# How do I run OpenIFS in parallel configuration using several nodes?

**URL:** <https://forum.ecmwf.int/t/how-do-i-run-openifs-in-parallel-configuration-using-several-nodes/12778>\
**Category:** OpenIFS\
**Tags:** openifs-configuration-and-namelists, openifs-48r1, openifs-parallel-configuration\
**Created:** [25 April 2025 11:16 UTC](https://forum.ecmwf.int/t/how-do-i-run-openifs-in-parallel-configuration-using-several-nodes/12778 "2025-04-25T11:16:37Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Marcus\_Koehler](https://forum.ecmwf.int/user_avatar/forum.ecmwf.int/marcus_koehler/32/1000_2.png) [@Marcus\_Koehler](https://forum.ecmwf.int/u/Marcus_Koehler)\
**Post date:** [25 April 2025 11:16 UTC](https://forum.ecmwf.int/t/how-do-i-run-openifs-in-parallel-configuration-using-several-nodes/12778/1 "2025-04-25T11:16:37Z")

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We have received several questions about how OpenIFS 48r1 can be used across multiple nodes on an HPC system. It may be helpful to summarise this here:

**First case:**  
You want to run the model **interactively** , i.e. you execute the `oifs-run` script directly from the command line in your terminal.  
(Note: Unless your system is very powerful, this will only work for low grid resolutions and short test runs.)

- First, edit the `exp-config.h` file in your experiment directory, and set `OIFS_NPROC` and `OIFS_NTHREAD` to the number of MPI tasks and OpenMP threads you wish to use.
- Also in this file, set the `LAUNCH` variable to the command that starts the model binary for parallel runs on your system. This might be `mpirun`, `aprun`, or `srun`—your local computing support team can advise you on which is appropriate.  
You do **not** need to include the OpenIFS executable name in this variable—only the command and its required options, e.g.: `LAUNCH="mpirun -np ${OIFS_NPROC}"`
- The `oifs-run` script will check whether the `LAUNCH` variable has been set. If it hasn’t, it will attempt to create it using its default launcher settings, found at line 349ff in the script.
- A configuration for the hpc2020 machine at ECMWF is included in the script. You may add your own system’s configuration there if you prefer not to set the `LAUNCH` command directly in `exp-config.h`.

**Second case:**  
You want to run the model as a **batch job** —this is the more common method for running OpenIFS.

- You will use a job script that calls the `oifs-run` script. We provide an example tailored for the hpc2020 system at ECMWF: `$OIFS_RUN_SCRIPT/run-oifs.ecmwf-hpc2020.job`
- The header lines in this script request the resources for your job. The example provided (for the SLURM batch scheduler used at ECMWF) requests 8 nodes by default.  
You may need to adjust these headers to match your own HPC system. Your local computing support can assist with this.
- In the job script, you should specify the location of your `oifs-config` file.
- You do **not** need to edit `OIFS_NPROC` and `OIFS_NTHREAD` in your experiment’s `exp-config.h` file, as the job script will override these values before calling `oifs-run`.
- In your experiment’s `exp-config.h` file, set the `LAUNCH` variable to the basic command without any flags, e.g. `"srun"`.

More information can be found in the [OpenIFS 48r1 User Guide - OpenIFS - ECMWF Confluence Wiki](https://confluence.ecmwf.int/display/OIFS/OpenIFS+48r1+User+Guide).

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_Note: The description above applies to version `openifs-48r1-20250214`._