9.1. Quick start: Running MPI applications

Although this section skips many details, it offers examples that will probably work in many environments.

Caution

Note that this section is a “Quick start” — it does not attempt to be comprehensive or describe how to build Open MPI in all supported environments. The examples below may therefore not work exactly as shown in your environment.

Please consult the other sections in this chapter for more details, if necessary.

Open MPI supports both mpirun and mpiexec (they are exactly equivalent) to launch MPI applications. For example:

shell$ mpirun -n 2 mpi-hello-world
# or
shell$ mpiexec -n 2 mpi-hello-world
# or
shell$ mpiexec -n 1 mpi-hello-world : -n 1 mpi-hello-world

are all equivalent. For simplicity, the rest of this documentation will simply refer to mpirun.

Error

TODO Link to the mpirun(1) page here.

Note that the mpirun command supports a large number of options. Be sure to see the mpirun man page for much more information.

9.1.1. Launching on a single host

It is common to develop MPI applications on a single laptop or workstation. In such cases, use mpirun and specify how many MPI processes you want to launch via the -n option:

shell$ mpirun -n 6 mpi-hello-world
Hello world, I am 0 of 6 (running on my-laptop))
Hello world, I am 1 of 6 (running on my-laptop)
...
Hello world, I am 5 of 6 (running on my-laptop)

If you do not specify the -n option, mpirun will default to launching as many MPI processes as there are processor cores (not hyperthreads) on the machine.

9.1.2. Launching in a non-scheduled environments (via ssh)

In general, Open MPI requires the following to launch and run MPI applications:

  1. You must be able to login to remote nodes non-interactively (e.g., without entering a password or passphrase).

  2. Open MPI’s executables must be findable (e.g., in your PATH).

  3. Open MPI’s libraries must be findable (e.g., in your LD_LIBRARY_PATH).

mpirun accepts a --hostfile parameter to specify a hostfile containing one hostname per line:

shell$ cat my-hostfile.txt
node1.example.com
node2.example.com
node3.example.com slots=2
node4.example.com slots=10

The optional slots attribute tells Open MPI the maximum number of processes that can be allocated to that node. If slots is not provided, Open MPI — by default — uses the number of processor cores (not hyperthreads) on that node.

Assuming that each of the 4 nodes in my-hostfile.txt have 16 cores:

shell$ mpirun --hostfile my-hostfile.txt mpi-hello-world
Hello world, I am 0 of 44 (running on node1.example.com)
Hello world, I am 1 of 44 (running on node1.example.com)
...
Hello world, I am 15 of 44 (running on node1.example.com)
Hello world, I am 16 of 44 (running on node2.example.com)
Hello world, I am 17 of 44 (running on node2.example.com)
...
Hello world, I am 31 of 44 (running on node2.example.com)
Hello world, I am 32 of 44 (running on node3.example.com)
Hello world, I am 33 of 44 (running on node3.example.com)
Hello world, I am 34 of 44 (running on node4.example.com)
...
Hello world, I am 43 of 44 (running on node4.example.com)

You can see the breakdown of how many processes Open MPI launched on each node:

  • node1: 16, because no slots was specified

  • node2: 16, because no slots was specified

  • node3: 2, because slots=2 was specified

  • node2: 10, because slots=10 was specified

9.1.3. Launching in scheduled environments

In scheduled environments (e.g., in a Slurm job, or PBS/Pro, or LSF, or any other schedule), the user tells the scheduler how many MPI processes to launch, and the scheduler decides which hosts to use. The scheduler then passes both pieces of information (the number of processes and the hosts to use) to Open MPI.

There are two ways to launch in a scheduled environment. Nominally, they both achieve the same thing: they launch MPI processes. Them main user-observable difference between the two methods is that mpirun has many more features than scheduler direct launchers.

9.1.3.1. Using Open MPI’s mpirun

Note

Technically, Open MPI’s mpirun is a thin layer around the PRRTE prun. Hence, most of the functionality described here is really about prun. For simplicity, however, this docmentation will describe everything in terms of mpirun.

Error

TODO Link to mpirun(1) here.

When using the full-featured mpirun in a scheduled environment, there is no need to specify a hostfile or number of MPI processes to launch. mpirun will receive this information directly from the scheduler. Hence, if you want to launch an MPI job that completely “fills” your scheduled allocation (i.e., one MPI process for each slot in the scheduled allocation), you can simply:

# Write a script that runs your MPI application
shell$ cat my-slurm-script.sh
#!/bin/sh
# There is no need to specify -n or --hostfile because that
# information will automatically be provided by Slurm.
mpirun mpi-hello-world

You then submit the my-slurm-script.sh script to Slurm for execution:

# Use -n to indicate how many MPI processes you want to run.
# Slurm will pick the specific hosts which will be used.
shell$ sbatch -n 40 my-slurm-script.sh
Submitted batch job 1234
shell$

After Slurm job 1234 completes, you can look at the output file to see what happened:

shell$ cat slurm-1234.out
Hello world, I am 0 of 40 (running on node37.example.com)
Hello world, I am 1 of 40 (running on node37.example.com)
Hello world, I am 2 of 40 (running on node37.example.com)
...
Hello world, I am 39 of 40 (running on node19.example.com)

Note that the Slurm scheduler picked the hosts on which the processes ran.

The above example shows that simply invoking mpirun mpi-hello-world — with no other CLI options — obtains the number of processes to run and hosts to use from the scheduler.

Error

TODO Link to mpirun(1) here.

mpirun has many more features not described in this Quick Start section. For example, while uncommon in scheduled environments, you can use -n and/or --hostfile to launch in subsets of the overall scheduler allocation. See the mpirun man page for more details.

9.1.3.2. Using the scheduler to “direct launch” (without mpirun)

Some schedulers (such as Slurm) have the ability to “direct launch” MPI processes without using Open MPI’s mpirun. For example:

shell$ srun -n 40 mpi-hello-world
Hello world, I am 0 of 40 (running on node14.example.com)
Hello world, I am 1 of 40 (running on node14.example.com)
Hello world, I am 2 of 40 (running on node14.example.com)
...
Hello world, I am 39 of 40 (running on node203.example.com)
shell$

Similar to the prior example, this example launches 40 copies of mpi-hello-world, but it does so via the Slurm srun command without using mpirun.