Can I run containers on the HPC systems?

The best-known container implementation is doubtlessly Docker. However, due to security concerns HPC sites typically don’t allow users to run Docker containers.

Fortunately, Singularity addresses container related security issues, so Singularity images can be used on the cluster. Since a Singularity image can be built from a Docker container, that should not be a severe limitation.

When should I use containers?

If the software you intend to use is available on the VSC infrastructure, don’t use containers. This software has been built to use specific hardware optimizations, while software deployed via containers is typically built for the common denominator.

Good use cases include:

  • Containers can be useful to run software that is hard to install on HPC systems, e.g., GUI applications, legacy software, and so on.
  • Containers can be useful to deal with compatibility issues between Linux flavors.
  • You want to create a workflow that can run on VSC infrastructure, but can also be burst to a third-party compute cloud (e.g., AWS or Microsoft Azure) when required.
  • You want to maximize the period your software can be run in a reproducible way.

How can I create a Singularity image?

You have three options to build images, locally on your machine, in the cloud or on the VSC infrastructure.

Building on VSC infrastructure

Given that most build procedures require superuser privileges, your options on the VSC infrastructure are limited. You can build an image from a Docker container, e.g., to build an image that contains a version of TensorFlow and has Jupyter as well, use:

$ export SINGULARITY_TMPDIR=$VSC_SCRATCH/singularity_tmp
$ mkdir -p $SINGULARITY_TMPDIR
$ export SINGULARITY_CACHEDIR=$VSC_SCRATCH/singularity_cache
$ mkdir -p $SINGULARITY_CACHEDIR
$ singularity build tensorflow.sif docker://tensorflow/tensorflow:latest-jupyter

Warning

Don’t forget to define and create the $SINGULARITY_TMPDIR and $SINGULARITY_CACHEDIR since if you fail to do so, Singularity will use directories in your home directory, and you will exceed the quota on that file system.

Also, images tend to be very large, so store them in a directory where you have sufficient quota, e.g., $VSC_DATA.

This approach will serve you well if you can use either prebuilt images or Docker containers. If you need to modify an existing image or container, you should consider the alternatives.

Note

Creating image files may take considerable time and resources. It is good practice to do this on a compute node, rather than on a login node.

Local builds

The most convenient way to create an image is on your own machine, since you will have superuser privileges, and hence the most options to chose from. At this point, Singularity only runs under Linux, so you would have to use a virtual machine when using Windows or macOS. For detailed instructions, see the Singularity installation documentation.

Besides building images from Docker containers, you have the option to create them from a definition file, which allows you to completely customize your image. We provide a brief introduction to Singularity definition files, but for more details, we refer you to the Singularity definition file documentation.

When you have a Singularity definition file, e.g., my_image.def, you can build your image file my_image.sif:

your_machine> singularity build my_image.sif my_image.def

Once your image is built, you can transfer it to the VSC infrastructure to use it.

Warning

Since Singularity images can be very large, transfer your image to a directory where you have sufficient quota, e.g., $VSC_DATA.

Remote builds

You can build images on the Singularity website, and download them to the VSC infrastructure. You will have to create an account at Sylabs. Once this is done, you can use Sylabs Remote Builder to create an image based on a Singularity definition. If the build succeeds, you can pull the resulting image from the library:

$ export SINGULARITY_CACHEDIR=$VSC_SCRATCH/singularity_cache
$ mkdir -p $SINGULARITY_CACHEDIR
$ singularity pull library://gjbex/remote-builds/rb-5d6cb2d65192faeb1a3f92c3:latest

Warning

Don’t forget to define and create the $SINGULARITY_CACHEDIR since if you fail to do so, Singularity will use directories in your home directory, and you will exceed the quota on that file system.

Also, images tend to be very large, so store them in a directory where you have sufficient quota, e.g., $VSC_DATA.

Remote builds have several advantages:

  • you only need a web browser to create them, so this approach is platform-independent,
  • they can easily be shared with others.

However, local builds still offer more flexibility, especially when some interactive setup is required.

Singularity definition files

Below is an example of a Singularity definition file:

Bootstrap: docker
From: ubuntu:xenial

%post
    apt-get update
    apt-get install -y grace

%runscript
    /usr/bin/grace

The resulting image will be based on the Ubuntu Xenial Xerus distribution (16.04). Once it is bootstrapped, the command in the %post section of the definition file will be executed. For this example, the Grace plotting package will be installed.

Note

This example is intended to illustrate that very old software that is no longer maintained can successfully be run on modern infrastructure. It is by no means intended to encourage you to start using Grace.

Singularity definition files are very flexible. For more details, we refer you to the Singularity definition file documentation.

An important advantage of definition files is that they can easily be shared, and improve reproducibility.

How can I run a Singularity image?

Once you have an image, there are several options to run the container.

  1. You can invoke any application that is in the $PATH of the container, e.g., for the image containing Grace:

    $ singularity  exec  grace.sif  xmgrace
    
  2. In case the definition file specified a %runscript directive, this can be executed using:

    $ singularity  run  grace.sif
    
  3. The container can be run as a shell:

    $ singularity  shell  grace.sif
    

By default, your home directory in the container will be mounted with the same path as it has on the host. The current working directory in the container is that on the host in which you invoked singularity.

Note

Although you can move to a parent directory of the current working directory in the container, you will not see its contents on the host. Only the current working directory and its sub-directories on the host are mounted.

Additional host directories can be mounted in the container as well by using the -B option. Mount points are created dynamically (using overlays), so they do not have to exist in the image. For example, to mount the $VSC_SCRATCH directory, you would use:

$ singularity  exec  -B $VSC_SCRATCH:/scratch  grace.sif  xmgrace

Your $VSC_SCRATCH directory is now accessible from within the image in the directory /scratch.

Note

If you want existing scripts to work from within the image without having to change paths, it may be convenient to use identical mount points in the image and on the host, e.g., for the $VSC_DATA directory:

$ singularity  exec  -B $VSC_DATA:$VSC_DATA  grace.sif  xmgrace

Or, more concisely:

$ singularity  exec  -B $VSC_DATA  grace.sif  xmgrace

The host environment variables are defined in the image, hence scripts that use those will work.

Can I use singularity images in a job?

Yes, you can. Singularity images can be part of any workflow, e.g., the following script would create a plot in the Grace container:

#!/bin/bash -l
#PBS -l nodes=1:ppn=1
#PBS -l walltime=00:30:00

cd $PBS_O_WORKDIR
singularity exec grace.sif gracebat -data data.dat \
                                    -batch plot.bat

Ensure that the container has access to all the required directories by providing additional bindings if necessary.

Can I run parallel applications using a Singularity image?

For shared memory applications there is absolutely no problem.

For distributed applications it is highly recommended to use the same implementation and version of the MPI libraries on the host and in the image. You also want to install the appropriate drivers for the interconnect, as well as the low-level communication libraries, e.g., ibverbs.

For this type of scenario, it is probably best to contact user support.

Note

For distributed applications you may expect some mild performance degradation.

Can I run a service from a Singularity image?

Yes, it is possible to run services such as databases or web applications that are installed in Singularity images.

For this type of scenario, it is probably best to contact user support.