MPI convention for index of rows and columns

Question

I am using MPI for solving PDE. For this, I breakdown the 2D domain into different cells (size of each of these cells is "xcell,ycell" with xcell = size_x_domain/(number of X subdomains) and ycell = size_y_domain/(number of Y subdomains).

So, I am running the code with number of processes = (number of X subdomains)*(number of Y subdomains)

The gain relatively to sequential version is that I communicate between each process representing the sub-domains.

Here a figure illustrating my breakdown with 8 processes (2 subdomains for X and 4 for Y) :

(xs,xe) represent x_start and x_end of the cell, (ys,ye) represent y_start and y_end of the cell

I would like to know if I have to set, into x(i,j) array, i the index as row index and j as column index ?

Is it a general rule to put the first index for row and the second one for column ? ( for example, in C, Fortran and Matlab language or maybe more)

Thanks for your help.

Answer 1

I'm not sure, but maybe try having a different flag for all 4 of the communications.