Maahi :
Hello All,
I have some questions regarding ex2.c in
/petsc/src/ksp/ksp/examples/tutorials/ex2.c.
1. The local sizes of the matrix was decided by PETSc using ierr =
MatSetSizes(A,PETSC_DECIDE,PETSC_DECIDE,m*n,m*n);CHKERRQ(ierr);
But without knowing the local sizes, how was the following function used which
requires the knowledge of how the big matrix was split into smaller chunks
within PETSc?
ierr = MatMPIAIJSetPreallocation(A,5,NULL,5,NULL);CHKERRQ(ierr);
This example sets a matrix for 2D 5-point PDE stencil, for which the maximum
non-zeros in each row is 5.
2. When using MatGetOwnershipRange(A,&Istart,&Iend);CHKERRQ(ierr), the
global index of first and last row of which local chunk of the big matrix is
it returning ? I mean if the big matrix was split into three processes, for
example, fist and last global row of which process is it returning?
Adding a line
ierr = MatGetOwnershipRange(A,&Istart,&Iend);CHKERRQ(ierr);
+ printf("Istart %d Iend %d\n",Istart,Iend);
I get
$ mpiexec -n 3 ./ex2
Istart 0 Iend 19
Istart 19 Iend 38
Istart 38 Iend 56
Norm of error 0.000522061 iterations 9
The matrix global size = 56
Hong
Your replies are highly appreciated
Thanks,
Maahi Talukder
Clarkson University
