What a supercomputer is

Last updated on 2026-10-26 | Edit this page

Overview

Questions

  • What is different about a supercomputer and the computer on your desk?
  • How do you log in to Snellius, and how is the course cluster different?

Objectives

  • Describe a supercomputer as a login node, a scheduler, and compute nodes.
  • Identify a login hostname and a compute hostname from a written example.
  • Connect to Snellius with SSH after the SURF user portal step.
  • Connect to the course cluster with the address in your welcome note.
  • Predict what a long computation does to a shared login node.

A room of computers, with a door

A supercomputer is not a bigger laptop. It is many computers in one room. They share a door, a queue, and file systems. You talk to the door. A scheduler decides when your work may run on a computer in the room.

You will meet three roles.

  • The login node is the door. You reach it with SSH, or with a web shell. It is shared with everyone else who is logged in. It is the right place to edit a script, look at a small file, and submit a job. It is the wrong place to run the job.
  • The scheduler is the queue. On Snellius and on the course cluster the scheduler is Slurm. You describe the resources you need. Slurm starts the work when those resources are free.
  • A compute node is where the work runs. You do not SSH there for a normal batch job. Slurm starts your script on that node and writes the text output to a file.
How a job moves. The nodes drawn are laptop, login node, scheduler, and compute node.
How a job moves. The nodes drawn are laptop, login node, scheduler, and compute node.

Snellius

Snellius is the Dutch national supercomputer, operated by SURF. The public description is at https://www.surf.nl/en/services/compute/snellius-the-national-supercomputer. Before the first SSH login you sign in to the SURF user portal, CUA. The account email comes from SURF and names your login.

From your own computer:

BASH 

ssh YOUR_LOGIN@snellius.surf.nl

Your prompt is on a login node. It looks like YOUR_LOGIN@int4:~$. The int names are login nodes. A compute node has a different name. If the network you are on is not trusted, SURF documents a separate door node in the service desk. Use that host when the welcome mail tells you to.

You can also work in a browser through Open OnDemand. That page gives you a shell, a file manager, and a few applications. The shell is still a login node.

Callout

A password prompt on the login node is normal. A job does not start because you logged in. Logging in only gives you the door.

The course cluster

This course also uses a small cluster built from SURF Research Cloud workspaces. The template is Magic Castle: https://github.com/ComputeCanada/magic_castle. Your trainer sends a setup link. That link creates your password. There is one login node, and there is no CUA account.

BASH 

ssh YOUR_LOGIN@MANAGER

MANAGER is the address in the welcome note. It is not snellius.surf.nl. The nodes have two cores each, with more cores on the GPU node. Ethernet stands in for the fast network Snellius uses. The partition names rome and thin_course point at the same CPU nodes. The gpu partition holds the GPU cards.

There is no project budget on the course cluster. accinfo is not installed. sacct still reports core-seconds in CPUTimeRAW. On Snellius your project does have a budget, counted in system billing units. You do not need the budget command for the exercises in this lesson.

A faded login

Fill the blank for the machine you are actually using.

BASH 

ssh ________@________

On Snellius the host is snellius.surf.nl and the name is your CUA login. On the course cluster both values come from the welcome note.

The sample SSH config in episodes/files/ssh-config-sample is the Snellius shape:

BASH 

Host snellius
  HostName snellius.surf.nl
  User YOUR_LOGIN
Challenge

Predict the hostname

You are about to run hostname after logging in. Write down what you think the name will start with, before you run it.

On Snellius, which of these is a login node: int4, or tcn123?

int4 is a login node. Names that start with int are the shared door. A name like tcn123 is a compute node. You should see an int name when you run hostname on Snellius just after SSH. On the course cluster there is one login node, and its name is the one in your welcome note, not an int name.

Challenge

Check a Snellius SSH config

From the lesson directory, run the offline check:

BASH 

bash episodes/files/exercises/login-checklist.sh

SH 

hostname=snellius.surf.nl

What does the script refuse to accept?

It reads HostName from episodes/files/ssh-config-sample and exits with an error if that value is not snellius.surf.nl. A course-cluster address belongs in a different Host block, not in this one.

Challenge

Log in for real

On Snellius or the course cluster, run:

BASH 

hostname
sinfo -s
who

In the EESSI client container there is no Slurm, so sinfo will not exist. Run hostname there anyway, and compare it with the cluster. Setup for the container is on the setup page.

On Snellius, hostname starts with int and sinfo -s lists partitions such as rome and genoa. On the course cluster, sinfo -s shows the small partitions, including thin_course or rome. who shows other people on the login node. That is the reminder: the door is shared.

The container prints its own hostname. That name is the container, not a SURF node. You use it for the software exercises, not as a substitute for sinfo.

What you do next

You have a shell on a shared door. The next episode loads the software you will run. The episode after that sends the work through Slurm so it does not stay on this door.

Key Points
  • A supercomputer is a login node, a scheduler, and compute nodes.
  • ssh YOUR_LOGIN@snellius.surf.nl reaches Snellius after you have a CUA account.
  • The course cluster is a Magic Castle workspace. Its address is in your welcome note.
  • hostname on a Snellius login node starts with int.
  • A long computation does not belong on the login node.