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Usage of Singularity

Usage of Singularity

Kurzinformation

Singularity is a container virtualization software specifically designed for HPC environments. You may imagine containers as lightweight virtual operating systems with preinstalled and preconfigured software that can be run just like any other program on a host system. You could, for example, run software in an Ubuntu environment within our CentOS compute cluster. This helps overcome portability issues with software that has very specific dependencies or was not built to be run under RHEL-based distributions.

 

Please note:

We are currently testing Singularity with selected use cases. If you need to run software on the cluster that profits from containerization and are interested in using singularity, please contact the IT-ServiceDesk (servicedesk@itc.rwth-aachen.de). Software that has already been containerized for Docker can often be ported to Singularity with virtually no effort.
If you want to run software via Singularity, please read the Best Practices first!


 Detailinformation

The Container Environment

Usual containers isolate applications from the host system. This poses problems in the context of HPC because users are running jobs across multiple nodes using special interconnects that need software support. Singularity, however, allows containerized multi-node MPI jobs and leveraging the Intel OmniPath fabric. Whilst you do not have access to the host operating system, within the container you may still do the following things:

  • Access all your personal directories ($HOME, $WORK, $HPCWORK). Within most containers (and all containers supplied by us) you may access these via the aforementioned variables as usual. You may thus comfortably share data between containerized and native applications.
  • Access other nodes through the network and run multi-node jobs. You may also switch frontends although we do not recommend this workflow.
  • Use MPI implementations installed on the ho
 

Limitations

Notably, you will not have access to the module system within the container!

 

Dual Use of the $HOME Directory

Not only do you have access to your home directory within the container but it will also, by default, serve as the container's home directory for every container that you execute. This means that configuration files stored within your home directory, such as application config files (most notably Zsh!) will be used within the container as well. This can prove both advantageous and disadvantageous since a shared configuration may make working within the container more comfortable but at the same time introduce settings that are incompatible with the containerized environment. 
Shell-based compatibility issues are mitigated by Singularity's default behavior of invoking containers with /bin/sh. You may invoke another shell by specifying its path via the "--shell" argument. The shell needs to exist within the container image which is usually the case for bash but not for zsh. 
Python software within containers should make use of virtual environments or package managers like conda to avoid hard-to-trace side effects.
 

If you wish to use an empty home directory within a container instead, please add the "--no-home" flag to your container invocation. This requires you to start the container from a path that is not within your home directory. You can also use a different directory as your temporary home directory via "--home /path/on/host".

 

Usage

We are using sandbox directories only on the cluster, i.e. the containers are not single files as you may be used to. Users cannot build custom containers on the cluster itself since this requires elevated privileges. Additionally, during the testing phase users may only run images that have been reviewed by the HPC team. Before you run any Singularity tests or batch jobs, please read the Best Practices below to avoid common pitfalls.

 

Singularity and MPI

Singularity supports two models for MPI usage. The first, which we very strongly recommend you use, uses the host MPI implementation to handle the actual communication between processes and utilization of the Intel Omni-Path interconnect between compute nodes. The container must simply contain binary-compatible MPI libraries, which often comes down to installing a similar or even the same MPI implementation in the container. This is called the "hybrid model". Notably, it allows you to use our regular intelmpi and openmpi modules for use with your container. Another option is the "bind model" which involves binding the host MPI implementations into the container. This requires additional effort and should not be used unless necessary.

Using the hybrid model is very easy if the containerized MPI version is supported. Instead of running the MPI wrapper inside the container, you run the container inside the wrapper, i.e. "srun singularity run my_container". For an example batch script, see down below.

 

Run a container

There are three standard ways of running a singularity container: The shell subcommand, the run and the exec subcommand.

  • The shell subcommand allows you to start an interactive shell within the container and is intended for use on frontends or within interactive batch jobs.
  • The run subcommand triggers the execution of a pre-defined runscript within the container. Container creators may choose to provide a default workflow which can be accessed this way.
  • The exec subcommand allows users to run custom shell commands in a containerized shell. This is the default way of using Singularity within batch scripts.
 

Using Singularity to start a container
singularity shell $HOME/my_container
singularity run $HOME/my_container
singularity exec $HOME/my_container cat /etc/os-release
singularity exec $HOME/my_container $HOME/my_exec_script.sh

 

Use GPUs inside the container

Providing access to GPUs inside containers is a non-trivial task. Luckily, Singularity supports this with a simple command line argument. To use NVidia GPUs simply add the "--nv" option after your desired subcommand like so:

singularity exec --nv tensorflow-gpu.sif $HOME/my_tensorflow_script.sh


Naturally the --nv flag will only work correctly on systems that actually have a GPU installed. If run on a non-GPU host, Singularity will issue a warning but still execute the container.

 

CUDA

Singularity will use the host's CUDA installation where possible. This works well for a lot of applications that support a recent CUDA version.

 

Singularity Containers As Modules

We provide several selected software modules for use with Singularity beneath the CONTAINERS tree. To load them, please follow this procedure:

module load CONTAINERS
module load tensorflow
singularity shell $R_CONTAINER

As usual, you may get an overview of all available modules via "module avail".

 

Example Batch Scripts
 

Serial Example

#!/usr/local_rwth/bin/zsh

### Job name
#SBATCH --job-name=SINGULARITY_EXAMPLE

### File / path where STDOUT will be written, %J is the job id
#SBATCH --output=singularity-job-out.%J

### Request the time you need for execution. The full format is D-HH:MM:SS
### You must at least specify minutes or days and hours and may add or
### leave out any other parameters
#SBATCH --time=30

### Request memory you need for your job in MB
#SBATCH --mem-per-cpu=2000

### Request number of hosts
#SBATCH --nodes=1

### Request number of CPUs
#SBATCH --cpus-per-task=4

### Change to the work directory
cd $HOME/jobdirectory

### Execute the container
### myexecscript.sh contains all the commands that should be run inside the container

singularity exec /path/to/my/container $HOME/myexecscript.sh

 

MPI Example

#!/usr/local_rwth/bin/zsh

### Job name
#SBATCH --job-name=SINGULARITY_MPI_EXAMPLE

### File / path where STDOUT will be written, %J is the job id
#SBATCH --output=singularity-job-out.%J

### Request the time you need for execution. The full format is D-HH:MM:SS
### You must at least specify minutes or days and hours and may add or
### leave out any other parameters
#SBATCH --time=30

### Request memory you need for your job in MB
#SBATCH --mem-per-cpu=2000

### Request number of CPUs
#SBATCH --ntasks=4

### Change to the work directory
cd $HOME/jobdirectory

### Execute the container
### myexecscript.sh contains all the commands that should be run inside the container

srun singularity exec /path/to/my/container $HOME/myexecscript.sh

 

Converting Docker Images for Singularity

Please note: 

While you can download images like this during the testing phase, you will not be able to run the containers due to path constraints.

 

Pull Docker Image From External Resource

Singularity's pull command allows pulling arbitrary docker containers and converting them to singularity SIF containers in a single step. Container registries or software documentation will often explain how to retrieve a container like so:
 

docker pull nvcr.io/nvidia/tensorflow:20.06-tf2-py3


This tells docker to pull the container "tensorflow" in version "20.12-tf2-py3" from the NVidia container registry. You can create a sandbox container from a docker resource without any special privileges like this:

singularity build --sandbox docker://nvcr.io/nvidia/tensorflow:20.12-tf2-py3


The prefix "docker://" tells singularity that the following URI points to a docker image and should be treated as such.

 

Pull Image From Nvidia Container Registry

This snippet shows the full process from pulling to executing an image from the NVCR.

# Pull Tensorflow 20.12
singularity build --sandbox docker://nvcr.io/nvidia/tensorflow:20.12-tf2-py3

# Inspect a container with an interactive shell and GPU support
singularity shell --nv tensorflow_20.12-tf2-py3.sif

# Execute a predefined script in a container with GPU support, e.g. within SLURM
singularity exec --nv tensorflow_20.12-tf2-py3.sif ./myexecscript.sh

 

Build Singularity Images on top of Docker Images

Please note: 

You need elevated privileges, i.e. the ability to run singularity as root, to build containers. Therefore, users can not build singularity recipes on the cluster but have to resort to other machines.

Singularity supports building containers on top of docker images via the "docker" bootstrapping option. A stub for this purpose would look like this:

Bootstrap: docker
From: ubuntu:18.04

# Use the image "ubuntu/18.04" from the docker registry as the foundation for this container

%post
    export DEBIAN_FRONTEND=noninteractive
    apt-get update
    apt-get install -y extra_packages
    rm -rf /var/lib/apt/lists/*

%help
    An example of how to bootstrap a singularity container

 

 Best Practices for Singularity Usage

Singularity's default workflow to carry over the host process's environment and employing the user's host home directory can lead to several problems that can be hard to debug for unwary users. Please make sure that you implement these best practices where applicable to your use case.

  • Handling of Modules
    • Except for IntelMPI and OpenMPI modules are not supported within Singularity environments. Loading modules alters a shell's environment, however, and these changes are carried over to a Singularity container invoked within this shell. This is but a cosmetic issue as long as the extra environment variables are not being used within the container. However, in some instances - notably compiler modules - the changes may cause software to break, e.g. CC being set to "icc". To avoid such issues we recommend unloading all modules that are not needed for the container before starting Singularity. If your program does not rely on MPI, you may use "module purge".
       
  • Using Compatible MPI Versions
    • Containers run via MPI need to be provided with a compatible MPI implementation. This can usually be achieved by choosing a compatible version from our module tree, loading the module and starting Singularity via the proper MPI wrapper (see above for an MPI batch example). If the container has been provided by a third party it should contain information on the MPI version against which the program was linked. It should be noted that for OpenMPI versions below 3.0.0 compatibility is only guaranteed for versions matching exactly.
       
  • Running a Container Without $WORK or $HPCWORK
    • By default both $WORK and $HPCWORK are mapped into each container run on the cluster giving you access to all personal directories you would have access to in a native environment. In case of filesystem problems this prevents the startup of containers which may be undesired if either of the filesystems is not used by a job. To circumvent this, consider the following line:
      singularity shell --contain --bind=/dev,$HOME,$WORK my_image
      The --contain flag disables binding of many host paths and the subsequent bind argument rebinds all relevant directories. In this case we have excluded $HPCWORK. If you want to start a container without $WORK, you can simply exchange the $WORK bind with $HPCWORK.
       
  • Running Python Software in a Container
    • In general containers images will provide all necessary Python modules in a default system path which normally takes precedence over any locally installed modules. In this case potentially conflicting module installations within your $HOME directory will not cause any problems. However, some images - mainly those distributed for Docker - might store modules in a custom location that needs to be added to the PYTHONPATH environment variable in order to use the image as intended. You may find further information on this topic here. A properly configured Singularity image will take of such issues by setting the PYTHONPATH accordingly. If this is not being done, you should make sure to start the container with an empty PYTHONPATH variable, e.g. by executing "unset PYTHONPATH".

      You should abstain from any software installations to default locations while inside a container as this can easily break existing or upcoming software installations. The PIP tool should only be used if you are fully aware of the consequences and if you see the need to use it, you are probably doing something wrong. Likewise sharing venv or conda environments between containers and the host system is almost guaranteed to lead to problems.
 

Further Questions

If you have any questions that were not (fully) answered above or have any suggestions for improvements, please contact us via servicedesk@itc.rwth-aachen.de . If your questions regard Singularity itself, you may find the official User Guide helpful.

zuletzt geändert am 12.04.2021

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