[MPICH] MPI_Dimms_create() and square numbers

[William Gropp]

Thanks for the example.  This isn't the intended default behavior;  
for systems with a particular underlying topology, there is a  
mechanism for that system to override the default Dims_create (e.g.,  
on a system with a mesh interconnect that might be a power of 2 in  
each direction).  We'll have a look at improving the default case.

Bill

On Jul 14, 2007, at 6:37 PM, Andrea Di Blas wrote:

> hello,
>
>
> I am wondering about the strategy implemented by MPI_Dimms_create() to
> best balance the grid size in all dimensions.
>
>
> for instance, when sizing a bidimensional cartesian communicator  
> with a
> square number p of processes, without enforcing constraints,  
> sometimes the
> two sizes are the sqrt(p) as (I would have) expected (e.g. with p =  
> 4 or p
> = 16), while other times they are not (e.g. with p = 9 or p = 100).
> the same behavior happenes with cubes.
>
> I wonder if I am doing something wrong, or if it is intentional,  
> and if it
> is so, why.  thanks in advance.
>
>
> regards,
>
>
> 	andrea
>
>
> /*---------------------------------------------------------*/
> CODE: (for the cube case)
>
>
> for(i = 2; i < 12; ++i)
> {	grid_size[0] = grid_size[1] = 0; grid_size[2] = 0;
> 	MPI_Dims_create(i*i*i, 3, grid_size);
> 	printf("\nMPI_Dims_create(%4d, 3, grid_size) = %3d, %3d, %3d", i*i*i,
> 			grid_size[0], grid_size[1], grid_size[2]);
> }
>
>
>
> OUTPUT: (for the square case)
>
> MPI_Dims_create(  4, 2, grid_size) =  2, 2
> MPI_Dims_create(  9, 2, grid_size) =  9, 1
> MPI_Dims_create( 16, 2, grid_size) =  4, 4
> MPI_Dims_create( 25, 2, grid_size) = 25, 1
> MPI_Dims_create( 36, 2, grid_size) = 18, 2
> MPI_Dims_create( 49, 2, grid_size) = 49, 1
> MPI_Dims_create( 64, 2, grid_size) =  8, 8
> MPI_Dims_create( 81, 2, grid_size) =  9, 9
> MPI_Dims_create(100, 2, grid_size) = 50, 2
> MPI_Dims_create(121, 2, grid_size) = 121, 1
>
>
> OUTPUT: (for the cube case)
>
> MPI_Dims_create(   8, 3, grid_size) =   4,   2,   1
> MPI_Dims_create(  27, 3, grid_size) =   9,   3,   1
> MPI_Dims_create(  64, 3, grid_size) =   4,   4,   4
> MPI_Dims_create( 125, 3, grid_size) = 125,   1,   1
> MPI_Dims_create( 216, 3, grid_size) = 108,   2,   1
> MPI_Dims_create( 343, 3, grid_size) = 343,   1,   1
> MPI_Dims_create( 512, 3, grid_size) =   8,   8,   8
> MPI_Dims_create( 729, 3, grid_size) =   9,   9,   9
> MPI_Dims_create(1000, 3, grid_size) = 500,   2,   1
> MPI_Dims_create(1331, 3, grid_size) = 1331,   1,   1
>
>
>
> -- 
> Andrea  Di Blas                     UCSC
>                    School of Engineering
> Tel: (831) 459 4193  Fax: (831) 459 4829
>