[petsc-users] KSP and matrix-free matrix (shell)

[Zhang, Hong]

When you use shell matrix, i.e., matrix-free, you cannot it as a preconditioner. Add option '-pc_type none'.
Hong

On Thu, Oct 18, 2018 at 10:57 AM Florian Lindner <mailinglists at xgm.de<mailto:mailinglists at xgm.de>> wrote:
Hello,

I try to use the KSP solver package together with a shell matrix:


  MyContext mycontext; // an empty struct, not sure it it's needed?
  Mat s;
  ierr = MatCreateShell(PETSC_COMM_WORLD, size, size, PETSC_DECIDE, PETSC_DECIDE, &mycontext, &s);
  ierr = MatShellSetOperation(s, MATOP_MULT, (void(*)(void))usermult); CHKERRQ(ierr);

To simulate a meaningfull usermult, I use MatMult on an actual existing matrix of same dimensions:

extern PetscErrorCode usermult(Mat m ,Vec x, Vec y)
{
  PetscErrorCode ierr = 0;
  ierr = MatMult(matrix, x, y);
  printf("Call\n");
  return ierr;
}

Btw, what is the significance of the Mat m argument here?

matrix is created like:

  ierr = MatCreate(PETSC_COMM_WORLD, &matrix); CHKERRQ(ierr);
  ierr = MatSetSizes(matrix, size, size, PETSC_DECIDE, PETSC_DECIDE); CHKERRQ(ierr);
  ierr = MatSetFromOptions(matrix); CHKERRQ(ierr);
  ierr = MatSetUp(matrix); CHKERRQ(ierr);


  MatMult(s, b, x);

works. The usermult function is called.

But trying to use a KSP gives an error:

  KSP solver;
  KSPCreate(PETSC_COMM_WORLD, &solver);
  KSPSetFromOptions(solver);
  KSPSetOperators(solver, s, s);


error:

[0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
[0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/linearsolvertable.html for possible LU and Cholesky solvers
[0]PETSC ERROR: Could not locate a solver package. Perhaps you must ./configure with --download-<package>
[0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for trouble shooting.
[0]PETSC ERROR: Petsc Release Version 3.9.3, unknown
[0]PETSC ERROR: ./a.out on a arch-linux2-c-opt named asaru by lindnefn Thu Oct 18 17:39:52 2018
[0]PETSC ERROR: Configure options --with-debugging=0 COPTFLAGS="-O3 -march=native -mtune=native" CXXOPTFLAGS="-O3 -march=native -mtune=native" FOPTFLAGS="-O3 -march=native -mtune=native" --download-petsc4py --download-mpi4py --with-mpi-dir=/opt/mpich
[0]PETSC ERROR: #1 MatGetFactor() line 4328 in /home/lindnefn/software/petsc/src/mat/interface/matrix.c
[0]PETSC ERROR: #2 PCSetUp_ILU() line 142 in /home/lindnefn/software/petsc/src/ksp/pc/impls/factor/ilu/ilu.c
[0]PETSC ERROR: #3 PCSetUp() line 923 in /home/lindnefn/software/petsc/src/ksp/pc/interface/precon.c
[0]PETSC ERROR: #4 KSPSetUp() line 381 in /home/lindnefn/software/petsc/src/ksp/ksp/interface/itfunc.c
[0]PETSC ERROR: #5 KSPSolve() line 612 in /home/lindnefn/software/petsc/src/ksp/ksp/interface/itfunc.c

Do I need to MatShellSetOperations additional operations? Like MATOP_ILUFACTOR? How can I know what operations to implement?

Best Thanks,
Florian


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