I implemented a peer-to-peer connection in MPI using MPI_Open_port and MPI_Comm_accept. I run a server and a client program using
rafael@server1:~$ mpirun server
rafael@server2:~$ mpirun client
on different computers. I noticed that
rafael@server1:~$ ./server
rafael@server2:~$ ./client
also works just fine. Do you know if there is any difference between starting an MPI executable with and without mpirun? 
Of course, I can't give additional parameters (e.g. mpirun --mca btl self,openib), and the rank of all processes is 0, which is all totally fine. But is there anything less obvious?
 
     
     
    