[petsc-users] MatMult scalability problem

[Matthew Knepley]

On Wed, May 2, 2012 at 12:01 PM, Javier Fresno <jfresno at infor.uva.es> wrote:

>
>
> I have a very simple Petsc program that multiplies a matrix and a vector
> several times. It works fine but it has some scalability issues. I execute
> it in a shared memory machine with 16 processors and it only runs 5 or 6
> times faster (only taking into account the MatMult call). I have programmed
> the same algorithm with C and MPI and it shows a proper speedup (around 14
> or 15). The matrices I use have millions of non zero elements, so I think
> they are big enough.
>
> What can I do to get the same speedup that in the manual C version?
>
> I enclose an except of the code. Thank you in advance.
>

We need the output of -log_summary to say anything about performance. Also,
we need to know what the
machine architecture is.

    Matt


> Javier
>
>
>
> /**
>  * Main function
>  */
> int main(int argc, char ** argv){
>
>    // Initialize Petsc
>    PetscInitialize(&argc, &argv, (char *) 0, NULL);
>
>    // Timers
>    PetscLogDouble t_start, t_end;
>
>    // File Viewer
>    PetscViewer fd;
>    PetscViewerBinaryOpen(PETSC_**COMM_WORLD,"matrix_file",FILE_**
> MODE_READ,&fd);
>
>    // M matrix
>    Mat M;
>    MatCreate(PETSC_COMM_WORLD,&M)**;
>    MatSetFromOptions(M);
>    MatLoad(M,fd);
>    PetscViewerDestroy(&fd);
>    MatAssemblyBegin(M,MAT_FINAL_**ASSEMBLY);
>    MatAssemblyEnd(M,MAT_FINAL_**ASSEMBLY);
>
>    PetscInt n, m, local_n, local_m;
>    MatGetSize(M,&n,&m);
>    MatGetLocalSize(M,&local_n,&**local_m);
>
>    // b and c vectors
>    Vec b,c;
>    VecCreate(PETSC_COMM_WORLD,&b)**;
>    VecSetFromOptions(b);
>    VecSetSizes(b,local_n,n);
>
>    VecCreate(PETSC_COMM_WORLD,&c)**;
>    VecSetFromOptions(c);
>    VecSetSizes(c,local_n,n);
>
>    init_vector_values(b);
>
>    VecAssemblyBegin(b);
>    VecAssemblyEnd(b);
>
>
>    // Main computation
>    PetscGetTime(&t_start);
>    int i;
>    for(i=0; i<iter/2; i++){
>        MatMult(M,b,c);
>        MatMult(M,c,b);
>    }
>    PetscGetTime(&t_end);
>
>    PetscPrintf(PETSC_COMM_WORLD,"**Comp time: %lf\n",t_end-t_start);
>
>    PetscFinalize();
>
>    return 0;
> }
>
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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