PABLO: fix parallel 2:1 balance

The first 2:1 balancing was only checking internal octants, however
the balancing algortihm uses marker information from the neighbours
of the checked octants and the neighbours could be ghosts octants.
It is therefore necessary to exchange the adaption markers before
starting balancing a partitioned tree.

Author: andrea-iob

src/PABLO/ParaTree.cpp

@@ -5791,38 +5791,49 @@ namespace bitpit {
             (*m_log) << " " << endl;
         }
 
-        // Process internal octants
-        if (verbose){
-            (*m_log) << " Processing internal octants" << endl;
-        }
-
-        m_octree.localBalance(balanceNewOctants, true, false);
-
+        bool initialStep = true;
+        while (true) {
 #if BITPIT_ENABLE_MPI==1
-        // Propagate marker changes across processes
-        if (!m_serial) {
-            if (verbose){
-                (*m_log) << " Propagating marker changes across processes" << endl;
-            }
-
-            while (true) {
-                // Exchange markers across processes
+            // Exchange markers across processes
+            if (!m_serial) {
                 bool markersUpdated = commMarker();
-                MPI_Allreduce(MPI_IN_PLACE, &markersUpdated, 1, MPI_C_BOOL, MPI_LOR, m_comm);
-                if (!markersUpdated) {
-                    break;
+                if (!initialStep) {
+                    MPI_Allreduce(MPI_IN_PLACE, &markersUpdated, 1, MPI_C_BOOL, MPI_LOR, m_comm);
+                    if (!markersUpdated) {
+                        break;
+                    }
                 }
+            }
+#endif
+
+            // Balance 2:1 the tree
+            //
+            // After the first step only ghost octants are checked for balance, that's because
+            // starting from the second step we only need to propagate marker information across
+            // the processes.
+            bool checkInterior = initialStep;
+            bool checkGhosts   = true;
 
-                // Process ghost octants to propagate marker changes applied by other processes
-                bool balanceUpdated = m_octree.localBalance(balanceNewOctants, false, true);
+            bool balanceUpdated = m_octree.localBalance(balanceNewOctants, checkInterior, checkGhosts);
+#if BITPIT_ENABLE_MPI==1
+            if (!m_serial) {
                 MPI_Allreduce(MPI_IN_PLACE, &balanceUpdated, 1, MPI_C_BOOL, MPI_LOR, m_comm);
                 if (!balanceUpdated) {
                     break;
                 }
+            } else {
+                break;
             }
-        }
+#else
+            BITPIT_UNUSED(balanceUpdated);
+            break;
 #endif
 
+            // This is not anymore the first balance step
+            initialStep = false;
+
+        }
+
         // Print footer
         if (verbose){
             (*m_log) << " 2:1 Balancing reached " << endl;