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The Singularity Community Edition container system (v3.8.4 or newer) is available on MeluXina, and is provided as a module that can be loaded on compute nodes:

module load Singularity-CE/3.8.4
singularity --help


  • Once built, container images are immutable. For development and fine-tuning, the "Sandbox" mode should be preferred (see singularity help build).
  • By default, containers have access only to the user home directory. If any data is required from project or project-scratch directories, the relevant paths must be explicitly bind-mounted into the container.
    • See singularity help run for the --bind option
    • Also the upstream documentation on bind paths and mounts
  • When running GPU-enabled applications on MeluXina GPU nodes, containers must be run using the --nv flag that enables NVIDIA support within the container.
  • MPI-enabled applications that are containerized can be run provided that a compatible MPI implementation is used in the outer (MeluXina) software stack and in the inner (container-based) application environment.
    • See the upstream documentation on MPI support
    • On MeluXina, the Slurm srun parallel launcher should be used e.g. to launch containerized applications linked to OpenMPI
  • For improved security we are using Singularity in non-privileged mode, which unpacks the containers in a temporary sandbox upon execution.

Running existing containers

Singularity Hub containers

As a first test, we will download a pre-existing "Hello World" container from the Singularity Hub registry and run it:

(compute)$ singularity pull shub://vsoch/hello-world
INFO:    Downloading shub image
59.8MiB / 59.8MiB [=============================================================================================================================================] 100 %

(compute)$ singularity run hello-world_latest.sif
RaawwWWWWWRRRR!! Avocado!

We can see that the container was downloaded as a Singularity Image File, and we can run the embedded command that prints a simple message. If a container has multiple applications inside, you can specify the command to run inside the container with singularity run <container> <command>.

Docker Hub containers

For a second example, we will take a reference Docker container from the Docker Hub which contains Python and run the embedded Python:

(compute)$ singularity pull docker://python:3.9.7-slim-bullseye
(compute)$ singularity run python_3.9.7-slim-bullseye.sif
Python 3.9.7 (default, Sep 28 2021, 18:41:28) 
[GCC 10.2.1 20210110] on linux
Type "help", "copyright", "credits" or "license" for more information.

GPU-enabled containers

Example of a GPU-enabled application that is being downloaded to a Project directory and ran on a MeluXina GPU node:

(login)$ salloc -A ACCOUNT -t 01:00:00 -p gpu -q short -N 1 -G 4
(compute)$ cd /project/home/lxp/test
(compute)$ module load Singularity-CE/3.8.4
(compute)$ singularity pull docker://tensorflow/tensorflow:latest-gpu
(compute)$ echo -e "import tensorflow as tf\nprint(tf.config.list_physical_devices('GPU'))" >
(compute)$ cat 
import tensorflow as tf

(compute)$ singularity run --nv --bind /project/home/lxp/test:/project/home/lxp/test tensorflow_latest-gpu.sif python3
[PhysicalDevice(name='/physical_device:GPU:0', device_type='GPU'), PhysicalDevice(name='/physical_device:GPU:1', device_type='GPU'), PhysicalDevice(name='/physical_device:GPU:2', device_type='GPU'), PhysicalDevice(name='/physical_device:GPU:3', device_type='GPU')]

Important elements above are the use of the --nv option to enable the container to use NVIDIA GPUs, and the use of --bind to enable the container to have access to a specific project folder (with the same location as on the MeluXina filesystem, such that any scripts referencing the path can be used without modification).

MPI-enabled containers

Example of running an MPI-enabled containerized application**:

(login)$ salloc -A ACCOUNT -t 1:00:00 -p cpu -q short -N 2 --ntasks-per-node=4
salloc: Nodes mel0*** are ready for job
(compute)$ module load Singularity-CE/3.8.4
(compute)$ module load OpenMPI/4.1.1-GCC-10.3.0
(compute)$ srun mpi-test-container.sif mpicode

Here, the mpicode application is run from inside the container, using the OpenMPI library from the MeluXina User Software Environment. The mpicode application must have been compiled using a compatible MPI library for the above to work. See the following section for an example of creating a container with MPI support.

Accessing data directories

To give a container access to your data hosted in a project directory (under Tier1/scratch and/or Tier2/Project home) you will need to ensure that the filesystem paths on the compute nodes are bind-mounted within the container, either through the SINGULARITY_BINDPATH environment variable or through the command line option --bind, which allow comma separated src[:dest[:opts]] tuples.

Example of enabling a container to use a project's home directory (e.g. /project/home/p299999):

(login)$ salloc -A ACCOUNT -t 01:00:00 -p cpu -q short -N 1 -n 1
(compute)$ module load Singularity-CE/3.8.4
(compute)$ singularity exec --bind "/mnt/tier2,/project/home/p2999999/" my_container.sif my_code

Example of enabling a container to use both the project home and scratch directories (e.g. /project/{home,scratch}/p299999):

(login)$ salloc -A ACCOUNT -t 01:00:00 -p cpu -q short -N 1 -n 1
(compute)$ module load Singularity-CE/3.8.4
(compute)$ export SINGULARITY_BINDPATH="/mnt/tier1,/project/scratch/p299999,/mnt/tier2,/project/home/p299999"
(compute)$ singularity exec my_container.sif my_code

Building Singularity images

There are two main approaches to building Singularity images:

  • Sandbox: you can build a container interactively within a sandbox environment (filesystem chroot), using a pre-existing image as base. This requires:

    • an existing container, e.g. downloaded from a registry: singularity build --sandbox my_container docker://ubuntu:latest
    • running a shell within and installing/configuring manually the packages and code inside, e.g. singularity shell --writable my_container
    • for 'production' use, the container should then be converted to an image file, e.g. singularity build my_ubuntu_container.sif my_container
  • From a Singularity Definition File: this is Singularity‚Äôs equivalent to building a Docker container from a Dockerfile, see examples below.

Creating a simple Singularity Definition File

A Singularity Definition File is a text file that contains a series of statements and instructions for building the container image, as shown below:

Bootstrap: docker
From: ubuntu:20.04

    apt-get -y update && apt-get install -y python

    python -c 'print("Hello World! Hello from our custom Singularity image!")'

The first two lines define a pre-existing container to use as base image that can then be customized. The Bootstrap: docker instruction is similar to prefixing an image path with docker:// when using the singularity pull command. A range of different bootstrap options are supported, From: ubuntu:20.04 defines that we will use an Ubuntu 20.04 base.

Next we have the %post section of the definition file, defining commands to be run within the image to customize it:

    apt-get -y update && apt-get install -y python

This section serves to provide shell commands performing a variety of tasks such as package installation, pulling data files from remote locations and setting local configurations within the image. In our example, we use the Ubuntu package manager to install python.

The %runscript section is used to define a script to be executed when the container is run with singularity run without any additional command.

Creating an MPI-enabled container

The following example shows how to build a container that includes libraries compatible with the software environment available on MeluXina (as of February 2022).
The example mpi-test-container.def definition file provides for a container with a Rocky Linux 8.5 base, with OpenMPI and the PMIx interface, Mellanox InfiniBand support (MOFED) and the OSU Micro-benchmarks as set of MPI-enabled applications.

BootStrap: yum
OSVersion: 8.5
Include: dnf

    export OMPI_DIR=/usr/local


    ## Prerequisites
    dnf update
    dnf install -y dnf-plugins-core
    dnf config-manager --set-enabled powertools
    dnf groupinstall -y 'Development Tools'
    dnf install -y wget git bash hostname gcc gcc-gfortran gcc-c++ make file autoconf automake libtool zlib-devel python3
    dnf install -y libmnl lsof numactl-libs ethtool tcl tk

    ## Packages required for OpenMPI and PMIx
    dnf install -y libnl3 libnl3-devel
    dnf install -y libevent libevent-devel
    dnf install -y munge munge-devel

    # Mellanox OFED matching MeluXina
    mkdir -p /tmp/mofed
    cd /tmp/mofed
    wget -c
    tar xf MLNX_OFED_LINUX-*.tgz
    cd MLNX_OFED_LINUX-5.4-
    ./mlnxofedinstall --basic --user-space-only --without-fw-update --distro rhel8.5 --force

    # PMIx
    mkdir -p /tmp/pmix
    cd /tmp/pmix
    wget -c
    tar xf pmix-3.2.3.tar.gz
    cd pmix-3.2.3
    ./configure --prefix=/usr/local --with-munge=/usr && \
    make -j
    make install

   # libfabric
   mkdir -p /tmp/libfabric
   cd /tmp/libfabric
   wget -c
   tar xf libfabric*tar.bz2
   cd libfabric-1.12.1
   ./configure --prefix=/usr/local && \
   make -j
   make install

    ## OpenMPI installation
    echo "Installing Open MPI"
    export OMPI_DIR=/usr/local
    export OMPI_VERSION=4.1.1
    export OMPI_URL="$OMPI_VERSION.tar.bz2"
    mkdir -p /tmp/ompi
    cd /tmp/ompi
    wget -c -O openmpi-$OMPI_VERSION.tar.bz2 $OMPI_URL && tar -xjf openmpi-$OMPI_VERSION.tar.bz2

    # Compile and install
    cd /tmp/ompi/openmpi-$OMPI_VERSION
   ./configure --prefix=$OMPI_DIR --with-pmix=/usr/local --with-libevent=/usr --with-ompi-pmix-rte --with-orte=no --disable-oshmem --enable-mpirun-prefix-by-default --enable-shared --with-ofi=/usr/local --without-verbs --with-hwloc
   make -j
   make install

   # Set env variables so we can compile our applications
   export PATH=$OMPI_DIR/bin:$PATH
   export MANPATH=$OMPI_DIR/share/man:$MANPATH

   ## Example MPI applications installation - OSU microbenchmarks
   cd /root
   wget -c
   tar xf osu-micro-benchmarks-5.7.tar.gz
   cd osu-micro-benchmarks-5.7/
   echo "Configuring and building OSU Micro-Benchmarks..."
   ./configure --prefix=/usr/local/osu CC=$(which mpicc) CXX=$(which mpicxx) CFLAGS=-I$(pwd)/util
   make -j
   make install

  echo "Container will run: /usr/local/osu/libexec/osu-micro-benchmarks/mpi/$*"
  exec /usr/local/osu/libexec/osu-micro-benchmarks/mpi/$*

To build this container you require Singularity on your workstation or server, then simply run:

sudo singularity build mpi-test-container.sif mpi-test-container.def

The container image mpi-test-container.sif can then be transferred to MeluXina, and a job launcher created to run it:

#!/bin/bash -l
#SBATCH -J MPIContainerTest
#SBATCH -p cpu
#SBATCH -q short
#SBATCH --ntasks-per-node=4
#SBATCH -t 0:5:0

module load Singularity-CE/3.8.4
module load OpenMPI/4.1.1-GCC-10.3.0

srun mpi-test-container.sif pt2pt/osu_mbw_mr

In the launcher above, the MPI application OSU "Multiple Bandwidth / Message Rate Test" is being run from the container using the system OpenMPI installation.
The launcher is then submitted to SLURM using sbatch