fix metis partitioning with too many processors

If you have more processors than cells to partition, METIS will behave
in a weird and undesired way:
- at some point all subdomains land on a single processor
- METIS produces an error message "***Cannot bisect a graph with 0
vertices!"

diff --git a/source/lac/sparsity_tools.cc b/source/lac/sparsity_tools.cc
index a3837ac34bd2ce7694b55179b351da1091963c45..813c4edd33e0319eea8767c8404dcbcf989c1288 100644 (file)
--- a/source/lac/sparsity_tools.cc
+++ b/source/lac/sparsity_tools.cc
@@ -82,13 +82,19 @@ namespace SparsityTools
     dummy;                                    // the numbers of edges cut by the
     // resulting partition
 
+    // We can not partition n items into more than n parts. METIS will
+    // generate non-sensical output (everything is owned by a single process)
+    // and complain with a message (but won't return an error code!):
+    // ***Cannot bisect a graph with 0 vertices!
+    // ***You are trying to partition a graph into too many parts!
+    nparts = std::min(n, nparts);
+
     // use default options for METIS
     idx_t options[METIS_NOPTIONS];
     METIS_SetDefaultOptions (options);
 
-    // one more nuisance: we have to copy our
-    // own data to arrays that store signed
-    // integers :-(
+    // one more nuisance: we have to copy our own data to arrays that store
+    // signed integers :-(
     std::vector<idx_t> int_rowstart(1);
     int_rowstart.reserve(sparsity_pattern.n_rows()+1);
     std::vector<idx_t> int_colnums;
@@ -106,12 +112,10 @@ namespace SparsityTools
     // Make use of METIS' error code.
     int ierr;
 
-    // Select which type of partitioning to
-    // create
+    // Select which type of partitioning to create
 
-    // Use recursive if the number of
-    // partitions is less than or equal to 8
-    if (n_partitions <= 8)
+    // Use recursive if the number of partitions is less than or equal to 8
+    if (nparts <= 8)
       ierr = METIS_PartGraphRecursive(&n, &ncon, &int_rowstart[0], &int_colnums[0],
                                       NULL, NULL, NULL,
                                       &nparts,NULL,NULL,&options[0],
@@ -124,14 +128,11 @@ namespace SparsityTools
                                  &nparts,NULL,NULL,&options[0],
                                  &dummy,&int_partition_indices[0]);
 
-    // If metis returns normally, an
-    // error code METIS_OK=1 is
-    // returned from the above
-    // functions (see metish.h)
+    // If metis returns normally, an error code METIS_OK=1 is returned from
+    // the above functions (see metish.h)
     AssertThrow (ierr == 1, ExcMETISError (ierr));
 
-    // now copy back generated indices into the
-    // output array
+    // now copy back generated indices into the output array
     std::copy (int_partition_indices.begin(),
                int_partition_indices.end(),
                partition_indices.begin());

diff --git a/tests/metis/metis_04.output b/tests/metis/metis_04.output
index 753e2b4819852ed7c9ed356a8869640490d87aff..b9c688d08cde0cd6f47f994d7455f039fcb1c1a2 100644 (file)
--- a/tests/metis/metis_04.output
+++ b/tests/metis/metis_04.output
@@ -14,27 +14,27 @@ DEAL::1 3 0 5 1 5 2
 DEAL:: procs = 7
 DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 8
-DEAL::1 5 0 7 3 4 2 
+DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 9
-DEAL::3 3 3 3 3 3 3 
+DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 10
-DEAL::4 4 4 4 4 4 4 
+DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 11
-DEAL::10 10 10 10 10 10 10 
+DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 12
-DEAL::11 11 11 11 11 11 11 
+DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 13
-DEAL::2 2 2 2 2 2 2 
+DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 14
-DEAL::9 9 9 9 9 9 9 
+DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 15
-DEAL::6 6 6 6 6 6 6 
+DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 16
-DEAL::7 7 7 7 7 7 7 
+DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 17
-DEAL::7 7 7 7 7 7 7 
+DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 18
-DEAL::8 8 8 8 8 8 8 
+DEAL::2 3 0 6 2 5 4 
 DEAL:: procs = 19
-DEAL::8 8 8 8 8 8 8 
+DEAL::2 3 0 6 2 5 4 
 DEAL::OK