spack/lib/spack/docs/build_settings.rst

.. Copyright Spack Project Developers. See COPYRIGHT file for details.

   SPDX-License-Identifier: (Apache-2.0 OR MIT)


.. _concretizer-options:

==========================================
Concretization Settings (concretizer.yaml)
==========================================

The ``concretizer.yaml`` configuration file allows to customize aspects of the
algorithm used to select the dependencies you install. The default configuration
is the following:

.. literalinclude:: _spack_root/etc/spack/defaults/concretizer.yaml
   :language: yaml

--------------------------------
Reuse already installed packages
--------------------------------

The ``reuse`` attribute controls how aggressively Spack reuses binary packages during concretization. The
attribute can either be a single value, or an object for more complex configurations.

In the former case ("single value") it allows Spack to:

1. Reuse installed packages and buildcaches for all the specs to be concretized, when ``true``
2. Reuse installed packages and buildcaches only for the dependencies of the root specs, when ``dependencies``
3. Disregard reusing installed packages and buildcaches, when ``false``

In case a finer control over which specs are reused is needed, then the value of this attribute can be
an object, with the following keys:

1. ``roots``: if ``true`` root specs are reused, if ``false`` only dependencies of root specs are reused
2. ``from``: list of sources from which reused specs are taken

Each source in ``from`` is itself an object:

.. list-table:: Attributes for a source or reusable specs
   :header-rows: 1

   * - Attribute name
     - Description
   * - type (mandatory, string)
     - Can be ``local``, ``buildcache``, or ``external``
   * - include (optional, list of specs)
     - If present, reusable specs must match at least one of the constraint in the list
   * - exclude (optional, list of specs)
     - If present, reusable specs must not match any of the constraint in the list.

For instance, the following configuration:

.. code-block:: yaml

   concretizer:
     reuse:
       roots: true
       from:
       - type: local
         include:
         - "%gcc"
         - "%clang"

tells the concretizer to reuse all specs compiled with either ``gcc`` or ``clang``, that are installed
in the local store. Any spec from remote buildcaches is disregarded.

To reduce the boilerplate in configuration files, default values for the ``include`` and
``exclude`` options can be pushed up one level:

.. code-block:: yaml

   concretizer:
     reuse:
       roots: true
       include:
       - "%gcc"
       from:
       - type: local
       - type: buildcache
       - type: local
         include:
         - "foo %oneapi"

In the example above we reuse all specs compiled with ``gcc`` from the local store
and remote buildcaches, and we also reuse ``foo %oneapi``. Note that the last source of
specs override the default ``include`` attribute.

For one-off concretizations, the are command line arguments for each of the simple "single value"
configurations. This means a user can:

.. code-block:: console

   % spack install --reuse <spec>

to enable reuse for a single installation, or:

.. code-block:: console

   spack install --fresh <spec>

to do a fresh install if ``reuse`` is enabled by default.

.. seealso::

   FAQ: :ref:`Why does Spack pick particular versions and variants? <faq-concretizer-precedence>`

------------------------------------------
Selection of the target microarchitectures
------------------------------------------

The options under the ``targets`` attribute control which targets are considered during a solve.
Currently the options in this section are only configurable from the ``concretizer.yaml`` file
and there are no corresponding command line arguments to enable them for a single solve.

The ``granularity`` option can take two possible values: ``microarchitectures`` and ``generic``.
If set to:

.. code-block:: yaml

   concretizer:
     targets:
       granularity: microarchitectures

Spack will consider all the microarchitectures known to ``archspec`` to label nodes for
compatibility. If instead the option is set to:

.. code-block:: yaml

   concretizer:
     targets:
       granularity: generic

Spack will consider only generic microarchitectures. For instance, when running on an
Haswell node, Spack will consider ``haswell`` as the best target in the former case and
``x86_64_v3`` as the best target in the latter case.

The ``host_compatible`` option is a Boolean option that determines whether or not the
microarchitectures considered during the solve are constrained to be compatible with the
host Spack is currently running on. For instance, if this option is set to ``true``, a
user cannot concretize for ``target=icelake`` while running on an Haswell node.

---------------
Duplicate nodes
---------------

The ``duplicates`` attribute controls whether the DAG can contain multiple configurations of
the same package. This is mainly relevant for build dependencies, which may have their version
pinned by some nodes, and thus be required at different versions by different nodes in the same
DAG.

The ``strategy`` option controls how the solver deals with duplicates. If the value is ``none``,
then a single configuration per package is allowed in the DAG. This means, for instance, that only
a single ``cmake`` or a single ``py-setuptools`` version is allowed. The result would be a slightly
faster concretization, at the expense of making a few specs unsolvable.

If the value is ``minimal`` Spack will allow packages tagged as ``build-tools`` to have duplicates.
This allows, for instance, to concretize specs whose nodes require different, and incompatible, ranges
of some build tool. For instance, in the figure below the latest `py-shapely` requires a newer `py-setuptools`,
while `py-numpy` still needs an older version:

.. figure::  images/shapely_duplicates.svg
   :scale: 70 %
   :align: center

Up to Spack v0.20 ``duplicates:strategy:none`` was the default (and only) behavior. From Spack v0.21 the
default behavior is ``duplicates:strategy:minimal``.

--------
Splicing
--------

The ``splice`` key covers config attributes for splicing specs in the solver.

"Splicing" is a method for replacing a dependency with another spec
that provides the same package or virtual. There are two types of
splices, referring to different behaviors for shared dependencies
between the root spec and the new spec replacing a dependency:
"transitive" and "intransitive". A "transitive" splice is one that
resolves all conflicts by taking the dependency from the new node. An
"intransitive" splice is one that resolves all conflicts by taking the
dependency from the original root. From a theory perspective, hybrid
splices are possible but are not modeled by Spack.

All spliced specs retain a ``build_spec`` attribute that points to the
original Spec before any splice occurred. The ``build_spec`` for a
non-spliced spec is itself.

The figure below shows examples of transitive and intransitive splices:

.. figure:: images/splices.png
   :align: center

The concretizer can be configured to explicitly splice particular
replacements for a target spec. Splicing will allow the user to make
use of generically built public binary caches, while swapping in
highly optimized local builds for performance critical components
and/or components that interact closely with the specific hardware
details of the system. The most prominent candidate for splicing is
MPI providers. MPI packages have relatively well-understood ABI
characteristics, and most High Performance Computing facilities deploy
highly optimized MPI packages tailored to their particular
hardware. The following config block configures Spack to replace
whatever MPI provider each spec was concretized to use with the
particular package of ``mpich`` with the hash that begins ``abcdef``.

.. code-block:: yaml

   concretizer:
     splice:
       explicit:
       - target: mpi
         replacement: mpich/abcdef
         transitive: false

.. warning::

   When configuring an explicit splice, you as the user take on the
   responsibility for ensuring ABI compatibility between the specs
   matched by the target and the replacement you provide. If they are
   not compatible, Spack will not warn you and your application will
   fail to run.

The ``target`` field of an explicit splice can be any abstract
spec. The ``replacement`` field must be a spec that includes the hash
of a concrete spec, and the replacement must either be the same
package as the target, provide the virtual that is the target, or
provide a virtual that the target provides. The ``transitive`` field
is optional -- by default, splices will be transitive.

.. note::

   With explicit splices configured, it is possible for Spack to
   concretize to a spec that does not satisfy the input. For example,
   with the config above ``hdf5 ^mvapich2`` will concretize to user
   ``mpich/abcdef`` instead of ``mvapich2`` as the MPI provider. Spack
   will warn the user in this case, but will not fail the
   concretization.

.. _automatic_splicing:

^^^^^^^^^^^^^^^^^^
Automatic Splicing
^^^^^^^^^^^^^^^^^^

The Spack solver can be configured to do automatic splicing for
ABI-compatible packages. Automatic splices are enabled in the concretizer
config section

.. code-block:: yaml

   concretizer:
     splice:
       automatic: True

Packages can include ABI-compatibility information using the
``can_splice`` directive. See :ref:`the packaging
guide<abi_compatibility>` for instructions on specifying ABI
compatibility using the ``can_splice`` directive.

.. note::

   The ``can_splice`` directive is experimental and may be changed in
   future versions.

When automatic splicing is enabled, the concretizer will combine any
number of ABI-compatible specs if possible to reuse installed packages
and packages available from binary caches. The end result of these
specs is equivalent to a series of transitive/intransitive splices,
but the series may be non-obvious.