# Grid Creation¶

To run Nyx 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).