zoltan: added a test that detects bad implementations of MPI_MINLOC. #…

…8798

packages/zoltan/test/TestMPI/CMakeLists.txt

@@ -1,3 +1,10 @@
+TRIBITS_ADD_EXECUTABLE_AND_TEST(
+ mpiMinLoc
+ SOURCES mpiMinLoc.c
+ COMM mpi
+ FAIL_REGULAR_EXPRESSION "FAIL"
+ PASS_REGULAR_EXPRESSION "PASS"
+ )
 
 TRIBITS_ADD_EXECUTABLE_AND_TEST(
  canarySelfMessages

packages/zoltan/test/TestMPI/mpiMinLoc.c

@@ -0,0 +1,54 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <mpi.h>
+
+/* Zoltan's gold-standard answers were generated with MPI that returns the
+ * lowest numbered rank in MPI_Allreduce with MPI_MINLOC when the compared
+ * values are equal on several processors. 
+ *
+ * The MPI standard indicates that MPI_MINLOC should return the lowest
+ * numbered rank; see, for example, 
+ * https://www.mpi-forum.org/docs/mpi-1.1/mpi-11-html/node79.html
+ *
+ * Some implementations of MPI (e.g., Spectrum MPI) incorrectly return
+ * the largest rank with the minimum value.
+ *
+ * Some Zoltan PHG tests will fail if the lowest rank is not returned.
+ * Zoltan_PHG_CoarsePartition tries several coarse partitions and selects
+ * the best wrt hyperedge cut. When two processors compute equally good
+ * but different coarse partitions, the choice of a different coarse
+ * partition leads to a different final partition. The final partition is
+ * correct, but it doesn't match the gold-standard answers and, thus, causes
+ * Zoltan tests to fail. 
+ *
+ * See, for example, https:/trilinos/Trilinos/issues/8798
+ *
+ * This test detects the condition needed in MINLOC to match the gold-standard
+ * answers. If this test fails, Zoltan will operate correctly and produce
+ * correct partitions, but the Zoltan tests may report failures.
+ */
+
+int main(int narg, char **arg)
+{
+ struct {
+ float val;
+ int rank;
+ } local, global;
+ int me;
+
+ MPI_Init(&narg, &arg);
+ MPI_Comm_rank(MPI_COMM_WORLD, &me);
+
+ local.val = 1.0;
+ local.rank = me;
+
+ MPI_Allreduce(&local, &global, 1, MPI_FLOAT_INT, MPI_MINLOC, MPI_COMM_WORLD);
+
+ printf("%d MINLOC value %f on rank %d\n\n", me, global.val, global.rank);
+
+ if (global.rank == 0) printf("PASSED\n\n");
+ else printf("FAILED\n\n");
+
+ MPI_Finalize();
+ return 0;
+}