Document quadratic bottleneck induced by data structure

git-svn-id: https://svn.dealii.org/trunk@15852 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/deal.II/include/dofs/dof_constraints.h b/deal.II/deal.II/include/dofs/dof_constraints.h
index 0cf61059ee98776c6c8b8780876af4c7ca972744..f68562e45c859d2a209ab80ad7af812b805cc2b9 100644 (file)
--- a/deal.II/deal.II/include/dofs/dof_constraints.h
+++ b/deal.II/deal.II/include/dofs/dof_constraints.h
@@ -683,6 +683,26 @@ class ConstraintMatrix : public Subscriptor
                                      * Same function as above, but
                                      * condenses square compressed
                                      * sparsity patterns.
+                                     *
+                                     * Given the data structure used
+                                     * by CompressedSparsityPattern,
+                                     * this function becomes
+                                     * quadratic in the number of
+                                     * degrees of freedom for large
+                                     * problems and can dominate
+                                     * setting up linear systems when
+                                     * several hundred thousand or
+                                     * millions of unknowns are
+                                     * involved and for problems with
+                                     * many nonzero elements per row
+                                     * (for example for vector-valued
+                                     * problems or hp finite
+                                     * elements). In this case, it is
+                                     * advisable to use the
+                                     * CompressedSetSparsityPattern
+                                     * class instead, see for example
+                                     * @ref step_27 "step-27" and
+                                     * @ref step_31 "step-31".
                                      */
     void condense (CompressedSparsityPattern &sparsity) const;
 
@@ -698,6 +718,26 @@ class ConstraintMatrix : public Subscriptor
                                      * Same function as above, but
                                      * condenses square compressed
                                      * sparsity patterns.
+                                     *
+                                     * Given the data structure used
+                                     * by BlockCompressedSparsityPattern,
+                                     * this function becomes
+                                     * quadratic in the number of
+                                     * degrees of freedom for large
+                                     * problems and can dominate
+                                     * setting up linear systems when
+                                     * several hundred thousand or
+                                     * millions of unknowns are
+                                     * involved and for problems with
+                                     * many nonzero elements per row
+                                     * (for example for vector-valued
+                                     * problems or hp finite
+                                     * elements). In this case, it is
+                                     * advisable to use the
+                                     * BlockCompressedSetSparsityPattern
+                                     * class instead, see for example
+                                     * @ref step_27 "step-27" and
+                                     * @ref step_31 "step-31".
                                      */
     void condense (BlockCompressedSparsityPattern &sparsity) const;