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Portable test run of ESPResSo on EuroHPC systems via EESSI

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Since 14 June 2024, ESPResSo v4.2.2 is available in the EESSI production repository, optimized for the 8 CPU targets that are fully supported by version 2023.06 of EESSI. This allows running ESPResSo effortlessly on the EuroHPC systems where EESSI is already available, like Vega and Karolina.

On 27 June 2024, an additional installation of ESPResSo v4.2.2 that is optimized for Arm A64FX processors was added, which enables also running ESPResSo efficiently on Deucalion, even though EESSI is not available yet system-wide on Deucalion (see below for more details).

With the portable test for ESPResSo that is available in the EESSI test suite we can easily evaluate the scalability of ESPResSo across EuroHPC systems, even if those systems have different system architectures.

Simulating Lennard-Jones fluids using ESPResSo

Lennard-Jones fluids model interacting soft spheres with a potential that is weakly attractive at medium range and strongly repulsive at short range. Originally designed to model noble gases, this simple setup now underpins most particle-based simulations, such as ionic liquids, polymers, proteins and colloids, where strongly repulsive pairwise potentials are desirable to prevent particles from overlapping with one another. In addition, solvated systems with atomistic resolution typically have a large excess of solvent atoms compared to solute atoms, thus Lennard-Jones interactions tend to account for a large portion of the simulation time. Compared to other potentials, the Lennard-Jones interaction is inexpensive to calculate, and its limited range allows us to partition the simulation domain into arbitrarily small regions that can be distributed among many processors.

Portable test to evaluate performance of ESPResSo

To evaluate the performance of ESPResSo, we have implemented a portable test for ESPResSo in the EESSI test suite; the results shown here were collected using version 0.3.2.

After installing and configuring the EESSI test suite on Vega, Karolina, and Deucalion, running the Lennard-Jones (LJ) test case with ESPResSo 4.2.2 available in EESSI can be done with:

reframe --name "ESPRESSO_LJ.*%module_name=ESPResSo/4.2.2"

This will automatically run the LJ test case with ESPResSo across all known scales in the EESSI test suite, which range from single core up to 8 full nodes.

Performance + scalability results on Vega, Karolina, Deucalion

The performance results of the tests are collected by ReFrame in a detailed JSON report.

The parallel performance of ESPResSo, expressed in particles integrated per second, scales linearly with the number of cores. On Vega using 8 nodes (1024 MPI ranks, one per physical core), ESPResSo 4.2.2 can integrate the equations of motion of roughly 615 million particles every second. On Deucalion using 8 nodes (384 cores), we observe a performance of roughly 62 million particles integrated per second.

Performance of ESPResSo 4.2.2 on Vega, Karolina, Deucalion

Plotting the parallel efficiency of ESPResSo 4.2.2 (weak scaling, 2000 particles per MPI rank) on the three EuroHPC systems we used shows that it decreases approximately linearly with the logarithm of the number of cores.

Parallel efficiency of ESPResSo 4.2.2 on Vega, Karolina, Deucalion

Running ESPResSo on Deucalion via EESSI + cvmfsexec

While EESSI is already available system-wide on both Vega and Karolina for some time (see here and here for more information, respectively), it was not available yet on Deucalion when these performance experiments were run.

Nevertheless, we were able to leverage the optimized installation of ESPResSo for A64FX that is available in EESSI since 27 June 2024, by leveraging the cvmfsexec tool, and by creatively implementing two simple shell wrapper scripts.

cvmfsexec wrapper script

The first wrapper script can be used to run a command in a subshell in which the EESSI CernVM-FS repository ( is mounted via cvmfsexec. This script can be used by regular users on Deucalion, it does not require any special privileges beyond the Linux kernel features that cvmfsexec leverages, like user namespaces.

Contents of ~/bin/

if [ -d /cvmfs/ ]; then
    # run command directly, EESSI CernVM-FS repository is already mounted
    # run command via in subshell where EESSI CernVM-FS repository is mounted,
    # via cvmfsexec which is set up in a unique temporary directory
    mkdir -p /tmp/$USER
    tmpdir=$(mktemp -p /tmp/$USER -d)
    cd $tmpdir
    git clone > $tmpdir/git_clone.out 2>&1
    cd cvmfsexec
    ./makedist default > $tmpdir/cvmfsexec_makedist.out 2>&1
    cd $orig_workdir
    $tmpdir/cvmfsexec/cvmfsexec -- "$@"
    # cleanup
    rm -rf $tmpdir

Do make sure that this script is executable:

chmod u+x ~/bin/

A simple way to test this script is to use it to inspect the contents of the EESSI repository:

~/bin/ ls /cvmfs/

or to start an interactive shell in which the EESSI repository is mounted:

~/bin/ /bin/bash -l

The job scripts that were submitted by ReFrame on Deucalion leverage to set up the environment and get access to the ESPResSo v4.2.2 installation that is available in EESSI (see below).

orted wrapper script

In order to get multi-node runs of ESPResSo working without having EESSI available system-wide, we also had to create a small wrapper script for the orted command that is used by Open MPI to start processes on remote nodes. This is necessary because mpirun launches orted, which must be run in an environment in which the EESSI repository is mounted. If not, MPI startup will fail with an error like "error: execve(): orted: No such file or directory".

This wrapper script must be named orted, and must be located in a path that is listed in $PATH.

We placed it in ~/bin/orted, and add export PATH=$HOME/bin:$PATH to our ~/.bashrc login script.

Contents of ~/bin/orted:


# first remove path to this orted wrapper from $PATH, to avoid infinite loop
orted_wrapper_dir=$(dirname $0)
export PATH=$(echo $PATH | tr ':' '\n' | grep -v $orted_wrapper_dir | tr '\n' ':')

~/bin/ orted "$@"

Do make sure that also this orted wrapper script is executable:

chmod u+x ~/bin/orted

If not, you will likely run into an error that starts with:

An ORTE daemon has unexpectedly failed after launch ...

Slurm job script

We can use the script in a Slurm job script on Deucalion to initialize the EESSI environment in a subshell in which the EESSI CernVM-FS repository is mounted, and subsequently load the module for ESPResSo v4.2.2 and launch the Lennard-Jones fluid simulation via mpirun:

Job script (example using 2 full 48-core nodes on A64FX partition of Deucalion):

#SBATCH --ntasks=96
#SBATCH --ntasks-per-node=48
#SBATCH --cpus-per-task=1
#SBATCH --time=5:0:0
#SBATCH --partition normal-arm
#SBATCH --export=None
#SBATCH --mem=30000M
~/bin/ << EOF
source /cvmfs/
module load ESPResSo/4.2.2-foss-2023a
mpirun -np 96 python3

(the Python script is available in the EESSI test suite, see here)