Which way to decompose domain/grid

[Wee-Beng Tay]

Hi,

I'm working on a 2D Cartesian grid and I'm going to decompose the grid for
MPI for my CFD Fortran code. The grid size is in the ratio of 110 x 70. I
wonder how I should decompose the grid - horizontally or vertically?

For e.g., for 2 processors, to 2 55x70 grids, or 2 110x35 grids.

I thought that communication between grids will be less if I do 55x70
because communication will only involve 70 values. However, if it's 110x35,
it'll involve 110 values

Hence does it matter in PETSc how the decomposition is done?

On the other hand, since Fortran is column major and hence I do the
calculation:

do j=1,size_y

do i=1,size_x

f(i,j)=....

end do

end do

I'll need to "package" the 70 values in a chunk for efficient sending.
However, if it's in 110x35, I can use mpi_isend directly since it's
contagious data.

So is there a better option since there seems to be a conflict? I read about
the use of DMMG. Will this problem be dealt with much better if I use DMMG
instead?

Thank you very much
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