[petsc-users] incomplete LU preconditioner

[Danesh Daroui]

Thanks, I changed my code to use same preconditioner. Some more
questions:

1. I changed the level from 1730 to 28 and I expected "no convergence"
or at convergence at more iterations or higher residual, but nothing
changed. What can be the reason?

2. Using same preconditioner I see no change in the solution time.

In all cases I get the convergence at max iteration 2 with very low
residual, but it is a bit slow.

Thanks,

D.


On Tue, 2011-06-07 at 10:49 -0500, Barry Smith wrote:
> On Jun 7, 2011, at 10:44 AM, Danesh Daroui wrote:
> 
> > 
> > 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?
> 
>    We've found that providing a high-quality drop tolerance ILU is far from trivial and haven't had the time/person with expertise to devote the time needed 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?
> 
>    Since you are using the linear solvers directly. If you change the right hand side but NOT the matrix you simply call KSPSolve() again with the new right hand side.
> 
>    If you are changing the matrix but do not want to update the preconditioner call KSPSetOperators() with the new matrix but use a final argument of SAME_PRECONDITIONER
> 
>    Barry
> 
> > 
> > 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.
> >>> 
> >>> 
> >> 
> >> 
> > 
> > 
> 
>