2 files changed, 122 insertions, 0 deletions
diff --git a/lib/spack/docs/packaging_guide.rst b/lib/spack/docs/packaging_guide.rst
index 60c15895d1..fa3f5f43e2 100644
--- a/lib/spack/docs/packaging_guide.rst
+++ b/lib/spack/docs/packaging_guide.rst
@@ -1,3 +1,5 @@
+.. _packaging-guide:
+
 Packaging Guide
 =====================
 
@@ -615,6 +617,8 @@ Spack will fail with a constraint violation, because the version of
 MPICH requested is too low for the ``mpi`` requirement in ``foo``.
 
 
+.. _abstract-and-concrete:
+
 Abstract & concrete specs
 ------------------------------------------
 
diff --git a/lib/spack/docs/site_configuration.rst b/lib/spack/docs/site_configuration.rst
index e9ca9dea51..ae52df2254 100644
--- a/lib/spack/docs/site_configuration.rst
+++ b/lib/spack/docs/site_configuration.rst
@@ -6,6 +6,124 @@ Site-specific configuration
 Temporary space
 ----------------------------
 
+By default, Spack will try to do all of its building in temporary
+space.  There are two main reasons for this.  First, Spack is designed
+to run out of a user's home directory, and on may systems the home
+directory is network mounted and potentially not a very fast
+filesystem.  We create build stages in a temporary directory to avoid
+this.  Second, many systems impose quotas on home directories, and
+``/tmp`` or similar directories often have more available space.  This
+helps conserve space for installations in users' home directories.
+
+You can customize temporary directories by editing
+``lib/spack/spack/globals.py``.  Specifically, find this part of the file:
+
+.. code-block:: python
+
+   # Whether to build in tmp space or directly in the stage_path.
+   # If this is true, then spack will make stage directories in
+   # a tmp filesystem, and it will symlink them into stage_path.
+   use_tmp_stage = True
+
+   # Locations to use for staging and building, in order of preference
+   # Use a %u to add a username to the stage paths here, in case this
+   # is a shared filesystem.  Spack will use the first of these paths
+   # that it can create.
+   tmp_dirs = ['/nfs/tmp2/%u/spack-stage',
+               '/var/tmp/%u/spcak-stage',
+               '/tmp/%u/spack-stage']
+
+The ``use_tmp_stage`` variable controls whether Spack builds
+**directly** inside the ``var/spack/`` directory.  Normally, Spack
+will try to find a temporary directory for a build, then it *symlinks*
+that temporary directory into ``var/spack/`` so that you can keep
+track of what temporary directories Spack is using.
+
+The ``tmp_dirs`` variable is a list of paths Spack should search when
+trying to find a temporary directory.  They can optionally contain a
+``%u``, which will substitute the current user's name into the path.
+The list is searched in order, and Spack will create a temporary stage
+in the first directory it finds to which it has write access.  Add
+more elements to the list to indicate where your own site's temporary
+directory is.
+
 
 Concretization policies
 ----------------------------
+
+When a user asks for a package like ``mpileaks`` to be installed,
+Spack has to make decisions like what version should be installed,
+what compiler to use, and how its dependencies should be configured.
+This process is called *concretization*, and it's covered in detail in
+:ref:`its own section <abstract-and-concrete>`.
+
+The default concretization policies are in the
+:py:mod:`spack.concretize` module, specifically in the
+:py:class:`spack.concretize.DefaultConcretizer` class.  These are the
+important methods used in the concretization process:
+
+* :py:meth:`concretize_version(self, spec) <spack.concretize.DefaultConcretizer.concretize_version>`
+* :py:meth:`concretize_architecture(self, spec) <spack.concretize.DefaultConcretizer.concretize_architecture>`
+* :py:meth:`concretize_compiler(self, spec) <spack.concretize.DefaultConcretizer.concretize_compiler>`
+* :py:meth:`choose_provider(self, spec, providers) <spack.concretize.DefaultConcretizer.choose_provider>`
+
+The first three take a :py:class:`Spec <spack.spec.Spec>` object and
+modify it by adding constraints for the version.  For example, if the
+input spec had a version range like `1.0:5.0.3`, then the
+``concretize_version`` method should set the spec's version to a
+*single* version in that range.  Likewise, ``concretize_architecture``
+selects an architecture when the input spec does not have one, and
+``concretize_compiler`` needs to set both a concrete compiler and a
+concrete compiler version.
+
+``choose_provider()`` affects how concrete implementations are chosen
+based on a virtual dependency spec.  The input spec is some virtual
+dependency and the ``providers`` index is a :py:class:`ProviderIndex
+<spack.packages.ProviderIndex>` object.  The ``ProviderIndex`` maps
+the virtual spec to specs for possible implementations, and
+``choose_provider()`` should simply choose one of these.  The
+``concretize_*`` methods will be called on the chosen implementation
+later, so there is no need to fully concretize the spec when returning
+it.
+
+The ``DefaultConcretizer`` is intendend to provide sensible defaults
+for each policy, but there are certain choices that it can't know
+about.  For example, one site might prefer ``OpenMPI`` over ``MPICH``,
+or another might prefer an old version of some packages.  These types
+of special cases can be integrated with custom concretizers.
+
+Writing a custom concretizer
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To write your own concretizer, you need only subclass
+``DefaultConcretizer`` and override the methods you want to change.
+For example, you might write a class like this to change *only* the
+``concretize_version()`` behavior:
+
+.. code-block:: python
+
+   from spack.concretize import DefaultConcretizer
+
+   class MyConcretizer(DefaultConcretizer):
+       def concretize_version(self, spec):
+           # implement custom logic here.
+
+Once you have written your custom concretizer, you can make Spack use
+it by editing ``globals.py``.  Find this part of the file:
+
+.. code-block:: python
+
+   #
+   # This controls how things are concretized in spack.
+   # Replace it with a subclass if you want different
+   # policies.
+   #
+   concretizer = DefaultConcretizer()
+
+Set concretizer to *your own* class instead of the default:
+
+.. code-block:: python
+
+   concretizer = MyConcretizer()
+
+The next time you run Spack, your changes should take effect.