Updated installation instructions for optional MPI and GPU components.

docs/source/openmp_mpi.rst

@@ -16,7 +16,24 @@ MPI
 
 The Message Passing Interface (MPI) has been utilised to implement a simple task farm that can be used to distribute a series of models as independent tasks. This can be useful in many GPR simulations where a B-scan (composed of multiple A-scans) is required. Each A-scan can be task-farmed as a independent model. Within each independent model OpenMP threading will continue to be used (as described above). Overall this creates what is know as a mixed mode OpenMP/MPI job.
 
-By default the MPI task farm functionality is turned off. It can be switched on using the ``-mpi`` command line flag. MPI requires an installation of the ``mpi4py`` Python package, which itself depends on an underlying MPI installation, usually `OpenMPI <http://www.open-mpi.org>`_. On Microsoft Windows ``mpi4py`` requires `Microsoft MPI 6 <https://www.microsoft.com/en-us/download/details.aspx?id=47259>`_.
+By default the MPI task farm functionality is turned off. It can be used with the ``-mpi`` command line option, which specifies the total number of MPI tasks, i.e. master + workers, for the MPI task farm. This option is most usefully combined with ``-n`` to allow individual models to be farmed out using a MPI task farm, e.g. to create a B-scan with 60 traces and use MPI to farm out each trace: ``(gprMax)$ python -m gprMax user_models/cylinder_Bscan_2D.in -n 60 -mpi 61``.
+
+Extra installation steps for MPI task farm usage
+------------------------------------------------
+
+The following steps provide guidance on how to install the extra components to allow the MPI task farm functionality with gprMax:
+
+1. Install a flavour of MPI on your system.
+
+Linux/macOS
+^^^^^^^^^^^
+It is recommended to use `OpenMPI <http://www.open-mpi.org>`_.
+
+Microsoft Windows
+^^^^^^^^^^^^^^^^^
+It is recommended to use `Microsoft MPI <https://docs.microsoft.com/en-us/message-passing-interface/microsoft-mpi>`_. Download and install both the .exe and .msi files.
+
+2. Install the ``mpi4py`` Python module. Open a Terminal (Linux/macOS) or Command Prompt (Windows), navigate into the top-level gprMax directory, and if it is not already active, activate the gprMax conda environment :code:`conda activate gprMax`. Run :code:`pip install mpi4py`
 
 HPC job scripts
 ===============