HP-MPI User's Guide (11th Edition)

Understanding HP-MPI
Running applications on HP-UX and Linux
Chapter 380
For example, if your appfile contains
-h voyager -np 10 send_receive
-h enterprise -np 8 compute_pi
HP-MPI assigns ranks 0 through 9 to the 10 processes running send_receive and
ranks 10 through 17 to the 8 processes running compute_pi.
You can use this sequential ordering of process ranks to your advantage when
you optimize for performance on multihost systems. You can split process groups
according to communication patterns to reduce or remove interhost
communication hot spots.
For example, if you have the following:
• A multi-host run of four processes
• Two processes per host on two hosts
• There is higher communication traffic between ranks 0—2 and 1—3.
You could use an appfile that contains the following:
-h hosta -np 2 program1
-h hostb -np 2 program2
However, this places processes 0 and 1 on hosta and processes 2 and 3 on hostb,
resulting in interhost communication between the ranks identified as having
slow communication:
A more optimal appfile for this example would be
-h hosta -np 1 program1
-h hostb -np 1 program2
-h hosta -np 1 program1
-h hostb -np 1 program2
hosta
process 0
process 1
hostb
process 2
process 3
Slow communication