[mpich-discuss] MPI_DIMS_CREATE

[Andreas Schäfer]

Good catch. If I'm not mistaken then the line

    n_root = 1 << (n_root / 2);

in src/mpi/topo/dims_create.c sets an overly strict boundary as the
division by 2 will round down. In the test case n_root will be set to
16, which is smaller than 19. The following line should fix this,
while being only marginally slower (with prime number density being
low for large n...)

    n_root = 1 << (n_root / 2 + 1);

Cheers
-Andreas


On 09:10 Tue 31 Mar     , Valery wrote:
> Dear All
> 
> I noticed that MPI_DIMS_CREATE cannot split eg
> 361
> into a 19 * 19 processor grid. Is that a feature?
> 
> The code follows. 
> 
> I used
> MPICH-3.1.4
> gcc-4.9.2
> 
> valery
> 
> 
> cat mpi_dims.f90
> program test
>   use mpi
>   implicit none
>   integer :: ierr, nnodes, ndims, dims(2), i
>   call MPI_INIT( ierr )
>   ndims = 2
>   do i = 1, 200
>      nnodes = i**2
>      dims(:) = 0
>      call MPI_DIMS_CREATE( nnodes, ndims, dims, ierr )
>      if( dims(1) /= i ) write(*,*) i, dims
>   enddo
>   call MPI_FINALIZE(ierr)
> end program test
> 
> mpif90 mpi_dims.f90
> 
> 
> mpiexec -n 1 ./a.out 
>           19         361           1
>           41        1681           1
>           43        1849           1
>           71        5041           1
>           73        5329           1
>           79        6241           1
>           83        6889           1
>           89        7921           1
>          137       18769           1
>          139       19321           1
>          149       22201           1
>          151       22801           1
>          157       24649           1
>          163       26569           1
>          167       27889           1
>          173       29929           1
>          179       32041           1
>          181       32761           1
> 
> 
>  		 	   		  

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-- 
==========================================================
Andreas Schäfer
HPC and Grid Computing
Chair of Computer Science 3
Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany
+49 9131 85-27910
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http://www.libgeodecomp.org
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