Defining Process Grids

When an S3L array is defined, the programmer has the choice of either defining a process grid explicitly, using the S3L_set_process_grid function, or letting S3L define one using an internal algorithm. The following F77 example how to specify a two-dimensional process grid that is defined over a set of eight processes having MPI ranks 0 through 7. The process grid has extents of 2x4 and is assigned column-major ordering.

Example 2-1

#include <s3l/s3l-c.h>
#include <s3l/s3l_errno-c.h>
int
S3L_free(a)
    S3L_array_t        *a
include `s3l/s3l-f.h'
integer*8 pg
integer*4 rank
integer*4 pext(2),process_list(8)
integer*4 i,ier

rank = 2
pext(1) = 2
pext(2) = 4
do i=1,8
    process_list(i)=i-1
end do
call s3l_set_process_grid(pg,rank,S3L_MAJOR_COLUMN,
    pext,8,process_list,ier)

A process grid can be defined over the full set of processes being used by an application or over any subset of those processes. This flexibility can be useful when circumstances call for setting up a process grid that does not include all available processes.

For example, if an application will be running in a two-node cluster where one node has 14 CPUs and the other has 10, better load balancing may be achieved by defining the process grid to have 10 processes in each node.

For more information about explicitly defining process grids, see "S3L_set_process_grid " or the S3L_set_process_grid(3) man page.