities you can run
spack solve zlib.
Setting default requirements
You can also set default requirements for all packages under
packages: all: require: '%clang'
which means every spec will be required to use
clang as a compiler.
Requirements on variants for all packages are possible too, but note that they are only enforced for those packages that define these variants, otherwise they are disregarded. For example:
packages: all: require: - "+shared" - "+cuda"
will just enforce
zlib, which has a boolean
shared variant but
Constraints in a single spec literal are always considered as a whole, so in a case like:
packages: all: require: "+shared +cuda"
the default requirement will be enforced only if a package has both a
shared variant, and will never be partially enforced.
all represents a default set of requirements -
if there are specific package requirements, then the default requirements
all are disregarded. For example, with a configuration like this:
packages: all: require: - 'build_type=Debug' - '%clang' cmake: require: - 'build_type=Debug' - '%gcc'
cmake to use
gcc and all other nodes (including
dependencies) to use
clang. If enforcing
build_type=Debug is needed also
cmake, it must be repeated in the specific
Setting requirements on virtual specs
A requirement on a virtual spec applies whenever that virtual is present in the DAG. This can be useful for fixing which virtual provider you want to use:
packages: mpi: require: 'mvapich2 %gcc'
With the configuration above the only allowed
mpi provider is
Requirements on the virtual spec and on the specific provider are both applied, if present. For instance with a configuration like:
packages: mpi: require: 'mvapich2 %gcc' mvapich2: require: '~cuda'
you will use
mvapich2~cuda %gcc as an
In some cases package requirements can be too strong, and package preferences are the better option. Package preferences do not impose constraints on packages for particular versions or variants values, they rather only set defaults. The concretizer is free to change them if it must, due to other constraints, and also prefers reusing installed packages over building new ones that are a better match for preferences.
Most package preferences (
can only be set globally under the
all section of
packages: all: compiler: [firstname.lastname@example.org, clang@12:, oneapi@2023:] target: [x86_64_v3] providers: mpi: [mvapich2, mpich, openmpi]
These preferences override Spack’s default and effectively reorder priorities when looking for the best compiler, target or virtual package provider. Each preference takes an ordered list of spec constraints, with earlier entries in the list being preferred over later entries.
In the example above all packages prefer to be compiled with
to target the
x86_64_v3 microarchitecture and to use
mvapich2 if they
version preferences can be set under
package specific sections of the
packages: opencv: variants: +debug gperftools: version: [2.2, 2.4, 2.3]
In this case, the preference for
opencv is to build with debug options, while
gperftools prefers version 2.2 over 2.4.
Any preference can be overwritten on the command line if explicitly requested.
Preferences cannot overcome explicit constraints, as they only set a preferred
ordering among homogeneous attribute values. Going back to the example, if
email@example.com: was requested, then Spack will install version 2.4
since the most preferred version 2.2 is prohibited by the version constraint.
Spack can be configured to assign permissions to the files installed by a package.
packages.yaml file under
permissions, the attributes
group control the package
permissions. These attributes can be set per-package, or for all
all. If permissions are set under
all and for a
specific package, the package-specific settings take precedence.
write attributes take one of
packages: all: permissions: write: group group: spack my_app: permissions: read: group group: my_team
The permissions settings describe the broadest level of access to
installations of the specified packages. The execute permissions of
the file are set to the same level as read permissions for those files
that are executable. The default setting for
user. In the example above, installations of
my_app will be installed with user and group permissions but no
world permissions, and owned by the group
my_team. All other
packages will be installed with user and group write privileges, and
world read privileges. Those packages will be owned by the group
group attribute assigns a Unix-style group to a package. All
files installed by the package will be owned by the assigned group,
and the sticky group bit will be set on the install prefix and all
directories inside the install prefix. This will ensure that even
manually placed files within the install prefix are owned by the
assigned group. If no group is assigned, Spack will allow the OS
default behavior to go as expected.
Assigning Package Attributes
You can assign class-level attributes in the configuration:
packages: mpileaks: # Override existing attributes url: http://www.somewhereelse.com/mpileaks-1.0.tar.gz # ... or add new ones x: 1
Attributes set this way will be accessible to any method executed
in the package.py file (e.g. the
install() method). Values for these
attributes may be any value parseable by yaml.
These can only be applied to specific packages, not “all” or virtual packages.