Section: 5.4 Collective operations
| 4 | pair4 |
| 5 | pipe0 |
| 6 | pipe1 |
def | 7 | safe |
8 smp | ||
By looping through these alternatives the performance of IS varies:
algorithm 0: Mop/s total = 95.60
algorithm 1: Mop/s total = 78.37
algorithm 2: Mop/s total = 34.44
algorithm 3: Mop/s total = 61.77
algorithm 4: Mop/s total = 41.00
algorithm 5: Mop/s total = 49.14
algorithm 6: Mop/s total = 85.17
algorithm 7: Mop/s total = 60.22
algorithm 8: Mop/s total = 48.61
For this particular combination of
5.4.1 Finding the best algorithm
Consider the image processing example from Chapter 4 which containes four collective operations. All of these can be tuned with respect to algorithm according to the following pattern:
user% for a in <range>; do
\>;
\>;mpimon <application>
For example, trying out the alternative algorithms for MPI_Reduce with two processes can be done as follows (assuming Bourne Again Shell [bash]:
user% for a in 0 1 2 3 4 5 6 7 8; do \>; SCAMPI_REDUCE_ALGORITHM=$a
\>;
Given that the application then reports the timing of the relevant parts of the code a best choice can be made. Note however that with multiple collective operations working in the same program there may be interference between the algorithms. Also, the performance of the implementations is interconnect dependent.
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