Merge pull request idaholab#28624 from idaholab/revert-28452-condense…

…d-eigen-system-27262-refactor

Revert "Condensed eigen system refactor"

framework/include/systems/NonlinearEigenSystem.h

@@ -173,6 +173,12 @@ class NonlinearEigenSystem : public NonlinearSystemBase
 
   void residualAndJacobianTogether() override;
 
+  /**
+   * Initialize the condensed matrices. This is a no-op if there are no
+   * constraints in the DofMap
+   */
+  void initializeCondensedMatrices();
+
   virtual void postInit() override;
   virtual void reinit() override;
 
@@ -197,6 +203,10 @@ class NonlinearEigenSystem : public NonlinearSystemBase
   bool _precond_matrix_includes_eigen;
   // Libmesh preconditioner
   Preconditioner<Number> * _preconditioner;
+
+  /// The number of degrees of freedom constrained at the libMesh level, e.g. via hanging node or
+  /// periodic boundary constraints
+  dof_id_type _num_constrained_dofs;
 };
 
 #else

framework/src/systems/NonlinearEigenSystem.C

@@ -113,7 +113,8 @@ NonlinearEigenSystem::NonlinearEigenSystem(EigenProblem & eigen_problem, const s
     _work_rhs_vector_AX(addVector("work_rhs_vector_Ax", false, PARALLEL)),
     _work_rhs_vector_BX(addVector("work_rhs_vector_Bx", false, PARALLEL)),
     _precond_matrix_includes_eigen(false),
-    _preconditioner(nullptr)
+    _preconditioner(nullptr),
+    _num_constrained_dofs(0)
 {
   SlepcEigenSolver<Number> * solver =
       cast_ptr<SlepcEigenSolver<Number> *>(_eigen_sys.eigen_solver.get());
@@ -190,18 +191,46 @@ NonlinearEigenSystem::postAddResidualObject(ResidualObject & object)
   }
 }
 
+void
+NonlinearEigenSystem::initializeCondensedMatrices()
+{
+  if (!(_num_constrained_dofs = dofMap().n_constrained_dofs()))
+    return;
+
+  _eigen_sys.initialize_condensed_dofs();
+  const auto m = cast_int<numeric_index_type>(_eigen_sys.local_non_condensed_dofs_vector.size());
+  auto M = m;
+  _communicator.sum(M);
+  if (_eigen_sys.has_condensed_matrix_A())
+  {
+    _eigen_sys.get_condensed_matrix_A().init(M, M, m, m);
+    // A bit ludicrously MatCopy requires the matrix being copied to to be assembled
+    _eigen_sys.get_condensed_matrix_A().close();
+  }
+  if (_eigen_sys.has_condensed_matrix_B())
+  {
+    _eigen_sys.get_condensed_matrix_B().init(M, M, m, m);
+    _eigen_sys.get_condensed_matrix_B().close();
+  }
+  if (_eigen_sys.has_condensed_precond_matrix())
+  {
+    _eigen_sys.get_condensed_precond_matrix().init(M, M, m, m);
+    _eigen_sys.get_condensed_precond_matrix().close();
+  }
+}
+
 void
 NonlinearEigenSystem::postInit()
 {
   NonlinearSystemBase::postInit();
-  _eigen_sys.initialize_condensed_matrices();
+  initializeCondensedMatrices();
 }
 
 void
 NonlinearEigenSystem::reinit()
 {
   NonlinearSystemBase::reinit();
-  _eigen_sys.initialize_condensed_matrices();
+  initializeCondensedMatrices();
 }
 
 void
@@ -216,7 +245,7 @@ NonlinearEigenSystem::solve()
   // We apply initial guess for only nonlinear solver
   if (_eigen_problem.isNonlinearEigenvalueSolver())
   {
-    if (_eigen_sys.have_condensed_dofs())
+    if (_num_constrained_dofs)
     {
       subvec = solution().get_subvector(_eigen_sys.local_non_condensed_dofs_vector);
       _eigen_sys.set_initial_space(*subvec);
@@ -251,7 +280,7 @@ NonlinearEigenSystem::solve()
   if (n_converged_eigenvalues)
     getConvergedEigenpair(_eigen_problem.activeEigenvalueIndex());
 
-  if (_eigen_problem.isNonlinearEigenvalueSolver() && _eigen_sys.have_condensed_dofs())
+  if (_eigen_problem.isNonlinearEigenvalueSolver() && _num_constrained_dofs)
     solution().restore_subvector(std::move(subvec), _eigen_sys.local_non_condensed_dofs_vector);
 }
 
@@ -261,7 +290,7 @@ NonlinearEigenSystem::attachSLEPcCallbacks()
   // Tell libmesh not to close matrices before solve
   _eigen_sys.get_eigen_solver().set_close_matrix_before_solve(false);
 
-  if (_eigen_sys.have_condensed_dofs())
+  if (_num_constrained_dofs)
   {
     // Condensed Matrix A
     if (_eigen_sys.has_condensed_matrix_A())