[petsc-users] A Beginner Question

[Stefan Kolb]

Hi all,
i have written a littel test program, to check how PETSc stores a matrix across 
two processes.

static char help[] = "PETSc test";

#include "stdio.h"

#include "petscksp.h"

int main(int argc,char **args)
{
        Mat                             A;
        PetscInt                        N=100,M=100, row_start = 0, 
column_start = 0, row_end = 0, column_end = 0, rank = 0, local_rows = 0, 
local_columns = 0,nproc=0;
        PetscErrorCode          ierr;
        PetscScalar             value =0.;
        
        PetscInitialize(&argc,&args,(char *)0,help);
        
        MPI_Comm_rank(PETSC_COMM_WORLD,&rank);
        MPI_Comm_size(PETSC_COMM_WORLD,&nproc);
        
        ierr = PetscPrintf(PETSC_COMM_WORLD,"Number of Processes 
%d\n",nproc);CHKERRQ(ierr);
        ierr = MatCreate(PETSC_COMM_WORLD,&A); CHKERRQ(ierr);
        ierr = MatSetSizes(A,PETSC_DECIDE, PETSC_DECIDE,N,M); CHKERRQ(ierr);
        ierr = MatSetFromOptions(A);CHKERRQ(ierr);
        ierr = MatGetOwnershipRange(A,&row_start,&row_end);CHKERRQ(ierr);
        ierr = 
MatGetOwnershipRangeColumn(A,&column_start,&column_end);CHKERRQ(ierr);
        ierr = MatGetLocalSize(A,&local_rows,&local_columns);CHKERRQ(ierr);

        printf("process=%d \t row_start=%d \t row_end=%d \t column_start=%d \t 
column_end=%d \t local_rows=%d \t 
local_columns=%d\n",rank,row_start,row_end,column_start,column_end,local_rows,local_columns);

        ierr = MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
        ierr = MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);

        ierr = MatDestroy(A); CHKERRQ(ierr);
        
        ierr = PetscFinalize();CHKERRQ(ierr);
        return 0;
}

The output of the program (using 2 processes) is:
Number of Processes 2
process=0        row_start=0     row_end=50      column_start=0          
column_end=50   local_rows=50   local_columns=50
process=1        row_start=50    row_end=100     column_start=50         
column_end=100          local_rows=50   local_columns=50

So if the output of my program is right the process with rank=0 owns a matrix 
with 50 rows and 50 columns in the same way the process with rank=1. This cant 
be right because on both processes only one half of the global matrix is 
stored. Wat is the error in my test programm?
I tried to set the local size of the matrix by hand with the command 
MatSetSizes(A,50, 100,N,M) but then i get an error from PETSc. How can i tell 
PETSc to store the first 50 rows on process 0 and the next 50 rows on process 1?

Regards,
Stefan