Add Connectivity.java example for testing MPI process communication

This example uses the Java bindings for MPI to perform pairwise send/recv
calls between processes to verify inter-process connectivity. Uses
MPI.COMM_WORLD and sendRecv to avoid deadlocks.

Signed-off-by: Vasista Dhyasani <vasi.dhyasani@gmail.com>

Author: vdhyasani17

examples/Connectivity.java

@@ -0,0 +1,78 @@
+/*
+ * Test the connectivity between all processes
+ */
+
+import mpi.*;
+import java.nio.IntBuffer;
+
+class Connectivity {
+    public static void main(String args[]) throws MPIException {
+         MPI.Init(args);
+ 
+         /*
+          * MPI.COMM_WORLD is the communicator provided when MPI is
+          * initialized. It contains all the processes that are created
+          * upon program execution.
+          */
+         int myRank = MPI.COMM_WORLD.getRank();
+         int numProcesses = MPI.COMM_WORLD.getSize();
+         boolean verbose = false;
+         String processorName = MPI.getProcessorName();
+ 
+         for (String arg : args) {
+             if (arg.equals("-v") || arg.equals("--verbose")) {
+                 verbose = true;
+                 break;
+             }
+         }
+ 
+         for (int i = 0; i < numProcesses; i++) {
+             /* Find current process */
+             if (myRank == i) {
+                 /* send to and receive from all higher ranked processes */
+                 for (int j = i + 1; j < numProcesses; j++) {
+                     if (verbose)
+                         System.out.printf("Checking connection between rank %d on %s and rank %d\n", i, processorName,
+                                j);
+ 
+                     /*
+                      * rank is the Buffer passed into sendRecv to send to rank j.
+                      * rank is populated with myRank, which is the data to send off
+                      * peer is the Buffer received from rank j to current rank
+                      */
+                     IntBuffer rank = MPI.newIntBuffer(1);
+                     IntBuffer peer = MPI.newIntBuffer(1);
+                     rank.put(0, myRank);
+ 
+                     /*
+                      * To avoid deadlocks, use combined sendRecv operation.
+                      * This performs a send and recv as a combined atomic operation
+                      * and allow MPI to efficiently handle the requests internally.
+                      */
+                     MPI.COMM_WORLD.sendRecv(rank, 1, MPI.INT, j, myRank, peer, 1, MPI.INT, j, j);
+                 }
+             } else if (myRank > i) {
+                 IntBuffer rank = MPI.newIntBuffer(1);
+                 IntBuffer peer = MPI.newIntBuffer(1);
+                 rank.put(0, myRank);
+ 
+                 /* receive from and reply to rank i */
+                 MPI.COMM_WORLD.sendRecv(rank, 1, MPI.INT, i, myRank, peer, 1, MPI.INT, i, i);
+             }
+         }
+ 
+         /* Wait for all processes to reach barrier before proceeding */
+         MPI.COMM_WORLD.barrier();
+ 
+         /*
+          * Once all ranks have reached the barrier,
+          * have only one process print out the confirmation message.
+          * In this case, we are having the "master" process print the message.
+          */
+         if (myRank == 0) {
+             System.out.printf("Connectivity test on %d processes PASSED.\n", numProcesses);
+         }
+ 
+         MPI.Finalize();
+     }
+}

examples/Makefile

@@ -60,6 +60,7 @@ EXAMPLES = \
 	 ring_oshmemfh \
 	 Ring.class \
 	 connectivity_c \
+	 Connectivity.class \
 	 oshmem_shmalloc \
 	 oshmem_circular_shift \
 	 oshmem_max_reduction \