From: Wolfgang Bangerth <bangerth@math.tamu.edu>
Date: Mon, 4 Apr 2005 18:40:20 +0000 (+0000)
Subject: Make the default value of preset_nonzero_locations = true.
X-Git-Tag: v8.0.0~14203
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4ff1dda208ff5dfa1c9644ae549bbc46b9aa2d99;p=dealii.git

Make the default value of preset_nonzero_locations = true.


git-svn-id: https://svn.dealii.org/trunk@10344 0785d39b-7218-0410-832d-ea1e28bc413d
---

diff --git a/deal.II/doc/news/changes.html b/deal.II/doc/news/changes.html
index 66c1ef43cb..f8b50394a9 100644
--- a/deal.II/doc/news/changes.html
+++ b/deal.II/doc/news/changes.html
@@ -230,6 +230,19 @@ inconvenience this causes.
 <h3>lac</h3>
 
 <ol>
+  <li> <p>
+       Fixed: The <code class="class">PETScWrappers::MPI::SparseMatrix</code>
+       class had functions that allow to take a pre-existing sparsity pattern
+       as the basis for entry allocation. These function had an option to
+       allow pre-setting these entries in the underlying data structures, but
+       this option was disabled by default because of unresolved questions
+       about its effectiveness. This has now been fixed: The code now properly
+       initializes these elements, and makes the resulting matrix much faster
+       to use.
+       <br> 
+       (WB, 2005/04/04)
+       </p>
+
   <li> <p>
        New: The <code class="class">ProductSparseMatrix</code>
        implements the product of two rectangular sparse matrices with
diff --git a/deal.II/lac/include/lac/petsc_parallel_sparse_matrix.h b/deal.II/lac/include/lac/petsc_parallel_sparse_matrix.h
index c71dde22e0..d2cfa586ef 100644
--- a/deal.II/lac/include/lac/petsc_parallel_sparse_matrix.h
+++ b/deal.II/lac/include/lac/petsc_parallel_sparse_matrix.h
@@ -261,29 +261,13 @@ namespace PETScWrappers
                                           * significantly more efficient to
                                           * get memory allocation right from
                                           * the start.
-                                          *
-                                          * Despite the fact that it would
-                                          * seem to be an obvious win, setting
-                                          * the @p preset_nonzero_locations
-                                          * flag to @p true doesn't seem to
-                                          * accelerate program. Rather on the
-                                          * contrary, it seems to be able to
-                                          * slow down entire programs
-                                          * somewhat. This is suprising, since
-                                          * we can use efficient function
-                                          * calls into PETSc that allow to
-                                          * create multiple entries at once;
-                                          * nevertheless, given the fact that
-                                          * it is inefficient, the respective
-                                          * flag has a default value equal to
-                                          * @p false.
                                           */
         SparseMatrix (const MPI_Comm                  &communicator,
                       const CompressedSparsityPattern &sparsity_pattern,
                       const std::vector<unsigned int> &local_rows_per_process,
                       const std::vector<unsigned int> &local_columns_per_process,
                       const unsigned int               this_process,
-                      const bool                       preset_nonzero_locations = false);
+                      const bool                       preset_nonzero_locations = true);
 
                                          /**
                                           * This operator assigns a scalar to
@@ -368,29 +352,13 @@ namespace PETScWrappers
                                           * significantly more efficient to
                                           * get memory allocation right from
                                           * the start.
-                                          *
-                                          * Despite the fact that it would
-                                          * seem to be an obvious win, setting
-                                          * the @p preset_nonzero_locations
-                                          * flag to @p true doesn't seem to
-                                          * accelerate program. Rather on the
-                                          * contrary, it seems to be able to
-                                          * slow down entire programs
-                                          * somewhat. This is suprising, since
-                                          * we can use efficient function
-                                          * calls into PETSc that allow to
-                                          * create multiple entries at once;
-                                          * nevertheless, given the fact that
-                                          * it is inefficient, the respective
-                                          * flag has a default value equal to
-                                          * @p false.
                                           */
         void reinit (const MPI_Comm                  &communicator,
                      const CompressedSparsityPattern &sparsity_pattern,
                      const std::vector<unsigned int> &local_rows_per_process,
                      const std::vector<unsigned int> &local_columns_per_process,
                      const unsigned int               this_process,
-                     const bool                       preset_nonzero_locations = false);
+                     const bool                       preset_nonzero_locations = true);
         
                                          /**
                                           * Return a reference to the MPI