Updating reduce_scatter decision function based on MX results up to 64 nodes and both 1ppn and 2ppn

configurations.

This commit was SVN r13945.

ompi/mca/coll/tuned/coll_tuned_decision_fixed.c

@@ -416,9 +416,12 @@ int ompi_coll_tuned_reduce_scatter_intra_dec_fixed( void *sbuf, void *rbuf,
                                                     struct ompi_op_t *op,
                                                     struct ompi_communicator_t *comm)
 {
-   int comm_size, i;
+   int comm_size, i, pow2;
    size_t total_message_size, dsize;
-   const size_t large_message_size = 512 * 1024;
+   const double a = 0.0012;
+   const double b = 8.0;
+   const size_t small_message_size = 12 * 1024;
+   const size_t large_message_size = 256 * 1024;
 
    OPAL_OUTPUT((ompi_coll_tuned_stream, "ompi_coll_tuned_reduce_scatter_intra_dec_fixed"));
 
@@ -438,7 +441,12 @@ int ompi_coll_tuned_reduce_scatter_intra_dec_fixed( void *sbuf, void *rbuf,
    for (i = 0; i < comm_size; i++) { total_message_size += rcounts[i]; }
    total_message_size *= dsize;
 
-   if (total_message_size <= large_message_size) {
+   /* compute the nearest power of 2 */
+   for (pow2 = 1; pow2 < comm_size; pow2 <<= 1);
+
+   if ((total_message_size <= small_message_size) ||
+       ((total_message_size <= large_message_size) && (pow2 == comm_size)) ||
+       (comm_size >= a * total_message_size + b)) {
       return 
          ompi_coll_tuned_reduce_scatter_intra_basic_recursivehalving(sbuf, 
                                                                      rbuf,