Do we *need* to always compute eigenvalues in SolverCG?

[bangerth]

A recent report on the ASPECT forum (https://community.geodynamics.org/t/issues-with-block-gmg-solver-scheme/3703) shows that SolverCG now requires LAPACK. That surprised me, and it turns out that it is because we are always computing the eigenvalue approximations:

template <typename VectorType>
DEAL_II_CXX20_REQUIRES(concepts::is_vector_space_vector<VectorType>)
inline void SolverCG<VectorType>::compute_eigs_and_cond(
  const std::vector<typename VectorType::value_type> &diagonal,
  const std::vector<typename VectorType::value_type> &offdiagonal,
  const boost::signals2::signal<void(const std::vector<double> &)>
                                              &eigenvalues_signal,
  const boost::signals2::signal<void(double)> &cond_signal)
{
  // Avoid computing eigenvalues unless they are needed.
  if (!cond_signal.empty() || !eigenvalues_signal.empty())                // ******************* apparently there are signals attached
    {
      TridiagonalMatrix<typename VectorType::value_type> T(diagonal.size(),
                                                           true);
      for (size_type i = 0; i < diagonal.size(); ++i)
        {
          T(i, i) = diagonal[i];
          if (i < diagonal.size() - 1)
            T(i, i + 1) = offdiagonal[i];
        }
      T.compute_eigenvalues();                                 // ***************** triggers call to LAPACK

      [...]

But I can't seem to find in ASPECT or deal.II where we are actually attaching signals that are monitoring the eigenvalues or condition number. Where is this place, and is this on purpose?

[tjhei]

But I can't seem to find in ASPECT or deal.II where we are actually attaching signals that are monitoring the eigenvalues or condition number. Where is this place, and is this on purpose?

Yes, we need eigenvalue estimates for the Chebyshev smoother for GMG:

aspect/source/simulator/stokes_matrix_free.cc

Line 1799 in 69b3924

mg_smoother_A[level].estimate_eigenvalues(temp_velocity);