Performance tuning for inter-node communication (#66)

This PR addresses two issues:
 - reduce the number of contexts supported by the host-interface by
   default to 1, we are not using those at the moment, and hence
   we now create fewer MPI_Win at the startup
 - introduces a micro-sleep in RO progress engine in case there are no
   pending requests. This leads significant performance improvements
   observed for inter-node communication with THor2 NICs.

src/reverse_offload/mpi_transport.cpp

@@ -25,6 +25,7 @@
 #include <functional>
 #include <utility>
 #include <vector>
+#include <unistd.h>
 
 #include "../host/host.hpp"
 #include "backend_ro.hpp"
@@ -591,6 +592,10 @@ void MPITransport::progress() {
     const int tag{1000};
     int flag{0};
     MPI_Status status{};
+
+    // Slowing the progress engine down a bit avoid hammering the memory subsystem.
+    // This leads to significant performance benefits
+    usleep (rocshmem_env_config.ro_progress_delay);
     NET_CHECK(MPI_Iprobe(MPI_ANY_SOURCE, tag, ro_net_comm_world, &flag, &status));
   } else {
     DPRINTF("Testing all outstanding requests (%zu)\n", requests.size());