[petsc-users] incomplete LU preconditioner

[Matthew Knepley]

On Tue, Jun 7, 2011 at 10:44 AM, Danesh Daroui <danesh.daroui at ltu.se> 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?
>

It is not very scalable, so it is only really useful in a serial code. We
tend to focus
on parallel algorithms which are scalable. We don't have any plans to
implement it,
but we do take code submissions.


> 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?
>

You would use KSPSetOperators() with SAME_PRECONDITIONER.

   Matt


>
> 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.
> > >
> > >
> >
> >
>
>
>


-- 
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
lead.
-- Norbert Wiener
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