ChIP-seq practical session

Running all analyses is computationally intensive and despite the power of the current laptops, jobs should be run on high-performance clusters (HPC).


slides are available as a pdf, click below

log in iris

iris is one of the High Performance Computer (HPC) of the UNI.

connect to the frontend

To connect to it, you need an account and an authorized ssh key. Actually, a pair of keys, one public and one private. The public key is sent over when connecting to the remote and compared to the authorized private key. A match allows the sender to log in. No password required.

After the setting up of your account, the following should work if you are using mac or GNU/Linux:

ssh iris-cluster

Otherwise, on Windows, use xmobaterm. In the terminal, log as your username, such as student01.

You should see the following prompt of the iris frontend:

     First reserve your nodes (using srun/sbatch(1))
aginolhac@access1.iris-cluster.uni.lux(11:30:46): ~ $

Note that you are on the access frontend.

The frontend is meant for browsing / transferring your files only and you MUST connect to a node for any computational work using the utility slurm described here. This program managed the queuing system and dispatch jobs among the resources according to the demands.

Softwares are organized into modules that provide you with the binaries but also all the environment required for their running processes. However, we will use a container that will ease our analyses.

The login procedure can be depicted as:


log in to a remote computer is great, all computation, heat generation is happening elsewhere but this comes with a price: disconnection.
This happens all the time. The way to get around it is to have a screen system that stores your terminal, commands, environment in which you can easily detach and re-attach.

Two systems exist, screen and tmux. Both work well, but tmux has a nicer interface IMHO.

a short tutorial is accessible here.

Briefly, on the access frontend, start a tmux instance with


to detach (press CTRL and B together, release then use the next key):

CTRL + B, then D

to re-attach:

tmux attach

or the alias

tmux at

Useful commands

once in an instance,

  • create a new tab
CTRL + B, then C
  • move to the next tab
CTRL + B, then N
  • move to the previous tab
CTRL + B, then P
  • rename to the current tab
CTRL + B, then ,

then type the new name



in all tabs kills the tmux session

Of note, tmux instances live until the frontend is rebooted.

connect to a computing node

Connecting to a computing node is anyway required to use modules.

You need to book resources by specifying how many cores, optionally if they are on a same node, the memory required, and a wall time clock. A job can never get extended.

Without entering into the details of submitting a job. The less you ask for, the more high up you are in the queue. Here is the explanation for the above command:

  • srun is for interactive, sbatch for passive
  • --time= following by hour:minute:second for wall time clock
  • --mem= with 12GB for booking 12 gigabytes
  • -c cores, A node is usually composed of 28 cores

Once logged in, the prompt changes for:


where you see the node you are logged to (here iris-001).

monitoring the resources used

On a shared cluster, you have to take care of three things:

  1. memory usage
  2. cores used
  3. disk space


Each node has On an interactive session, use the command htop to see if the memory is not full. If the system is swapping (using hard drives for memory storage) it becomes super slow and eventually stalled.

For passive sessions, you can use ganglia to check out the nodes you are using.


even if you book 10 cores, nothing will prevent you from starting 100 jobs. They will run but then tasks are distributed on the available resources. In this example, each task will use 1/10th of a core, then runs very slowly.
On an interactive session, use the command htop to see if a process is correctly using close to 100% of a core.

disk space

Like on your local machine, you need to check how much data you used. Using a command line, you could use

disk usage

du -sh ~

to display your disk usage (du) for your home folder (~). In a form readable by human (-h)

disk free

df -h

disk free scans all disks mounted. Could takes time to display the global usage. Please worry if only few Mb are available on the disk you are planning to write to.

check also your own quota with df-ulhpc on the frontend.

closing connection

When you are done, you can kill yourself your job by either doing CTRL + D or typing exit. That will free your booked resources for others. Once done, you will still log on the frontend and normally inside a tmux. The best is to detach from the tmux instance and log off from the gaia frontend using CTRL + D or typing exit.