% mpicc -o pingpong -O pingpong.c
% mpirun -np 2 pingpong 
Kind            n       time (sec)      Rate (MB/sec)
Isend/Irecv     1       0.000102        0.078604
Isend/Irecv     2       0.000102        0.157401
Isend/Irecv     4       0.000103        0.310640
Isend/Irecv     8       0.000131        0.489217
Isend/Irecv     16      0.000131        0.974203
Isend/Irecv     32      0.000148        1.726855
Isend/Irecv     64      0.000165        3.100228
Isend/Irecv     128     0.000199        5.148454
Isend/Irecv     256     0.000258        7.942731
Isend/Irecv     512     0.000365        11.211941
Isend/Irecv     1024    0.000584        14.018996
Isend/Irecv     2048    0.001073        15.274495
Isend/Irecv     4096    0.001706        19.211164
Isend/Irecv     8192    0.002760        23.745680
Isend/Irecv     16384   0.004859        26.975654
Isend/Irecv     32768   0.008890        29.487223
Isend/Irecv     65536   0.016999        30.841831
Isend/Irecv     131072  0.033209        31.575328
Isend/Irecv     262144  0.065526        32.005061
Isend/Irecv     524288  0.130904        32.040958
Isend/Irecv     1048576 0.261332        32.099458