[petsc-users] incomplete LU preconditioner

[Danesh Daroui]

Hi,

I want to use incomplete LU preconditioner. My code works OK when I use
no preconditioner and direct solver for test:


ierr=KSPCreate(PETSC_COMM_WORLD, &ksp);
ierr=KSPSetOperators(ksp, Mp, Mp, DIFFERENT_NONZERO_PATTERN);
ierr=KSPSetTolerances(ksp, 1.e-2/Msize, 1.e-50, PETSC_DEFAULT,
PETSC_DEFAULT);

// GMRES iterative solver is used
ierr=KSPSetType(ksp, KSPGMRES);

// no PETSc preconditioner is used
ierr=KSPGetPC(ksp, &prec);
ierr=PCSetType(prec, PCLU);

// set up the solver according to the options
ierr=KSPSetFromOptions(ksp);

ierr=KSPSetUp(ksp);

// solve the equation using an iterative solver
ierr=KSPSolve(ksp, bp, xp);


Again for some tests, at the first step, I want to use ILU but create
the exact LU and then change the parameters to tune the preconditioner
and I almost know what values to set according to my tests in MATLAB. I
use the code below to create the get exact LU, using ILU preconditioner:





ierr=KSPCreate(PETSC_COMM_WORLD, &ksp);
ierr=KSPSetOperators(ksp, Mp, Mp, DIFFERENT_NONZERO_PATTERN);
ierr=KSPSetTolerances(ksp, 1.e-2/Msize, 1.e-50, PETSC_DEFAULT,
PETSC_DEFAULT);

// GMRES iterative solver is used
ierr=KSPSetType(ksp, KSPGMRES);

// no PETSc preconditioner is used
ierr=KSPGetPC(ksp, &prec);
ierr=PCSetType(prec, PCILU);

PetscReal dt=1e-10;
PetscReal dtcol=0.1;
PetscInt maxrowcount=M;
PCFactorSetDropTolerance(prec, dt, dtcol, maxrowcount);

// set up the solver according to the options
ierr=KSPSetFromOptions(ksp);

ierr=KSPSetUp(ksp);

// solve the equation using an iterative solver
ierr=KSPSolve(ksp, bp, xp);


and my coefficient matrix is a square "MxM" matrix and I pass "M" as
"maxrowcount". Using this code, the solver never converges while it
solved the equation using previous code (direct solver). Can anybody let
me know how can I get exact LU using ILU and what parameters should I
change in the above code?

Regards,

D.