# Working in a HPC environment

Many researchers have access to institutional HPC facilities that allow time-sharing of the capacity of a large computer cluster between many users. Julia and COBREXA.jl work well within this environment, and the COBREXA analyses usually require only minimal additional customization to be able to find and utilize the resources available from the HPC.

When executed in a HPC environment, the analysis script must solve several relatively complex tasks:

• It needs to find out how many resources were allocated for the analysis
• It needs to add the remote workers precisely at the allocated places

Fortunately, the package ClusterManagers.jl does that for us. For simplicily, here we assume that your HPC is scheduled by Slurm, but other scheduling environments are supported in a very similar way.

## Interacting with Slurm

Adding of the Slurm-provided is done as follows:

• you import the ClusterManagers package
• you find how many processes to spawn from the environment from SLURM_NTASKS environment variable
• you use the function addprocs_slurm to precisely connect to your allocated computational resources

After adding the Slurm workers, you may continue as if the workers were added using normal addprocs –- typically you load the model and (for example) run the flux_variability_analysis as if you would use the local workers.

The Julia script that does a parallel analysis in a Slurm cluster may look as follows:

using COBREXA, Distributed, ClusterManagers, GLPK

# ... load models, prepare data, etc. ...

result = flux_variability_analysis(...; workers=workers())

# ... save the results into a file ...
What about the other HPC schedulers?

ClusterManagers.jl supports many other common HPC scheduling systems, including LFS, Sun Grid, SGE, PBS, and Scyld, in a way almost identical to Slurm. See the package documentation for details.

## Wrapping your script in a Slurm batch job

To be able to submit your script for later processing using the sbatch Slurm command, you need to wrap it in a small "batch" script that tells Slurm how many resources the process needs.

Assuming you have a Julia computation script written down in myJob.jl and saved on your HPC cluster's access node, the corresponding Slurm batch script (let's call it myJob.sbatch) may look as follows:

#!/bin/bash -l
#SBATCH -n 100           # the job will require 100 individual workers
#SBATCH -c 1             # each worker will sit on a single CPU
#SBATCH -t 30            # the whole job will take less than 30 minutes
#SBATCH -J myJob         # the name of the job

module load lang/Julia   # this is usually required to make Julia available to your job

julia myJob.jl

To run the computation, simply run sbatch myJob.sbatch on the cluster access node. The job will be scheduled and eventually executed. You may watch the output of commands sacct and squeue in the meantime, to see the progress.

Remember that you need to explicitly save the result of your Julia script computation to files, to be able to retrieve them later. Standard outputs of the jobs are often mangled and discarded. If you still want to collect the standard output of your Julia script, you may change the last line of the batch script to

julia myJob.jl > myJob.log

and collect the output from myJob.log later. This is convenient especially if your script logs various computation details using @info and similar macros.