Sets the local and global sizes, and checks to determine compatibility
PETSC_DECIDE is not used for the arguments ‘m’ and ‘n’, then the
user must ensure that they are chosen to be compatible with the
vectors. To do this, one first considers the matrix-vector product
‘y = A x’. The
m that is used in the above routine must match the
local size used in the vector creation routine
VecCreateMPI() for ‘y’.
n used must match that used as the local size in
VecCreateMPI() for ‘x’.
You cannot change the sizes once they have been set.
The sizes must be set before
MatSetUp() or MatXXXSetPreallocation() is called.