iPOP-UP training: hands-on

Date: 12/06/2023
Trainers: Olivier Kirsh, Julien Rey, Magali Hennion

Presentation

The slides of the presentation can be downloaded here.

Table of content

Connect to the cluster

ssh -o PubkeyAuthentication=no username@ipop-up.rpbs.univ-paris-diderot.fr

Security warning
Never leave your computer unsupervised with your session open and iPOP-UP server connected.

Warm-up

Where are you on the cluster?

pwd

Then explore the /shared folder

tree -L 1 /shared

/shared/banks folder contains commonly used data and resources. Explore it by yourself with commmands like ls or cd.

Can you see the first 10 lines of the mm10.fa file? (mm10.fa = mouse genomic sequence version 10)

There is a training project accessible to you, navigate to this folder and list what is inside.

Then go to one of your projects and create a folder named 230612_training. This is where you will do all the exercices.

Optional: use a file explorer

Using the file manager from GNOME, you can navigate easily on iPOP-UP file server.

  • Open the file manager Fichiers.
  • Click on Autres emplacements on the side bar.
  • In the bar Connexion à un serveur, type sftp://ipop-up.rpbs.univ-paris-diderot.fr/ and press the enter key.
  • Enter your login and password.

This way, you can modify your files directly using any local text editor.

Be careful
Never use word processor (like Microsoft Word or LibreOffice Writer) to modify your code and never copy/past code to/from those softwares. Use only text editors and UTF-8 encoding.

Tip
For other systems, please see the instructions for Windows or Linux.

Get information about the cluster

sinfo

Slurm sbatch command

sbatch allows you to send an executable file to be ran on a computation node.

Exercise 1: my first sbatch script

Starting from 01_02_flatter.sh, make a script named flatter.sh printing “What a nice training !”

Then run the script:

sbatch flatter.sh

The output that should have appeared on your screen has been diverted to slurm-xxxxx.out but this name can be changed using SBATCH options.

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Correction

Exercise 2: my first SBATCH option

Modify flatter.sh to add this line:

#SBATCH -o flatter.out

then run it. Anything different ?

Correction

Exercise 3: hostname

Run using sbatch the command hostname in a way that the sbatch outfile is called hostname.out.

What is the output ? How does it differ from typing directly hostname in the terminal and why ?

Correction

Useful sbatch options 1/2

Options Flag Function
−−partition -p partition to run the job (mandatory)
−−job-name -J give a job a name
−−output -o output file name
−−error -e error file name
−−chdir -D set the working directory before running
−−time -t limit the total run time (default : no limit)
−−mem   memory that your job will have access to (per node)

To find out more, the Slurm manual man sbatch or https://slurm.schedmd.com/sbatch.html.

Modules

A lot of tools are installed on the cluster. To list them, use one of the following commands.

module available
module avail
module av

You can limit the search for a specific tool, for example look for the different versions of multiqc on the cluster using module av multiqc.

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To load a tool

module load tool/1.3
module load tool1 tool2 tool3

To list the modules loaded

module list

To remove all loaded modules

module purge

Tip
Load your modules within your “sbatch” file for consistency.

Job handling and monitoring

Exercise 4: follow your jobs

The sleep command : do nothing (delay) for the set number of seconds.

Restart from 03_04_hostname_sleep.sh and launch a simple job that will launch sleep 600.

Correction

squeue

On your terminal, type

squeue

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ST Status of the job.
R = Running
PD = Pending

To see only iPOP-UP jobs

squeue -p ipop-up

To see only the jobs of untel

squeue -u untel

To see only your jobs

squeue --me

scancel

To cancel a job which you started, use the scancel command followed by the jobID (Number given by SLURM, visible in squeue)

scancel jobID

You can stop the previous sleep job with this command.

sacct

Re-run sleep.sh and type

sacct

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You can pass the option --format to list the information that you want to display, including memory usage, time of running,…
For instance

sacct --format=JobID,JobName,Start,Elapsed,CPUTime,NCPUS,NodeList,MaxRSS,ReqMeM,State

To see every options, run sacct --helpformat

Job efficiency : seff

After the run, the seff command allows you to access information about the efficiency of a job.

seff <jobid>

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Practical example

Exercise 5 : Alignment

Run an alignment using STAR version 2.7.5a starting from 05_06_star.sh.

  • The FASTQ files to align are in /shared/projects/training/test_fastq.
  • You need an index folder for STAR (version 2.7.5a) for the mouse mm39 genome, look for it in the banks.
  • You have to increase the RAM to 25G.

After the run

Check the resource that was used using seff.

Correction

Parallelization

Useful sbatch options 2/2

Options Default Function
−−nodes 1 Number of nodes required (or min-max)
−−nodelist   Select one or several nodes
−−ntasks-per-node 1 Number of tasks invoked on each node
−−mem 2GB Memory required per node
−−cpus-per-task 1 Number of CPUs allocated to each task
−−mem-per-cpu 2GB Memory required per allocated CPU
−−array   Submit multiple jobs to be executed with identical parameters

Multi-threading

Some tools allow multi-threading, i.e. the use of several CPUs to accelerate one task. It is the case of STAR with the --runThreadN option.

Exercise 6: Alignment, parallel

Modify the previous sbatch file to use 4 threads to align the FASTQ files on the reference. Run and check time and memory usage.

Use Slurm variables

The Slurm controller will set some variables in the environment of the batch script. They can be very useful. For instance, you can improve the previous script using $SLURM_CPUS_PER_TASK.

Correction

The full list of variables is visible here.

Some useful ones:

  • $SLURM_CPUS_PER_TASK
  • $SLURM_JOB_ID
  • $SLURM_JOB_ACCOUNT
  • $SLURM_JOB_NAME
  • $SLURM_JOB_PARTITION

Of note, Bash shell variables can also be used in the sbatch script:

  • $USER
  • $HOME
  • $HOSTNAME
  • $PWD
  • $PATH

Job arrays

Job arrays allow to start the same job a lot of times (same executable, same resources) on different files for example. If you add the following line to your script, the job will be launch 6 times (at the same time), the variable $SLURM_ARRAY_TASK_ID taking the value 0 to 5.

#SBATCH --array=0-5

Exercice 7 : Job array

Starting from 07_08_array_example.sh, make a simple script launching 6 jobs in parallel.

Correction

Exercice 8 : fair resource sharing

It is possible to limit the number of jobs running at the same time using %max_running_jobs in #SBATCH --array option.

Modify your script to run only 2 jobs at the time.

You will see using squeue command that some of the tasks are pending until the others are over.

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Correction

Job arrays examples

Take all files matching a patern in a directory

Example

#SBATCH --array=0-7   # if 8 files to proccess 
FQ=(*fastq.gz)  #Create a bash array
echo ${FQ[@]}   #Echos array contents
INPUT=$(basename -s .fastq.gz "${FQ[$SLURM_ARRAY_TASK_ID]}") #Each elements of the array are indexed (from 0 to n-1) for slurm 
echo $INPUT     #Echos simplified names of the fastq files

List or find files to process

If for any reason you can’t use bash array, you can alternatively use ls or find to identify the files to process and get the nth with sed (or awk).

#SBATCH --array=1-4   # If 4 files, as sed index start at 1
INPUT=$(ls $PATH2/*.fq.gz | sed -n ${SLURM_ARRAY_TASK_ID}p)
echo $INPUT

Job Array Common Mistakes

  • The index of bash arrays starts at 0
  • Don’t forget to have different output files for each task of the array
  • Same with your log names (\%a or \%J in the name will do the trick)
  • Do not overload the cluster! Please use \%50 (for example) at the end of your indexes to limit the number of tasks (here to 50) running at the same time. The 51st will start as soon as one finishes!
  • The RAM defined using #SBATCH --mem=25G is for each task

Complex workflows

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Use workflow managers such as Snakemake or Nextflow.

nf-core workflows can be used directly on the cluster.

Exercice 9: nf-core workflows

Starting from 09_nf-core.sh, write a script running ChIP-seq workflow on nf-core test data.

Some help can be found here. Please also see the full documentation.

Correction
[Of note, even with the test dataset, it takes a lot of time and resources!]

Useful resources

Thanks

  • iPOP-UP’s technical and steering committees

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BiBs 2024 parisepigenetics
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