Update gridding

.. role:: cpp(code)

   :language: c++

.. _Chap:Managing the Grid Hierarchy:

Managing the Grid Hierarchy

===========================

There are four separate parts of any strategy to balance computational

work ina  large-scale calculation with hybrid parallelism:

1) Creation of grids -- this includes defining the BoxArray on which

MultiFabs will be built at each level and defining the work estimates

2) Distribution of grids to MPI processes -- this uses the work estimate

defined above (or defaults to work estimate = number of cells) to define

the DistributionMapping with which MultiFabs at that level will be built.

3) When running on multicore machines with OpenMP: creation of grid tiles

(by defining fabarray_mfiter.tile_size), and if relevant, creation of 

particle tiles (by defining particle.tile_size)

4) When running on multicore machines with OpenMP: distribution of tiles to threads

Computational load balancing is based on several different steps:

If the application contains task parallelism, load balancing may require special 

care in task scheduling.  See the section on task parallelism.

#. The first component of load balancing is the creation of individual grids, 

   i.e. defining the :cpp:`BoxArray` on which :cpp:`MultiFabs` will be built at each level 

#. The second component is distribution of these grids to MPI processes, 

   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 

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