% mpicc -o pingpong -O pingpong.c
% mpirun -np 4 pingpong
Kind            n       time (sec)      Rate (MB/sec)
Send/Recv       1       0.001731        0.004623
Send/Recv       2       0.001696        0.009432
Send/Recv       4       0.002104        0.015211
Send/Recv       8       0.002233        0.028660
Send/Recv       16      0.001733        0.073871
Send/Recv       32      0.001886        0.135732
Send/Recv       64      0.002374        0.215682
Send/Recv       128     0.003464        0.295582
Send/Recv       256     0.004840        0.423155
Send/Recv       512     0.009620        0.425757
Send/Recv       1024    0.017626        0.464781
Send/Recv       2048    0.028715        0.570573
Send/Recv       4096    0.057779        0.567122
Send/Recv       8192    0.130371        0.502687
Send/Recv       16384   0.206248        0.635507
Send/Recv       32768   0.452411        0.579437
Send/Recv       65536   1.154206        0.454241
Send/Recv       131072  2.241706        0.467758
Send/Recv       262144  4.576753        0.458218
Send/Recv       524288  8.277655        0.506702
Send/Recv       1048576 15.917955       0.526990