Below are the numbers while using MPI_PROC_NULL. The trend is the same. I guess the next step is to start instrumenting the code. BG/P: ---- # MPI Message Rate Test with 4 Threads # nthreads Size Latency (us) Message Rate (MMPS) 1 0 1.02 0.981 2 0 1.21 0.825 3 0 2.48 0.403 4 0 3.78 0.265 Breadboard: ---------- # MPI Message Rate Test with 4 Threads # nthreads Size Latency (us) Message Rate (MMPS) 1 0 0.39 2.589 2 0 0.85 1.183 3 0 4.68 0.213 4 0 4.36 0.229 Thanks. -- Pavan -- Pavan Balaji http://www.mcs.anl.gov/~balaji