FUCHS Cluster Usage

The cluster was updated to a new operating system (AlmaLinux 9). Please help us to help you. In the beginning it is usally very stressful because we get bomarded with tickets, like my previous software ran but nothings works anymore. It can be nessecary to set up software e.g. Spack from the scratch. Also some old ssh-keys with the rsa cipher might not work anymore. Please provide us some time to rearrange our dokumentation. Please discuss problems first in your group, maybe team members already found a solution to your problem, before attacking us with tickets. As less tickets we get as faster we can proceed to provide a working cluster for everybody. Also see common errors.

FUCHS is a general-purpose compute cluster based on Intel CPU architectures running AlmaLinux 9 and SLURM. Please read the following instructions and ensure that this guide is fully understood before using the system.


An SSH client is required to connect to the cluster. On a Linux system the command is usually:

ssh <user_account>@fuchs.hhlr-gu.de

Am I connected to the right server? Please find our ssh-rsa fingerprint here:

fuchs.hhlr-gu.de fingerprints

Warnings - Security Breach - Keys etc.

You may receive a warning from your system that something with the security is wrong, “maybe somebody is evedropping”. Due to the upgrade of the operating system from Scientifc Linux 7.9 to AlmaLinux 9.2 the SSH server keys have changed. Please erase your old FUCHS key with
ssh-keygen -R fuchs.hhlr-gu.de

and accept the new fuchs.hhlr-gu.de key. Above you will find our unique ECDSA and ED25519 fingerprints. Some programs tend to display the fingerprint in the SHA256 or MD5 format. Just click on fuchs.hhlr-gu.de fingerprints above.

Please check with ssh-keyscan fuchs.hhlr-gu.de if the key above is the same. On Windows systems please use/install a Windows SSH client (e.g. PuTTY, MobaXterm, the Cygwin ssh package or the built-in ssh command).

After your first login you will get the message, that your password has expired and you have to change it. Please use the password provided by CSC at the prompt, choose a new one and retype it. You will be logged out automatically. Now you can login with your new password and work on the cluster.

Never run heavy calculations on the login node, i.e. CPU-time- or RAM-consuming processes. You can check the CPU-time limit (in seconds) by running

ulimit -t

on the command line. On the login node, any process that exceeds the CPU-time limit (e.g. a long running test program or a long running rsync) will be killed automatically.

Environment Modules

There are several versions of software packages installed on our systems. The same name for an executable (e.g. mpirun) and/or library file may be used by more than one package. The environment module system, with its module command, helps to keep them apart and prevents name clashes. You can list the module-managed software by running module avail on the command line. Other important commands are module load <name> (loads a module) and module list (lists the already loaded modules).

It's important to know, which modules you really need. Loading more than one MPI module at the same time will likely lead to overlapping.

If you want to know more about module commands, the module help command will give you an overview.

Working with Intel oneAPI

With the command 'module avail' you are able to see all available modules on the cluster. The intel/oneapi/xxx works a kind of different, because it functions like a container. Start loading it with module load intel/oneapi/xxx and see with module avail what is inside. Now start loading your preferred module.

To avoid erros please use versions numbers instead of latest.


module load intel/oneapi/2023.2.0
module avail
# new modules available within the Intel oneAPI modulefile
----------------------------------- /cluster/intel/oneapi/2023.2.0/modulefiles -----------------------------------
advisor/2023.2.0        dev-utilities/2021.10.0  icc32/2023.2.1                mkl32/2023.2.0
advisor/latest          dev-utilities/latest     icc32/latest                  mkl32/latest
ccl/2021.10.0           dnnl-cpu-gomp/2023.2.0   inspector/2023.2.0            mpi/2021.10.0
ccl/latest              dnnl-cpu-gomp/latest     inspector/latest              mpi/latest
compiler-rt/2023.2.1    dnnl-cpu-iomp/2023.2.0   intel_ipp_ia32/2021.9.0       mpi_BROKEN/2021.10.0
compiler-rt/latest      dnnl-cpu-iomp/latest     intel_ipp_ia32/latest         mpi_BROKEN/latest
compiler-rt32/2023.2.1  dnnl-cpu-tbb/2023.2.0    intel_ipp_intel64/2021.9.0    oclfpga/2023.2.0
compiler-rt32/latest    dnnl-cpu-tbb/latest      intel_ipp_intel64/latest      oclfpga/2023.2.1
compiler/2023.2.1       dnnl/2023.2.0            intel_ippcp_ia32/2021.8.0     oclfpga/latest
compiler/latest         dnnl/latest              intel_ippcp_ia32/latest       tbb/2021.10.0
compiler32/2023.2.1     dpct/2023.2.0            intel_ippcp_intel64/2021.8.0  tbb/latest
compiler32/latest       dpct/latest              intel_ippcp_intel64/latest    tbb32/2021.10.0
dal/2023.2.0            dpl/2022.2.0             itac/2021.10.0                tbb32/latest
dal/latest              dpl/latest               itac/latest                   vtune/2023.2.0
debugger/2023.2.0       icc/2023.2.1             mkl/2023.2.0                  vtune/latest
debugger/latest         icc/latest               mkl/latest

loaded  modulepath

Compiling Software

You can compile your software on the login nodes (or on any other node, inside a job allocation). Several compiler suites are available. While GCC version 11.3.1 is the built-in OS default, you can list additional compilers and libraries by running module avail:

  • GNU compilers
  • Intel compilers
  • MPI libraries

For the right compilation commands please consider:

C/C++, Fortran77, Fortran 95

Compilation commands for different compilers

To build and manage software which is not available via “module avail” and is not available as a built-in OS package, we recommend using Spack. Please read this small introduction on how to use Spack on the cluster. More information is available on the Spack webpage.


There are various storage systems available on the cluster. In this section we describe the most relevant:

  • your home directory /home/fuchs/<group>/<user> (NFS, slow),
  • your scratch directory /scratch/fuchs/<group>/<user> (parallel file system BeeGFS, fast),
  • the non-shared local storage (i.e. unique on each compute node) under /local/$SLURM_JOB_ID on each compute node (max. 1.4 TB per node, slow)

Please use your home directory for small permanent files, e.g. source files, libraries and executables.

Use the scratch space for large temporary job data and delete the data as soon as you no longer need it, e.g. when it's older than 30 days.

By default, the space in your home directory is limited to 30 GB, and in your scratch directory to 5 TB and/or 800000 inodes (which corresponds to approximately 200000+ files). You can check your homedir and scratch usage by running the quota command on a login node.

While the data in your home directory is backed up nightly (please ask, if you want us to restore anything from there, see also HRZ-Backup), there is no backup of your scratch directory.

If you need local storage on the compute nodes, you have to add the --tmp parameter to your job script (see SLURM section below). Set the amount of storage in megabytes, e.g. set --tmp=5000 to allocate 5 GB of local disk space. The local directory (/local/$SLURM_JOB_ID) is deleted after the corresponding job has finished. If, for some reason, you don't want the data to be deleted (e.g. for debugging), you can use salloc instead of sbatch and work interactively (see man salloc). Or, one can put an rsync at the end of the job script, in order to save the local data to /scratch just before the job exits:

mkdir /scratch/fuchs/<groupid>/<userid>/$SLURM_JOBID
scontrol show hostnames $SLURM_JOB_NODELIST | xargs -i ssh {} \
    rsync -a /local/$SLURM_JOBID/ \

All shared file systems are shared between users and jobs. There is no guarantee, that you always get the desired bandwidth and/or response time.

Although our storage systems are protected by RAID mechanisms, we can't guarantee the safety of your data. It is within the responsibility of the user to backup important files.

Running Jobs With SLURM

On our systems, compute jobs and resources are managed by SLURM (Simple Linux Utility for Resource Management). The compute nodes are organized in the partition (or queue) named fuchs.

PartitionNode type
fuchs Intel Broadwell CPU

Nodes are used exclusively, i.e. only whole nodes are allocated for a job and no other job can use the same nodes concurrently.

In this document we discuss several job types and use cases. In most cases, a compute job falls under one (or more than one) of the following categories:

For every compute job you have to submit a job script (unless working interactively using salloc or srun, see man page for more information). If jobscript.sh is such a script, then a job can be enqueued by running

sbatch jobscript.sh

on a login node. A SLURM job script is a shell script containing SLURM directives (options), i.e. pseudo-comment lines starting with


The SLURM options define the resources to be allocated for the job (and some other properties). Otherwise the script contains the “job logic”, i.e. commands to be executed.

Read More

The following instructions shall provide you with the basic information you need to get started with SLURM on our systems. However, the official SLURM documentation covers some more use cases (also in more detail). Please read the SLURM man pages (e.g. man sbatch or man salloc) and/or visit http://www.schedmd.com/slurmdocs. It's highly recommended.

Helpful SLURM link: SLURM FAQ

Your First Job Script

In fuchs you can allocate up to 120 nodes with two Intel Ivy Bridge CPUs with 20 cores (i.e. 40 HT threads). In the following example we allocate 60 CPU cores (i.e. three nodes) and 512 MB per core for 5 minutes (SLURM may kill the job after that time, if it's still running):

#SBATCH --job-name=foo
#SBATCH --partition=fuchs
#SBATCH --nodes=3
#SBATCH --ntasks=60
#SBATCH --cpus-per-task=1
#SBATCH --mem-per-cpu=512   
#SBATCH --time=00:05:00
#SBATCH --no-requeue
#SBATCH --mail-type=FAIL
#SBATCH –-extra-node-info=2:10:1
srun hostname
sleep 3

For SLURM, a CPU core (a CPU thread, to be more precise) is a CPU.
Prevent the job from being requeued after a failure.
Send an e-mail if sth. goes wrong.
Disable Hyper-Threading.

The srun command is responsible for the distribution of the program (hostname in our case) across the allocated resources, so that 20 instances of hostname will run on each of the allocated nodes concurrently. Please note, that this is not the only way to run or to distribute your processes. Other cases and methods are covered later in this document. In contrast, the sleep command is executed only on the head1) node.

Although nodes are allocated exclusively, you should always specify a memory value that reflects the RAM requirements of your job. The scheduler treats RAM as a consumable resource. As a consequence, if you omit the --nodes parameter (so that only the number of CPU cores is defined) and allocate more memory per core than there actually is on a node, you'll automatically get more nodes if the job doesn't fit in otherwise.2) Moreover, jobs are killed through SLURM's memory enforcement when using more memory than requested.

After saving the above job script as e.g. jobscript.sh, you can submit your job by running

sbatch jobscript.sh

on the command line. The job's output streams (stdout and stderr) will be joined and saved to slurm-ID.out, where ID is a SLURM job ID, which is assigned automatically. You can change this behavior by adding an --output and/or --error argument to the SLURM options.

Job Monitoring

For job monitoring (to check the current state of your jobs) you can use the squeue command. Depending on the current cluster utilization (and other factors), your job(s) may take a while to start. You can list the current queuing times by running sqtimes on the command line.

If you need to cancel a job, you can use the scancel command (please see the manpage, man scancel, for further details).

Goethe-HLR users

All users with an account at Goethe-HLR and FUCHS, have to add

#SBATCH --account=<your FUCHS group>
#SBATCH --partition=fuchs
to the job script, when they want to use the FUCHS cluster.

Node Type

On FUCHS one type of compute node is available. There are

NumberTypeVendorProcessorProcessor x Core (HT)RAM [GB]
194dual-socketIntelXeon Ivy Bridge E5-2670 v2 2×10 (2×20)128

Per-User Resource Limits

On FUCHS, you have the following limits for the partition fuchs:

MaxTime 21 days the maximum run time for jobs
MaxJobsPU 10 max. number of jobs a user is able to run simultaneously
MaxSubmitPU 20 max. number of jobs in running or pending state
MaxNodesPU 120 max. number of nodes a user is able to use at the same time
MaxArraySize 1001 the maximum job array size


On compute nodes you can use Hyper-Threading. That means, in addition to each physical CPU core a virtual core is available. SLURM identifies all physical and virtual cores of a node, so that you have 40 logical CPU cores on an Intel Ivy Bridge node. If you don't want to use HT, you can disable it by adding

Node typesbatch command
Ivy Bridge#SBATCH --extra-node-info=2:10:1

to your job script. Then you'll get half the threads per node (which will correspond to the number of cores). This can be beneficial in some cases (some jobs may run faster and/or more stable).

Bundling Single-Threaded Tasks

Note: Please also see the Job Arrays section below. Because only full nodes are given to you, you have to ensure, that the available resources are used efficiently. Please combine as many single-threaded jobs as possible into one. The limits for the number of combined jobs are given by the number of cores and the available memory. A simple job script to start 20 independent processes may look like this one:

#SBATCH --partition=fuchs
#SBATCH --nodes=1
#SBATCH --ntasks=20
#SBATCH --cpus-per-task=1
#SBATCH --mem-per-cpu=2000
#SBATCH --time=01:00:00
#SBATCH --mail-type=FAIL
# Replace by a for loop.
./program input01 >& 01.out &
./program input02 >& 02.out &
./program input20 >& 20.out &
# Wait for all child processes to terminate.

In this (SIMD) example we assume, that there is a program (called program) which is run 20 times on 20 different inputs (usually input files). Both output streams (stdout and stderr) of each process are redirected to a file N.out. A job script is always executed on the first allocated node, so we don't need to use srun, since exactly one node is allocated. Further we assume that the executable is located in the same directory where the job was submitted (that is the initial working directory).

Job Arrays

If you have a lot of single-core computations to run, job arrays are worth a look. Telling SLURM to run a job script as a job array will result in running that script multiple times (after the corresponding resources have been allocated). Each instance will have a distinct SLURM_ARRAY_TASK_ID variable defined in its environment.

Due to our full-node policy, you still have to ensure, that your jobs don't waste any resources. Let's say, you have 400 single-core tasks. In the following example 400 tasks are run inside a job array while ensuring that only 20-core nodes are used and that each node runs exactly 20 tasks in parallel.

#SBATCH --partition=fuchs
#SBATCH --nodes=1
#SBATCH --ntasks=20
#SBATCH --cpus-per-task=1
#SBATCH --mem-per-cpu=2000
#SBATCH --time=00:10:00
#SBATCH --array=0-399:20
#SBATCH --mail-type=FAIL
my_task() {
     # Print the given "global task number" with leading zeroes
     # followed by the hostname of the executing node.
     K=$(printf "%03d" $1)
     echo "$K: $HOSTNAME"
     # Do nothing, just sleep for 3 seconds.
     sleep 3
# Every 20-task block will run on a separate node.
for I in $(seq 20); do
     # This is the "global task number". Since we have an array of
     # 400 tasks, J will range from 1 to 400.
     # Put each task into background, so that tasks are executed
     # concurrently.
     my_task $J &
     # Wait a little before starting the next one.
     sleep 1
# Wait for all child processes to terminate.

If the task running times vary a lot, consider using the thread pool pattern. Have a look at GNU parallel, for instance.

OpenMP Jobs

For OpenMP jobs, set the --cpus-per-task parameter. As usual, you should also specify a --mem-per-cpu value. But in this case you have to divide the total RAM required by your program by the number of threads. E.g. if your application needs 4000 MB and you want to run 20 threads, then you have to set --mem-per-cpu=200 (4000/20 = 200). Don't forget to set the OMP_NUM_THREADS environment variable. Example:

#SBATCH --partition=fuchs
#SBATCH --ntasks=1
#SBATCH --cpus-per-task=20
#SBATCH --mem-per-cpu=200
#SBATCH --mail-type=ALL
#SBATCH --time=48:00:00

MPI Jobs

Remember: Nodes are used exclusively. Each node has 20 CPU cores. If you want to run a lot of small jobs (i.e. where more than one job could be run on a single node concurrently), consider running more than one computation within a job. Otherwise it will most likely result in a waste of resources and will lead to a longer queueing time (for you and others).

See also: http://www.schedmd.com/slurmdocs/faq.html#steps

As an example, we want to run a program that spawns 80 MPI ranks and where 1200 MB of RAM are allocated for each rank.

#SBATCH --partition=fuchs
#SBATCH --ntasks=80
#SBATCH --cpus-per-task=1
#SBATCH --mem-per-cpu=1200
#SBATCH --mail-type=ALL
#SBATCH --extra-node-info=2:10:1
#SBATCH --time=48:00:00
module load mpi/XXXX/....
mpirun ./your_mpi_program

Please note that some MPI installations support the srun command (instead of or in addition to mpirun), e.g.:
srun [--mpi=pmix] ./your_mpi_program

MPI implementations are typically designed to work seamlessly with job schedulers like Slurm. When you launch MPI tasks with mpirun (or srun) inside your job script, the MPI library uses the information provided by Slurm (via environment variables or other means) to determine the communication topology and allocate processes accordingly.

Hybrid Jobs: MPI/OpenMP

MVAPICH2 example script (20 ranks, 5 threads each and 200 MB per thread, i.e. 1 GB per rank; so, for 20*5 threads, you'll get five 20-core nodes):

#SBATCH --partition=fuchs
#SBATCH --ntasks=20
#SBATCH --cpus-per-task=5
#SBATCH --mem-per-cpu=200
#SBATCH --mail-type=ALL
#SBATCH --extra-node-info=2:10:1
#SBATCH --time=48:00:00
mpirun -np 20 ./example_program

Please note, that this is just an example. You may or may not run it as-it-is with your software, which is likely to have a different scalability.

You have to disable the core affinity when running hybrid jobs with MVAPICH2. Otherwise all threads of an MPI rank will be pinned to the same core. Our example now includes the command


which disables this feature. The OS scheduler is now responsible for the placement of the threads during the runtime of the program. But the OS scheduler can dynamically change the thread placement during the runtime of the program. This leads to cache invalidation, which degrades performance. This can be prevented by thread pinning.

Memory Allocation

Normally the memory available per CPU thread is calculated by the whole amount of RAM divided by the number of threads. For instance 128GB / 40 threads = 3.2GB per thread. Keep in mind that the FUCHS cluster provides two threads per core. Now imagine you need more memory, let's say 8192MB per task. Type scontrol show node=<node> and look for 'mem'. Another way is to login into an Ivy Bridge Node and type free -m. In both cases you will determine 128768M(B) of memory. Now we calculate how many processes we can launch on one node like this: 128782MB/8192MB=15.72… . With this result we can determine how many nodes we need. In the following example we like to have 69 processes à 8192MB.

#SBATCH --job-name=<your_job_name>
#SBATCH --partition=fuchs
#SBATCH --ntasks=69            # Whole amount of processes we have
#SBATCH --cpus-per-task=1      # Only one task per CPU

##SBATCH --mem-per-cpu=8192    # We can't use this argument here, because it ends up with an error message
                               # therefore it's commented out.

#SBATCH --mem=0                # Instead we use this argument, which provides us the full memory of the node  
#SBATCH --ntasks-per-node=15   # We have calculated and rounded that we need 15 process on one node

srun hostname

# If everythins works fine you were granted 5 nodes, for example 4 nodes à 14 tasks and 1 node à 13 tasks
# 56 tasks + 13 tasks = 69 tasks, as requested.

Local Storage

On each node there is up to 1.4 TB of local disk space (see also Storage). If you need local storage, you have to add the --tmp parameter to your SLURM script. Set the amount of storage in megabytes, e.g. set --tmp=5000 to allocate 5 GB of local disk space. The data in the local directory (/local/$SLURM_JOB_ID) is deleted after the corresponding batch job has finished.

The salloc Command

For interactive workflows you can use SLURM's salloc command. With salloc almost the same options can be used as with sbatch, e.g.:

[user@fuchs ~]$ salloc --nodes=4 --time=0:45:00 --mem=64g --partition=fuchs
salloc: Granted job allocation 1122971
salloc: Waiting for resource configuration
salloc: Nodes node27-[012-015] are ready for job
[user@fuchs ~]$ 

Now you can ssh into the nodes that were allocated for the job and run further commands, e.g.:

[user@fuchs ~]$ ssh node27-012
[user@node27-012 ~]$ hostname
[user@node27-012 ~]$ exit
Connection to node27-012 closed.

Or you can use srun for running a command on all allocated nodes in parallel:

[user@fuchs ~]$ srun hostname

Finally you can terminate your interactive job session by running exit, which will free the allocated nodes:

[user@fuchs ~]$ exit
salloc: Relinquishing job allocation 1122971

the first one of the three allocated nodes
please see Memory Allocation for the new Slurm behavior
public/usage/fuchs.txt · Last modified: 2024/07/05 11:26 by geier
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