You need to use MPI_Comm_spawn_multiple. Despite the name, it results in a single communicator being created by a single launch - it just allows you to specify multiple applications to run.

In this case, we use the same app, but give each element a different "host" info key to get the behavior we want. Looks something like this:

    MPI_Comm child;
    char *cmds[3] = {"myapp", "myapp", "myapp"};
    MPI_Info info[3];
    int maxprocs[] = { 1, 3, 1 };

  MPI_Info_set(info[0], "host", "m1");

  MPI_Info_set(info[1], "host", "m2");

  MPI_Info_set(info[2], "host", "m1");
        MPI_Comm_spawn_multiple(3, cmds, NULL, maxprocs, 
                                info, 0, MPI_COMM_WORLD,
                                &child, MPI_ERRCODES_IGNORE);

I won't claim the above is correct - but it gives the gist of the idea.

On May 16, 2011, at 12:19 PM, Thatyene Louise Alves de Souza Ramos wrote:


I have the same issue and I've been searching how to do this, but I couldn't find. 

What exactly must be the string in the host info key to do what Rodrigo described?

<<< Inside your master, you would create an MPI_Info key "host" that has a value 
<<< consisting of a string "host1,host2,host3" identifying the hosts you want 
<<< your slave to execute upon. Those hosts must have been included in 
<<< my_hostfile. Include that key in the MPI_Info array passed to your Spawn.

I tried to do what you said above but ompi ignores the repetition of hosts. Using Rodrigo's example I did:

host info key = "m1,m2,m2,m2,m3" and number of processes = 5 and the result was

m1 -> 2
m2 -> 2
m3 -> 1

and not

m1 -> 1
m2 -> 3
m3 -> 1

as I wanted.

Thanks in advance.

Thatyene Ramos

On Fri, May 13, 2011 at 9:16 PM, Ralph Castain <> wrote:
I believe I answered that question. You can use the hostfile info key, or you can use the host info key - either one will do what you require.

On May 13, 2011, at 4:11 PM, Rodrigo Silva Oliveira wrote:


I think I was not specific enough. I need to spawn the copies of a process in a unique mpi_spawn call. It is, I have to specify a list of machines and how many copies of the process will be spawned on each one. Is it possible?

I would be something like that:

machines     #copies
m1                1
m2                3
m3                1

After an unique call to spawn, I want the copies running in this fashion. I tried use a hostfile with the option slot, but I'm not sure if it is the best way.


m1 slots=1
m2 slots=3
m3 slots=1


Rodrigo Silva Oliveira
M.Sc. Student - Computer Science
Universidade Federal de Minas Gerais
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