# Grid Creation

To run MFIX-Exa you must specify `n_cell` in the inputs file – this is the number of cells spanning the domain in each coordinate direction at level 0.

Users often specify `max_grid_size` as well. The default load balancing algorithm then divides the domain in every direction so that each grid is no longer than `max_grid_size` in that direction. If not specified by the user, `max_grid_size` defaults to 128 in 2D and 32 in 3D (in each coordinate direction).

Another popular input is `blocking_factor`. The value of `blocking_factor` constrains grid creation in that in that each grid must be divisible by `blocking_factor`. Note that both the domain (at each level) and `max_grid_size` must be divisible by `blocking_factor` If not specified by the user, `blocking_factor` defaults to 8 in each coordinate direction. The typical purpose of `blocking_factor` is to ensure that the grids will be sufficiently coarsenable for good multigrid performance.

There is one more default behavior to be aware of. There is a boolean `refine_grid_layout` that defaults to true but can be over-ridden at run-time. If `refine_grid_layout` is true and the number of grids created is less than the number of processors (Ngrids < Nprocs), then grids will be further subdivided until Ngrids >= Nprocs.

Caveat: if subdividing the grids to achieve Ngrids >= Nprocs would violate the `blocking_factor` criterion then additional grids are not created and the number of grids will remain less than the number of processors

Note that `n_cell` must be given as three separate integers, one for each coordinate direction.

However, `max_grid_size` and `blocking_factor` can be specified as a single value applying to all coordinate directions, or as separate values for each direction.

> - if `max_grid_size` (or `blocking_factor`) is specified as multiple integers then the first integer applies to level 0, the second to level 1, etc. If you don’t specify as many integers as there are levels, the final value will be used for the remaining levels.

> - if different values of `max_grid_size` (or `blocking_factor`) are wanted for each coordinate direction, then `max_grid_size_x`, `max_grid_size_y` and `max_grid_size_z` (or `blocking_factor_x`, `blocking_factor_y` and `blocking_factor_z`) must be used. If you don’t specify as many integers as there are levels, the final value will be used for the remaining levels.

> - note that `max_grid_size` is just an upper bound; with `n_cell = 48` and `max_grid_size = 32`, we will typically have one grid of length 32 and one of length 16.

The grid creation process at level 0 proceeds as follows (if not using the KD-tree approach):

1. The domain is initially defined by a single grid of size `n_cell`.

2. If `n_cell` is greater than `max_grid_size` then the grids are subdivided until each grid is no longer than `max_grid_size` cells on each side. The `blocking_factor` criterion (ie that the length of each side of each grid is divisible by `blocking_factor` in that direction) is satisfied during this process.

3. Next, if `refine_grid_layout = true` and there are more processors than grids at this level, then the grids at this level are further divided until Ngrids >= Nprocs (unless doing so would violate the `blocking_factor` criterion).