dolfin team mailing list archive

["Garth N. Wells" <g.n.wells@xxxxxxxxxx>]

On Wed, 2006-03-22 at 08:47 -0600, Anders Logg wrote:
> On Wed, Mar 22, 2006 at 10:13:22AM +0000, Alexander Jarosch wrote:
> > I got a bit lost in the current updates and now I just have a short 
> > question. I set up my solver like this:
> > 
> >  // Discretize equation
> >  Matrix A;
> >  Vector x, b;
> >  FEM::assemble(*a, *L, A, b, mesh, bc);
> > 
> >  // Solve the linear system
> >  KrylovSolver solver(Preconditioner::ilu);
> >  KSP ksp;
> >  ksp = solver.solver();
> > 
> >  PC pc;
> >  KSPGetPC(ksp, &pc);
> >  PCFactorSetShiftNonzero(pc, PETSC_DECIDE);
> >  //PCFactorSetShiftPd(pc, PETSC_TRUE);
> > 
> >  solver.set("Krylov relative tolerance", 1e-14);
> > 
> >  solver.solve(A, x, b);
> > 
> > but Petsc still complains about zero Pivot. should the line 
> > "PCFactorSetShiftNonzero(pc, PETSC_DECIDE);" not take care of that.
> 
> The problem is that KrylovSolver::solve() reads all parameters from
> the (DOLFIN) database, including "Krylov relative tolerance", and sets
> them before each solve. The solve() function also sets the
> preconditioner. Therefore, setting options manually by PETSc calls
> might not always work.
> 
> The plan is to remove the KrylovSolver::solve() function if no one
> objects (Garth?). This will leave two options: either use the simple
> (but limited) DOLFIN wrapper or get the PETSc data structures and
> solve using PETSc calls:
> 
> Mat Amat = A.mat();
> Vec xvec = x.vec();
> 
> PETSc calls...
> 

Another option is to add PETSc options at the command line. This is
really easy,

  ./dolfin -pc_factor_shift_nonzero

Just make sure that you have 
  int main(int argc, char* argv[])
 	

> That said, I will (re-)add a parameter for "Krylov shift nonzero" so
> you can do this natively in DOLFIN.
> 
> I'll await comments from Garth and others before going ahead.
> 
> /Anders
> 

I think that removing KrylovSolver.solver() is the best approach from a
robustness point of view. KrylovSolver then doesn't have to worry that a
user might have messed with the PETSc solver directly. I do find the
direct access to ksp useful, but I can live without.

Garth  



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