[petsc-users] KSP ex2.c and Parallelization

[Smith, Barry F.]

   Maybe we could provide more guidance on how to proceed if we knew a bit more about the 1) problem you are trying to solve, 2) the discretization you are using and 3) the mesh you are using?

    From your code fragment with a m*n I am guessing you are solving something on a structured grid with m grid points in the x direction and n grid points in the y direction? 

  Barry


> On Feb 15, 2019, at 4:26 PM, Maahi Talukder via petsc-users <petsc-users at mcs.anl.gov> wrote:
> 
> Thank you so much for your reply. 
>  
> But in the definition of d_nz of MatMPIAIJSetPreallocation  , it is defined as 'number of nonzeros per row in DIAGONAL portion of local submatrix'. So I was wondering about how to find the Diagonal portion of the local submatrix that PETSc is splitting the big matrix into . So should it be the maximum number of nonzeros in each row of the big matrix instead of  the min number of nonzeros in the diagonal portion of the local matrix?
> 
> And so MatGetOwnershipRange returns the start and the end of all the smaller matrices that PETSc is splitting the big matrix into, right?
> 
> 
> Regards,
> Maahi Talukder
> 
> On Fri, Feb 15, 2019 at 2:43 PM Zhang, Hong <hzhang at mcs.anl.gov> wrote:
> 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 
> 
>