Start LAM/MPI once, before running any MPI programs.
It is not necessary to start MPI before compiling and linking MPI programs.
The LAM/MPI program "lamd" will run until halted.
[psiders@beowulf source] lamboot -v ~/lamhosts

   LAM 6.5.4/MPI 2 C++/ROMIO - University of Notre Dame

   Executing hboot on n0 (beowulf.d.umn.edu - 1 CPU)...
   Executing hboot on n1 (node8.d.umn.edu - 1 CPU)...
   Executing hboot on n2 (node7.d.umn.edu - 1 CPU)...
   Executing hboot on n3 (node6.d.umn.edu - 1 CPU)...
   Executing hboot on n4 (node5.d.umn.edu - 1 CPU)...
   Executing hboot on n5 (node4.d.umn.edu - 1 CPU)...
   Executing hboot on n6 (node2.d.umn.edu - 1 CPU)...

lamboot reads the list of available computers from the file specified on the "lamboot" command line. In the example above, that is lamhosts.
[psiders@beowulf source] cat ~/lamhosts.

   beowulf.d.umn.edu
   node8.d.umn.edu
   node7.d.umn.edu
   node6.d.umn.edu
   node5.d.umn.edu
   node4.d.umn.edu
   node2.d.umn.edu
   #node3.d.umn.edu machine is down
   #node1.d.umn.edu login fails

If any of the computers in lamhosts are down or refuse to allow remote command execution, lamboot will write a verbose error message containing suggestions.

The lamnodes command shows node assignments
[psiders@beowulf source] lamnodes