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HPC workflow with sequential jobs

 Copyright (c) 2013-2018 UL HPC Team <>


Make sure you have followed the tutorial "Getting started".


For many users, the typical usage of the HPC facilities is to execute 1 program with many parameters. On your local machine, you can just start your program 100 times sequentially. However, you will obtain better results if you parallelize the executions on a HPC Cluster.

During this session, we will see 3 use cases:

  • Exercise 1: Use the serial launcher (1 node, in sequential and parallel mode);
  • Exercise 2: Use the generic launcher, distribute your executions on several nodes (python script);
  • Exercise 3: Advanced use case, using a Java program: "JCell".

We will use the following github repositories:


Connect to the cluster access node, and set-up the environment for this tutorial

You can chose one of the 3 production cluster hosted by the University of Luxembourg.

For the next sections, note that you will use Slurm on Iris.

(yourmachine)$> ssh iris-cluster

If your network connection is unstable, use screen:

(access)$> screen

We will work in the home directory.

You can check the usage of your directories using the command df-ulhpc

(access)$> df-ulhpc
Directory                         Used  Soft quota  Hard quota  Grace period
---------                         ----  ----------  ----------  ------------
/home/users/hcartiaux             3.2G  100G        -           none

Note that the user directories are not yet all available on Iris, and that the quota are not yet enabled.

Create a sub directory $SCRATCH/PS2, and work inside it

(access)$> mkdir $SCRATCH/PS2
(access)$> cd $SCRATCH/PS2

In the following parts, we will assume that you are working in this directory.

Clone the repositories ULHPC/tutorials and ULHPC/launcher-scripts.git

(access)$> git clone
(access)$> git clone

In order to edit files in your terminal, you are expected to use your preferred text editor:

If you have never used any of them, nano is intuitive, but vim and emacs are more powerful.

With nano, you will only have to learn a few shortcuts to get started:

  • $ nano <path/filename>
  • quit and save: CTRL+x
  • save: CTRL+o
  • highlight text: Alt-a
  • Cut the highlighted text: CTRL+k
  • Paste: CTRL+u

Exercise 1: Object recognition with Tensorflow and Python Imageai

In this exercise, we will process some images from the OpenImages V4 data set with an object recognition tools.

Create a file which contains the list of parameters (random list of images):

(access)$>  find /work/projects/bigdata_sets/OpenImages_V4/train/ -print | head -n 10000 | sort -R | head -n 50 | tail -n +2 > $SCRATCH/PS2/param_file

Step 0: Prepare the environment

(access)$> srun -p interactive -N 1 --qos qos-interactive --pty bash -i

Load the default Python module

(node) module load lang/Python

(node) module list

Create a new python virtual env

(node) cd $SCRATCH/PS2
(node) virtualenv venv

Enable your newly created virtual env, and install the required modules inside

(node) source venv/bin/activate

(node) pip install tensorflow scipy opencv-python pillow matplotlib keras
(node) pip install

(node) exit

Step 1: Naive workflow

We will use the launcher (full path: $SCRATCH/PS2/launcher-scripts/bash/serial/

Edit the following variables:

  • MODULE_TO_LOAD must contain the list of modules to load before executing $TASK,
  • TASK must contain the path of the executable,
  • ARG_TASK_FILE must contain the path of your parameter file.
    (node)$> nano $SCRATCH/PS2/launcher-scripts/bash/serial/
Using Slurm on Iris

Launch the job, in interactive mode and execute the launcher:

(access)$> srun -p interactive -N 1 --qos qos-interactive --pty bash -i

(node)$> source venv/bin/activate
(node)$> $SCRATCH/PS2/launcher-scripts/bash/serial/

Or in passive mode (the output will be written in a file named BADSerial-<JOBID>.out)

(access)$> sbatch $SCRATCH/PS2/launcher-scripts/bash/serial/

You can use the command scontrol show job <JOBID> to read all the details about your job:

(access)$> scontrol show job 207001
JobId=207001 JobName=BADSerial
   UserId=hcartiaux(5079) GroupId=clusterusers(666) MCS_label=N/A
   Priority=8791 Nice=0 Account=ulhpc QOS=qos-batch
   JobState=RUNNING Reason=None Dependency=(null)
   Requeue=0 Restarts=0 BatchFlag=1 Reboot=0 ExitCode=0:0
   RunTime=00:00:23 TimeLimit=01:00:00 TimeMin=N/A
   SubmitTime=2018-11-23T10:01:04 EligibleTime=2018-11-23T10:01:04
   StartTime=2018-11-23T10:01:05 EndTime=2018-11-23T11:01:05 Deadline=N/A

And the command sacct to see the start and end date

(access)$> sacct --format=start,end --j 207004
              Start                 End
------------------- -------------------
2018-11-23T10:01:20 2018-11-23T10:02:31
2018-11-23T10:01:20 2018-11-23T10:02:31

In all cases, you can connect to a reserved node using the command srun and check the status of the system using standard linux command (free, top, htop, etc)

(access)$> srun -p interactive --qos qos-interactive --jobid <JOBID> --pty bash

During the execution, you can see the job in the queue with the command squeue:

(access)$> squeue
        207001     batch BADSeria        hcartiaux  R       2:27      1 iris-110

Using the system monitoring tool ganglia, check the activity on your node.

Step 2: Optimal method using GNU parallel (GNU Parallel)

We will use the launcher (full path: $SCRATCH/PS2/launcher-scripts/bash/serial/

Edit the following variables:

(access)$> nano $SCRATCH/PS2/launcher-scripts/bash/serial/


Submit the (passive) job with sbatch

(access)$> sbatch $SCRATCH/PS2/launcher-scripts/bash/serial/

Question: compare and explain the execution time with both launchers:

  • Naive workflow: time = ? CPU usage for the sequential workflow

  • Parallel workflow: time = ? CPU usage for the parallel workflow

Exercise 2: Watermarking images in Python

We will use another program, (full path: $SCRATCH/PS2/tutorials/basic/sequential_jobs/scripts/, and we will distribute the computation on 2 nodes with the launcher (full path: $SCRATCH/PS2/launcher-scripts/bash/generic/

This python script will apply a watermark to the images (using the Python Imaging library).

The command works like this:

python <path/to/watermark_image> <source_image>

We will work with 2 files:

  • copyright.png: a transparent images, which can be applied as a watermark
  • images.tgz: a compressed file, containing 30 JPG pictures (of the Gaia Cluster :) ).

Step 0: python image manipulation module installation

In an interactive job, install pillow in your home directory using this command:

(access IRIS)>$ srun -p interactive -N 1 --qos qos-interactive --pty bash -i

(node)>$ pip install --user pillow

Step 1: Prepare the input files

Copy the source files in your $SCRATCH directory.

(access)>$ tar xvf /mnt/isilon/projects/ulhpc-tutorials/sequential/images2.tgz -C $SCRATCH/PS2/
(access)>$ cp /mnt/isilon/projects/ulhpc-tutorials/sequential/ulhpc_logo.png $SCRATCH/PS2

(access)>$ cd $SCRATCH/PS2

Step 2: Create a list of parameters

We must create a file containing a list of parameters, each line will be passed to

ls -d -1 $SCRATCH/PS2/images/*.JPG | awk -v watermark=$SCRATCH/PS2/ulhpc_logo.png '{print watermark " " $1}' > $SCRATCH/PS2/generic_launcher_param
\_____________________________/   \_________________________________________________________________/ \_________________________________/
               1                                                    2                                                3
  1. ls -d -1: list the images
  2. awk ...: prefix each line with the first parameter (watermark file)
  3. >: redirect the output to the file $SCRATCH/generic_launcher_param

Step 3: Configure the launcher

We will use the launcher (full path: $SCRATCH/PS2/launcher-scripts/bash/generic/

Edit the following variables:

(access)$> nano $SCRATCH/PS2/launcher-scripts/bash/generic/

# number of cores needed for 1 task

Step 4: Submit the job

We will spawn 1 process per 2 cores on 2 nodes

On Iris, the Slurm job submission command is sbatch

(access IRIS)>$ sbatch $SCRATCH/PS2/launcher-scripts/bash/generic/

Step 5: Download the files

On your laptop, transfer the files in the current directory and look at them with your favorite viewer. Use one of these commands according to the cluster you have used:

(yourmachine)$> rsync -avz iris-cluster:/scratch/users/<LOGIN>/PS2/images .

Question: which nodes are you using, identify your nodes with the command sacct or Slurmweb

Exercise 3: Advanced use case, using a Java program: "JCell"

Let's use JCell, a framework for working with genetic algorithms, programmed in Java.

We will use 3 scripts:

  • (full path: $SCRATCH/PS2/tutorials/basic/sequential_jobs/scripts/

We want to execute Jcell, and change the parameters MutationProb and CrossoverProb. This script will install JCell, generate a tarball containing all the configuration files, and the list of parameters to be given to the launcher.

  • (full path: $SCRATCH/PS2/tutorials/basic/sequential_jobs/scripts/

This script is a wrapper, and will start one execution of jcell with the configuration file given in parameter. If a result already exists, then the execution will be skipped. Thanks to this simple test, our workflow is fault tolerant, if the job is interrupted and restarted, only the missing results will be computed.

  • (full path: $SCRATCH/PS2/launcher-scripts/bash/generic/

This script will drive the experiment, start and balance the java processes on all the reserved resources.

Step 1: Generate the configuration files:

Execute this script:

    (access)$> $SCRATCH/PS2/tutorials/basic/sequential_jobs/scripts/

This script will generate the following files in $SCRATCH/PS2/jcell:

  • config.tgz
  • jcell_param

Step 2: Edit the launcher configuration, in the file $SCRATCH/PS2/launcher-scripts/bash/generic/

This application is cpu-bound and not memory-bound, so we can set the value of NB_CORE_PER_TASK to 1. Using these parameters, the launcher will spawn one java process per core on all the reserved nodes.

    (access)$> nano $SCRATCH/PS2/launcher-scripts/bash/generic/

    # number of cores needed for 1 task

Step 3: Submit the job

On Iris, the Slurm job submission command is sbatch

(access IRIS)>$ sbatch $SCRATCH/PS2/launcher-scripts/bash/generic/

Step 4. Retrieve the results on your laptop:

Use one of these commands according to the cluster you have used:

    (yourmachine)$> rsync -avz iris-cluster:/scratch/users/<LOGIN>/PS2/jcell/results .

Question: check the system load and memory usage with Ganglia


At the end, please clean up your home and scratch directories :)

Please do not store unnecessary files on the cluster's storage servers:

(access)$> rm -rf $SCRATCH/PS2

For going further: