Re: [Libmesh-users] Setting the linear solver

[Roy S. <roy...@od...>]

On Wed, 9 Dec 2020, Laura Scarabosio wrote:

> Thank you for your explanations. I built libmesh by disabling mpi,
> so is the default solver still PETSc?

If you configured with PETSc (which can be configured with "mpiuni"
rather than a real MPI), it is.  If you didn't, it isn't.

You can look for HAVE_PETSC in libmesh_config.h to check for sure, but
if you don't know whether you configured with a no-MPI PETSc build,
then you probably didn't.

> Also, for some reasons I am not able to pass command line arguments,
> they don't work.

No, they depend on PETSc.

> Therefore I tried to do the modifications that you suggested in the
> code itself, and assumed that the default solver was Eigen. I tried

> LinearImplicitSystem * ls = dynamic_cast<LinearImplicitSystem*>(& eq_sys.get_system("Diffusion")); 

This cast isn't really useful unless you test the results - if you have a
LinearImplicitSystem then get_linear_solver() will give you what you want even
when called from the base class; if you don't then the dynamic_cast here will
just return nullptr.

> EigenSparseLinearSolver<double> * linearsolver = dynamic_cast< EigenSparseLinearSolver<double> *>(ls->get_linear_solver());                      

This cast shouldn't be necessary or helpful; set_solver_type() is a
method in the base class, not just the derived.

> linearsolver->set_solver_type(CG);
>
> and small variants, but I am not able to use the enum, CG gives me a
> compilation error.

The important thing is *which* compilation error, though, right?  Did
you include enum_solver_type.h?

> >> something as specific as SuperLU_Dist you have to get at through the
> >> PETSc interface, either by extracting the internal PETSc object from
> >> one of our solver shims and using their APIs

> Can I still use PETSc since I have no mpi or should I use Eigen?

You can use either.  I still like PETSc better even in serial but the
improvements for you might not be worth the hassle of downloading and
configuring it.

> Also, would you have an example to do what you said, since I cannot
> use the command line?

We have a few; see `grep set_solver_type examples/*/*/*`.
Look at miscellaneous_ex12.C, maybe?
---
Roy