Interactive node access

Sometimes it is desirable to have a direct interactive connection to a compute node — for example when testing or debugging code. Regular production runs, especially multi-day calculations, should use sbatch instead.

Two ways of interactive computing

LUNARC is committed to interactive HPC and supports two approaches:

• The LUNARC Desktop on Demand provides a graphical interface with interactive access to compute resources. You can launch applications directly or open a command-line terminal on your chosen resource. Search for Interactive Terminal (CPU only) or Accelerated Terminal (GPU support) in the application menu. See the dedicated Desktop on Demand guide.
• The interactive command — described on this page

We recommend the Desktop on Demand for most users, especially those new to LUNARC.

Starting an interactive session

Use the interactive command to request resources and start a session. The following example requests 4 cores on 1 node for 60 minutes, charged to project lu-test:

interactive -N 1 --ntasks-per-node=4 -t 60 -A lu-test

The session ends when the requested time expires or you type exit. Your account is charged for the full wall-clock duration regardless of how much computation you perform — 4 cores for 60 minutes costs 4 CPU-hours; 4 cores for 30 minutes costs 2 CPU-hours.

The interactive command accepts most sbatch options. See man sbatch for the full list.

Modules and environment variables

Loaded modules and environment variables are not always exported correctly to the interactive session. Once inside the session, run:

module purge

Then reload all the modules you need from scratch — even if module list shows them as still loaded, they may not function correctly.

Also check that any required environment variables still hold the expected values. If your software uses a setup script, re-run it inside the session.

Author: (LUNARC)

Last Updated: 2025-09-08