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Old   December 10, 2009, 04:42
Default Which way to decompose domain/grid
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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

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


end do

end do

If I choose 55x70, 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 it seems that there's conflict. So does it really matter how I decompose the grid?

Thank you very much
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