Add text about dual gridding

.. role:: cpp(code)

   :language: c++

.. role:: fortran(code)

   :language: fortran

.. _ss:dual_grid:

Dual Grid Approach

------------------

In MFiX-Exa the mesh work and particle work have very different requirements for load balancing.

Rather than using a combined work estimate to create the same grids for mesh and particle

data, we have the option to pursue a "dual grid" approach.

With this approach the mesh (:cpp:`MultiFab`) and particle (:cpp:`ParticleContainer`) data 

are allocated on different :cpp:`BoxArrays` with different :cpp:`DistributionMappings`.

This enables separate load balancing strategies to be used for the mesh and particle work.

The cost of this strategy, of course, is the need to copy mesh data onto temporary 

:cpp:`MultiFabs` defined on the particle :cpp:`BoxArrays` when mesh-particle communication

is required.

   i.e. defining the :cpp:`DistributionMapping` with which :cpp:`MultiFabs` at that level will be built.

   How we do this depends on what strategy we specify and what work estimate we use 

We note that we can load balance the mesh and particle data separately; see :ref:`dual_grid`

.. toctree::

   :maxdepth: 1

   ManagingGridHierarchy

   GridCreation

   DualGrid