[petsc-users] incomplete LU preconditioner

[Danesh Daroui]

So, it is not implemented? That's too bad! ILU with drop threshold is
very useful so I am wondering why it is not implemented yet? Do you have
any plan to implement it?

Setting levels did work OK for me. I have another question. If I want to
calculate the preconditioner once, and use it later to solve other
equations, since I know that the preconditioner would be OK because I
solve several equations in several iterations and I know that my
coefficient matrix will not change that much so calculating the
preconditioner at once would be OK. What routines should I call once and
how can I apply the pre-calculated preconditioner each time I want to
solve the equation?

Thanks,

D.



On Tue, 2011-06-07 at 10:28 -0500, Barry Smith wrote:
> Sorry for the confusion, it is our fault. We don't have a drop tolerance ILU hence PCFactorSetDropTolerance() isn't doing anything.
> 
>   You can run with -pc_factor_levels  M   or call PCFactorSetLevels(pc,M) in the code  where M is the size of the matrix.
> 
>    Barry
> 
> On Jun 7, 2011, at 10:20 AM, Danesh Daroui wrote:
> 
> > 
> > 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.
> > 
> > 
> 
>