[petsc-users] Starting vector

[Marc-André Dubois]

Hi all, I am currently developing a code in C++ and I try to use SLEPC in conjunction with PETSC to solve an eigenvalue problem. Considering I have a set of similar matrices, my strategy was to solve one problem, and then reuse the starting vector to accelerate the resolution of the next problems. I tested this strategy with a rudimentary code to validate the process, but it does not seems to work. I am afraid I do not fully understand some of the particularities of SLEPC or PETSC. The main.cpp fille of my code goes like this: (see at the bottom of this message). You can see that I create a matrix (A) and two eigensolver contexts (eps and eps2). First a try to solve the problem using eps. I then recall the invariant subspace and use it as a starting vector in eps2. This code seems to work, but eps2 takes systematically more time to converge than eps. Is there something wrong with this code or my understanding of how SLEPC/PETSC works? Thank you,

Marc






#include "slepceps.h"


int main(int argc, char *argv[])
{

    static char help[] = "Simple Hello World example program in SLEPc\n";


    Mat            A;           /* problem matrix */
    EPS            eps;         /* eigenproblem solver context */
    EPS            eps2;
    EPSType        type;
    PetscReal      error,tol,re,im;
    PetscScalar    kr,ki;
    Vec            xr,xi;
    PetscInt       n=30,i,Istart,Iend,nev,maxit,its,nconv;
    PetscErrorCode ierr;
PetscInt a,b,c;


    SlepcInitialize(&argc,&argv,(char*)0,help);

    ierr = PetscOptionsGetInt(NULL,"-n",&n,NULL);CHKERRQ(ierr);



    /*
       Create matrix
    */


     ierr = MatCreateDense(PETSC_COMM_WORLD,PETSC_DECIDE,PETSC_DECIDE,n,n,NULL,&A);
     CHKERRQ(ierr);

     ierr = MatGetOwnershipRange(A,&Istart,&Iend);CHKERRQ(ierr);
     for (i=Istart;i<Iend;i++) {
        if (i>0) { ierr = MatSetValue(A,i,i-1,-1.0,INSERT_VALUES);CHKERRQ(ierr); }
        if (i<n-1) { ierr = MatSetValue(A,i,i+1,-1.0,INSERT_VALUES);CHKERRQ(ierr); }
        ierr = MatSetValue(A,i,i,2.0,INSERT_VALUES);CHKERRQ(ierr);
      }
      ierr = MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
      ierr = MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);

      ierr = MatCreateVecs(A,NULL,&xr);CHKERRQ(ierr);
      ierr = MatCreateVecs(A,NULL,&xi);CHKERRQ(ierr);




    /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                  Create the eigensolver and set various options
       - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
    /*
       Create eigensolver context
    */
    ierr = EPSCreate(PETSC_COMM_WORLD,&eps);CHKERRQ(ierr);
    ierr = EPSSetOperators(eps,A,NULL);CHKERRQ(ierr);
    ierr = EPSSetProblemType(eps,EPS_HEP);CHKERRQ(ierr);
    its = 4000;
    ierr = EPSSetTolerances(eps,PETSC_DEFAULT,its);CHKERRQ(ierr);

    nev = (int)n/2;
    EPSSetType(eps,EPSKRYLOVSCHUR);
    EPSSetDimensions(eps,nev,n,n);
    EPSSetWhichEigenpairs(eps,EPS_SMALLEST_MAGNITUDE);
    EPSSetFromOptions(eps);


    ierr = EPSSolve(eps);CHKERRQ(ierr);

    ierr = EPSGetConverged(eps,&nconv);CHKERRQ(ierr);

    

PetscInt nbr = nconv;

    Vec subspace;
    VecCreate(PETSC_COMM_WORLD,&subspace);
    VecSetSizes(subspace,PETSC_DECIDE,n);
    VecSetFromOptions(subspace);

    Vec *sub;
    VecDuplicateVecs(subspace,nbr,&sub);

    EPSGetInvariantSubspace(eps,sub);

    ierr = EPSDestroy(&eps);CHKERRQ(ierr);




    ierr = EPSCreate(PETSC_COMM_WORLD,&eps2);CHKERRQ(ierr);
    ierr = EPSSetInitialSpace(eps2,nbr,sub);

    ierr = EPSSetOperators(eps2,A,NULL);CHKERRQ(ierr);
    ierr = EPSSetProblemType(eps2,EPS_HEP);CHKERRQ(ierr);
    its = 4000;
    ierr = EPSSetTolerances(eps2,PETSC_DEFAULT,its);CHKERRQ(ierr);

    nev = (int)n/2;
    ierr = EPSSetType(eps2,EPSKRYLOVSCHUR);CHKERRQ(ierr);
    ierr = EPSSetDimensions(eps2,nev,n,n);CHKERRQ(ierr);
    ierr = EPSSetWhichEigenpairs(eps2,EPS_SMALLEST_MAGNITUDE);CHKERRQ(ierr);
    ierr = EPSSetFromOptions(eps2);CHKERRQ(ierr);


    ierr = EPSSolve(eps2);CHKERRQ(ierr);


 //   ierr = EPSGetConverged(eps2,&nconv);CHKERRQ(ierr);


ierr = EPSDestroy(&eps2);CHKERRQ(ierr);



    ierr = MatDestroy(&A);CHKERRQ(ierr);
    ierr = VecDestroy(&xr);CHKERRQ(ierr);
    ierr = VecDestroy(&xi);CHKERRQ(ierr);
    ierr = SlepcFinalize();
    return 0;

}

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