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Julia

Julia is a general purpose software used widely in datascience and for data visualisation.

Important

This documentation is provided by an external party (i.e. not by the ARCHER2 service itself). Julia is not part of the officially supported software on ARCHER2. While the ARCHER2 service desk is able to provide support for basic use of this software (e.g. access to software, writing job submission scripts) it does not generally provide detailed technical support for the software and you may be directed to seek support from other places if the service desk cannot answer the questions.

First time installation

Note

There is no centrally installed version of Julia, so you will have to manually install it and any packages you may need. The following guide was tested on julia-1.6.6.

You will first need to download Julia into your work directory and untar the folder. You should then add the folder to your system path so you can use the julia executable. Finally, you need to tell Julia to install any packages in the work directory as opposed to the default home directory, which can only be accessed from the login nodes. This can be done with the following code

export WORK=/work/t01/t01/auser
cd $WORK

wget https://julialang-s3.julialang.org/bin/linux/x64/1.6/julia-1.6.6-linux-x86_64.tar.gz
tar zxvf julia-1.6.6-linux-x86_64.tar.gz
rm ./julia-1.6.6-linux-x86_64.tar.gz

export PATH="$PATH:$WORK/julia-1.6.6/bin"

mkdir ./.julia
export JULIA_DEPOT_PATH="$WORK/.julia"
export PATH="$PATH:$WORK/$JULIA_DEPOT_PATH/bin"

At this point you should have a working installation of Julia! The environment variables will however be cleared when you log out of the terminal. You can set them in the .bashrc file so that they're automatically defined every time you log in by adding the following lines to the end of the file ~/.bashrc

export WORK="/work/t01/t01/auser"
export JULIA_DEPOT_PATH="$WORK/.julia"
export PATH="$PATH:$WORK/julia-1.6.6/bin"
export PATH="$PATH:$JULIA_DEPOT_PATH/bin"

Installing packages and using environments

Julia has a built in package manager which can be used to install registered packages quickly and easily. Like with many other high level programming languages we can make use of environments to control dependencies etc.

To make an environment, first navigate to where you want your environment to be (ideally a subfolder of your /work/ directory) and create an empty folder to store the environment in. Then launch Julia with the --project flag.

cd $WORK
mkdir ./MyTestEnv
julia --project=$WORK/MyTestEnv
This launches Julia in the MyTestEnv environment. You can then install packages as usual using the normal commands in the Julia terminal. E.g.
using Pkg
Pkg.add("Oceananigans")

Configuring MPI.jl

The MPI.jl package doesn't use the system MPICH implementation by default. You can set it up to do this by following the steps below. First you will need to load the cray-mpich module and define some environment variables (see here for further details). Then you can launch Julia in an environment of your choice, ready to build.

module load cray-mpich/8.1.4
export JULIA_MPI_BINARY="system"
export JULIA_MPI_PATH=""
export JULIA_MPI_LIBRARY="/opt/cray/pe/mpich/8.1.4/ofi/cray/9.1/lib/libmpi.so"
export JULIA_MPIEXEC="srun"

julia --project=<<path to environment>>
Once in the Julia terminal you can build the MPI.jl package using the following code. The final line installs the mpiexecjl command which should be used instead of srun to launch mpi processes.
using Pkg
Pkg.build("MPI"; verbose=true)
MPI.install_mpiexecjl(command = "mpiexecjl", force = false, verbose = true)
The mpiexecjl command will be installed in the directory that JULIA_DEPOT_PATH points too.

Note

You only need to do this once per environment.

Running Julia on the compute nodes

Below is an example script for running Julia with mpi on the compute nodes

#!/bin/bash
# Slurm job options (job-name, compute nodes, job time)
#SBATCH --job-name=<<job-name>>
#SBATCH --time=00:19:00

#SBATCH --nodes=1
#SBATCH --tasks-per-node=24
#SBATCH --cpus-per-task=1

#SBATCH --qos=short
#SBATCH --reservation=shortqos

#SBATCH --account=<<your account>>
#SBATCH --partition=standard

# Setup the job environment (this module needs to be loaded before any other modules)
module load PrgEnv-cray
module load cray-mpich/8.1.4

# Set the number of threads to 1
#   This prevents any threaded system libraries from automatically
#   using threading.
export OMP_NUM_THREADS=1
export JULIA_NUM_THREADS=1

# Define some paths
export WORK=/work/t01/t01/auser

export JULIA="$WORK/julia-1.6.6/bin/julia"  # The julia executable
export PATH="$PATH:$WORK/julia-1.6.6/bin"  # The folder of the julia executable
export JULIA_DEPOT_PATH="$WORK/.julia"
export MPIEXECJL="$JULIA_DEPOT_PATH/bin/mpiexecjl"  # The path to the mpiexexjl executable

$MPIEXECJL --project=$WORK/MyTestEnv -n 24 $JULIA ./MyMpiJuliaScript.jl

The above script uses MPI but you can also use multithreading instead by setitng the JULIA_NUM_THREADS environment variable.