Merge pull request #16745 from kronbichler/gmres_cleanup

Cleanup of SolverGMRES and SolverFGMRES implementations

doc/news/changes/incompatibilities/20240312Kronbichler

@@ -0,0 +1,11 @@
+Changed: SolverGMRES::AdditionalData now controls the size of the Arnoldi
+basis through the new member variable
+SolverGMRES::AdditionalData::max_basis_size, rather than the number of
+temporary vectors (which is the basis size plus 2). As a result, the default
+value of the basis size is now 30, compared to 28 used before. The old
+variable SolverGMRES::AdditionalData::max_n_tmp_vectors is still available,
+but whenever SolverGMRES::AdditionalData::max_basis_size is set to a non-zero
+value (including the value set by the default constructor), the latter takes
+precedence.
+<br>
+(Martin Kronbichler, 2024/04/12)

doc/news/changes/minor/20240312Kronbichler

@@ -0,0 +1,9 @@
+Improved: The implementations of SolverGMRES and SolverFGMRES have been
+overhauled and made more similar. In particular, SolverFGMRES now uses the
+same internal algorithm to solve the minimization problem in the Arnoldi
+basis, employing Givens rotations in analogy to the setting used by
+SolverGMRES. Since the Arnoldi process is sensitive to roundoff errors, this
+change might slightly affect iteration counts (often giving slightly better
+results).
+<br>
+(Martin Kronbichler, 2024/04/12)

examples/step-12/step-12.cc

@@ -481,7 +481,7 @@ namespace Step12
     SolverControl solver_control(1000, 1e-6 * right_hand_side.l2_norm());
 
     SolverGMRES<Vector<double>>::AdditionalData additional_data;
-    additional_data.max_n_tmp_vectors = 100;
+    additional_data.max_basis_size = 100;
     SolverGMRES<Vector<double>> solver(solver_control, additional_data);
 
     // Here we create the preconditioner,

examples/step-22/doc/results.dox

@@ -544,7 +544,7 @@ $k=100$ temporary vectors:
                                     1e-6*system_rhs.l2_norm());
       GrowingVectorMemory<BlockVector<double> > vector_memory;
       SolverGMRES<BlockVector<double> >::AdditionalData gmres_data;
-      gmres_data.max_n_tmp_vectors = 100;
+      gmres_data.max_basis_size = 100;
 
       SolverGMRES<BlockVector<double> > gmres(solver_control, vector_memory,
                                               gmres_data);