mpi-multi-host

Docker example - mpi-multi-host

This example runs two MPI programs across two hosts/nodes. To do so, it uses Docker Compose to build a Docker image and start two containers based on that image.

This programs are:

1. A simple hello world program, which you can inspect (and alter) in helloworld_multihost.c.

2. A simple matmult (matrix multiplication) program, which you can inspect (and alter) in matmult_multihost.c.

Note that the containers are started in descending order (node2 is started before node1) in docker-compose.yml with some hackery to add an entry in node2 for node1's IP address in /etc/hosts. This is because Docker Compose supports one-way linking (i.e. node1 will have an entry in its /etc/hosts file for node2, but not the way around). For two-way linking I would suggest looking at serf and/or weave, which I don't cover here.

Note that the Dockerfile bases this image off the single-host image example. Make sure you have built that image (see its README) to do so before running any of this!

Building the image

# View available docker images
[~]: docker images

# Change into this directory
[~]: cd /path/to/docker-examples/mpi-multi-host

# Build the docker image for this example using Docker Compose
[/path/to/docker-examples/mpi-multi-host]: docker-compose build

# Watch it build. It may take some time the first time you run it.

# Convince yourself the above image has been built
[/path/to/docker-examples/mpi-multi-host]: docker images

# Notice that this time the image is named mpimultihost_node2, which is a name
# generated by Docker Compose based on the name of this directory and the name we gave the container.
# Also notice that in docker-compose.yml we only specify the image to build for the first node, and
# for node1 we use that built image again. Naming is a bit weird, but hopefully Docker Compose will
# evolve to be friendlier that way.

Running the application

# These commands assume you're in the /path/to/docker-examples/mpi-multi-host directory
# since docker-compose looks for docker-compose.yml in that directory.

# Run the app
[/path/to/docker-examples/mpi-multi-host]: docker-compose up

# Convince yourself the application has run
[~]: docker-compose ps

# Kill and remove the application
[~]: docker-compose kill && docker-compose rm

Notes

These containers will sip on your local computer's RAM and munch on CPU using Docker's defaults. You can, if you so wish, play around with docker run options --cpu-shares and --cpuset to limit how much of a CPU a container is allowed to use or which CPUs it can use respectively. You can also limit the memory it sips on using the --memory option. Check out the docs for more info and also here for a great example.