Add flag useFixedOrder to the coarsen method of AMGs ParallelIndicesCoarsener.

If set to true, the indices for the coarser grid will be created in a fixed order, making parallel runs reproducible but the runtime is possibly not ideal.
If set to false (which is the default), the order depends on the order of messages recieved from the processes responsible for the respective parts of the finer grid.
Then the indices on the coarser grid may differ from run to run.

dune/istl/paamg/indicescoarsener.hh

@@ -83,6 +83,10 @@ namespace Dune
        * @param aggregates The mapping of unknowns onto aggregates.
        * @param coarseInfo The information about the parallel data decomposition
        * on the coarse level.
+       * @param noAggregates
+       * @param useFixedOrder Flag indicating if creating indices for the coarser level
+       * should be done in a fixed order, i.e., the order in which the rows were sent.
+       * If set to true, this makes the runs reproducible but it might slow down performance.
        * @return The number of unknowns on the coarse level.
        */
       template<typename Graph, typename VM>
@@ -92,7 +96,8 @@ namespace Dune
               VM& visitedMap,
               AggregatesMap<typename Graph::VertexDescriptor>& aggregates,
               ParallelInformation& coarseInfo,
-              typename Graph::VertexDescriptor noAggregates);
+              typename Graph::VertexDescriptor noAggregates,
+              bool useFixedOrder = false);
 
     private:
       template<typename G, typename I>
@@ -187,7 +192,8 @@ namespace Dune
                                            const AggregatesMap<typename Graph::VertexDescriptor>& aggregates,
                                            ParallelIndexSet& coarseIndices,
                                            RemoteIndices& coarseRemote,
-                                           ParallelAggregateRenumberer<Graph,I>& renumberer);
+                                           ParallelAggregateRenumberer<Graph,I>& renumberer,
+                                           bool useFixedOrder);
 
     };
 
@@ -219,7 +225,8 @@ namespace Dune
               VM& visitedMap,
               AggregatesMap<typename Graph::VertexDescriptor>& aggregates,
               SequentialInformation& coarseInfo,
-              typename Graph::VertexDescriptor noAggregates);
+              typename Graph::VertexDescriptor noAggregates,
+              bool useFixedOrder = false);
     };
 
 #if HAVE_MPI
@@ -231,13 +238,14 @@ namespace Dune
                                            VM& visitedMap,
                                            AggregatesMap<typename Graph::VertexDescriptor>& aggregates,
                                            ParallelInformation& coarseInfo,
-                                           [[maybe_unused]] typename Graph::VertexDescriptor noAggregates)
+                                           [[maybe_unused]] typename Graph::VertexDescriptor noAggregates,
+                                           bool useFixedOrder)
     {
       ParallelAggregateRenumberer<Graph,typename ParallelInformation::GlobalLookupIndexSet> renumberer(aggregates, fineInfo.globalLookup());
       buildCoarseIndexSet(fineInfo, fineGraph, visitedMap, aggregates,
                           coarseInfo.indexSet(), renumberer);
       buildCoarseRemoteIndices(fineInfo.remoteIndices(), aggregates, coarseInfo.indexSet(),
-                               coarseInfo.remoteIndices(), renumberer);
+                               coarseInfo.remoteIndices(), renumberer, useFixedOrder);
 
       return renumberer;
     }
@@ -318,7 +326,8 @@ namespace Dune
                                                                  const AggregatesMap<typename Graph::VertexDescriptor>& aggregates,
                                                                  ParallelIndexSet& coarseIndices,
                                                                  RemoteIndices& coarseRemote,
-                                                                 ParallelAggregateRenumberer<Graph,I>& renumberer)
+                                                                 ParallelAggregateRenumberer<Graph,I>& renumberer,
+                                                                 bool useFixedOrder)
     {
       std::vector<char> attributes(static_cast<std::size_t>(renumberer));
 
@@ -375,7 +384,7 @@ namespace Dune
       // sync the index set and the remote indices to recompute missing
       // indices
       IndicesSyncer<ParallelIndexSet> syncer(coarseIndices, coarseRemote);
-      syncer.sync(renumberer);
+      syncer.sync(renumberer, useFixedOrder);
 
     }
 
@@ -390,7 +399,8 @@ namespace Dune
       [[maybe_unused]] VM& visitedMap,
       [[maybe_unused]] AggregatesMap<typename Graph::VertexDescriptor>& aggregates,
       [[maybe_unused]] SequentialInformation& coarseInfo,
-      [[maybe_unused]] typename Graph::VertexDescriptor noAggregates)
+      [[maybe_unused]] typename Graph::VertexDescriptor noAggregates,
+      [[maybe_unused]] bool useFixedOrder)
     {
       return noAggregates;
     }