6.6 Using LAM/MPI with OSCAR

Before we leave OSCAR, let's look at a programming example. You can use this to convince yourself that everything is really working. You can find several LAM/MPI examples in /usr/share/doc/lam-oscar-7.0/examples and the documentation in /opt/lam-7.0/share/lam/doc. (For MPICH, look in /opt/mpich-1.2.5.10-ch_p4-gcc/examples for code and /opt/mpich-1.2.5.10-ch_p4-gcc/doc for documentation.)

Log on as a user other than root and verify that LAM/MPI is selected using switcher.

[sloanjd@amy doc]$ switcher mpi --show

user:default=lam-7.0

system:exists=true

If necessary, change this and log off and back on.

If you haven't logged onto the individual machines, you need to do so now using ssh to register each machine with ssh. You could do this with a separate command for each machine.

[sloanjd@amy sloanjd]$ ssh node1

...

Using a shell looping command is probably better since it will ensure that you don't skip any machines and can reduce typing. With the Bash shell, the following command will initiate your logon to the machines node1 through node99, each in turn.

[sloanjd@amy sloanjd]$ for ((i=1; i<100; i++))

> do

>   ssh node${i}

> done

Just adjust the loop for a different number of machines. You will need to adjust the syntax accordingly for other shells. This goes fairly quickly and you'll need to do this only once.

Create a file that lists the individual machines in the cluster by IP address. For example, you might create a file called myhosts like the following:

[sloanjd@amy sloanjd]$ cat myhosts

172.16.1.1

172.16.1.2

172.16.1.3

172.16.1.4

172.16.1.5

This should contain the server as well as the clients.

Next, run lamboot with the file's name as an argument.

[sloanjd@amy sloanjd]$ lamboot myhosts

   

LAM 7.0/MPI 2 C++/ROMIO - Indiana University

You now have a LAM/MPI daemon running on each machine in your cluster.

Copy over the example you want to run, compile it with mpicc, and then run it with mpirun.

[sloanjd@amy sloanjd]$ cp /usr/share/doc/lam-oscar-7.0/examples/

alltoall/alltoall.c $HOME

[sloanjd@amy sloanjd]$ mpicc -o alltoall alltoall.c

[sloanjd@amy sloanjd]$ mpirun -np 4 alltoall

Rank 0 not sending to myself

Rank 1 sending message "1" to rank 0

Rank 2 sending message "2" to rank 0

...

You should see additional output. The amount will depend on the number of machines in myhosts. Happy coding, everyone!