Niflheim Getting Started

View the page contents in the menu bar on the left. See the Access to Niflheim section about getting access.

Overview of the Niflheim cluster

Login to Niflheim

See the Access to Niflheim section about getting access. Login to Niflheim is available using SSH only from within the DTU network. If you are outside of DTU, please log in to the DTU_VPN service or the G-databar first.

It is recommended to login to the node type identical to the compute nodes onto which you submit batch jobs, see Compute node partitions and the Hardware information page. This rule will ensure that if you build your own compiled code, it is going to run only on compatible compute node hardware, see Usage of binary compiled code.

Niflheim’s login nodes are:

  • sylg.fysik.dtu.dk, slid.fysik.dtu.dk, and slid2.fysik.dtu.dk:

    • Login node for partition xeon24el8 (RockyLinux 8 OS).

    • 24 CPU cores (Intel Xeon CPU E5-2650 v4 @ 2.20GHz Broadwell), 256 GB of RAM memory.

    • Refer to this as CPU_ARCH= broadwell_el8.

  • svol.fysik.dtu.dk and thul.fysik.dtu.dk:

    • Login node for partitions xeon40el8 as well as sm3090el8 (RockyLinux 8 OS).

    • Intel Xeon Scalable Gold Skylake CPUs with AVX512 vector instructions.

    • Refer to this as CPU_ARCH= skylake_el8.

  • surt.fysik.dtu.dk:

    • Login node for partitions xeon56, xeon32_4096, and a100 (RockyLinux 8 OS).

    • Please build all applications for xeon56 with the latest Intel MKL math library (see Software environment modules below)!

    • 56 CPU cores (Intel Xeon Gold 6348 CPU @ 2.60GHz IceLake with AVX512 vector instructions), 512 GB of RAM.

    • Refer to this as CPU_ARCH= icelake.

  • fjorm.fysik.dtu.dk:

    • Login node for partition epyc96 (RockyLinux 8 OS).

    • Please build all applications for epyc96 with the latest foss toolchain (see Software environment modules below)!

    • 16 CPU cores (AMD EPYC 9124 Genoa Zen4), 384 GB of RAM. Note that the epyc96 partition compute nodes have 96 CPU cores.

    • Refer to this as CPU_ARCH= epyc9004.

The login nodes fjorm, surt, svol, sylg, and thul must not be overloaded with heavy tasks, since this will disturb other users.

The login nodes slid2 and slid would be acceptable for more heavy testing of codes, but please bear in mind that the login nodes may be shared by many users, and no single user should monopolize any login nodes. Long tasks should always be submitted as batch jobs.

SSH setup

The SSH (Secure Shell) communication protocol is described widely on the Internet. The user’s SSH configuration files and cryptographic keys are stored in the directory $HOME/.ssh/ on Linux systems.

You are encouraged to configure SSH keys on your own PC so that you can login to Niflheim without entering a password:

The SSH public key from your PC can be appended to the file $HOME/.ssh/authorized_keys to enable password-less logins.

WARNING: DO NOT copy SSH keys from Niflheim to any external computer (for example, your PC) for reasons of security! The Niflheim SSH keys must only be used on the Niflheim system.

Optional: You may create SSH keys using this command on any Niflheim login node:

authorized_keys

The SSH key files will be created in the directory $HOME/.ssh/. This can be necessary if you use commercial MPI libraries which use SSH in stead of the recommended Slurm method for starting tasks.

Home directory and disk quota

Every user has a personal home directory on one of the Niflheim file servers, located in a file system allocated to the research group (for example, /home/energy/).

The home directory file servers have a daily backup of all changed files. To request a manual restore of lost files, please send mail to the address in the Niflheim support page.

To view your current disk quota:

quota -s

To view file systems mounted on the node (omitting temporary file systems):

df -Phx tmpfs

Usage of binary compiled code

Users of Niflheim should be aware of some important facts about different CPU types. More recent CPUs implement new machine instructions (for example, AVX or AVX2 vector instructions) which do not exist on older generations of CPUs. The general rules of CPU usage are:

  • Code compiled on newer CPUs may likely crash if executed on older CPUs.

  • Code compiled on older CPUs (older node types) is likely to run much slower on newer nodes because performance-enhancing vector instructions etc. are not used.

  • Do not run old binaries compiled on other and older systems (such as CentOS 7 Linux). Such binaries will run slowly or may likely crash.

Read more about CPU architectures and instructions here:

File transfer to and from Niflheim

If you need to transfer files to and from Niflheim, please use SSH’s transfer method scp (Secure Copy).

You can also synchronize directories between Niflheim and your local (CAMD) machine in a simple way by using rsync over an SSH connection. On your local machine you may find these commands useful:

From Niflheim to your local machine:
rsync -av -e ssh sylg.fysik.dtu.dk:niflheim_directory/ local_directory/

From your local machine to Niflheim:
rsync -av -e ssh local_directory/ sylg.fysik.dtu.dk:niflheim_directory/

(Note that trailing ``/`` is important with rsync - read the rsync man-page first).

Another useful option to rsync is –exclude-from=FILE that allows one to exclude files/directories specified in the file FILE. Note that paths in FILE must be relative to the root directory of the source, e.g. niflheim_directory/ in the first example above.

If the disk on your local machine is formatted as a Windows FAT/FAT32 filesystem (for example, on an external USB disk) we suggest using these flags with rsync:

rsync -rltv --modify-window=1 -e ssh sylg.fysik.dtu.dk:niflheim_directory/ USB-disk/

If the disk on your local machine is formatted as a Windows ExFAT filesystem (for example, on an external USB disk) use these options:

rsync -rltv -e ssh sylg.fysik.dtu.dk:niflheim_directory/ USB-disk/

NOTICE about ExFAT file systems:

  • ExFAT file systems do not support the concept of a symbolic_link (soft link) file.

  • File names must not contain “:” or other special characters, see www.ntfs.com. Such file names may be renamed using the Linux rename command.

Windows users may use WinSCP or FileZilla, to do scp or sftp operations.

Slurm batch queueing system

Here is a brief introduction to the usage of Slurm:

Compute node partitions

Slurm node partitions are the compute resource in Slurm which group nodes into logical and possibly overlapping sets.

To display the status of all available Slurm partitions use the showpartitions command (append -h for help).

Niflheim contains a number of node partitions with different types of CPU architecture hardware and the corresponding recommended login nodes:

Partition

CPU architecture

CPU cores

RAM memory

/tmp scratch disk

Login nodes

Linux_ OS

xeon24el8, xeon24el8_test, xeon24el8_week

Broadwell

24

254 GB

140 GB

sylg, slid, slid2

RockyLinux 8

xeon40el8

Skylake and Cascade_Lake.

40

380 GB

140 GB

thul, svol

RockyLinux 8

xeon40el8_768

Skylake

40

760 GB

140 GB

thul, svol

RockyLinux 8

xeon40el8_clx

Cascade_Lake

40

380 GB

140 GB

thul, svol

RockyLinux 8

sm3090el8

Skylake + GPUs

80 (40*2 with HT)

192 GB

800 GB

thul

RockyLinux 8

sm3090el8_768

Skylake + GPUs

80 (40*2 with HT)

768 GB

800 GB

thul

RockyLinux 8

xeon56

IceLake

56

512 GB

293 GB

surt

RockyLinux 8

epyc96

AMD EPYC Zen4 9474F

96

768 GB

1.7 GB

fjorm

RockyLinux 8

xeon32_4096

IceLake

32

4096 GB

14 TB

surt

RockyLinux 8

a100

IceLake + 4* A100 NVLink GPUs

128 (16*4 with HT)

512 GB

1.7 TB

surt

RockyLinux 8

Please notice the following points:

  • The default maximum time limit for jobs is 50 hours in all partitions. Some partitions will accept jobs up to 1 week (168 hours), please use the showpartitions command to view all available partitions. The xeon24el8_test partition has a 10 minute time limit and must be used only for development tests.

  • Please use all CPU cores in the most modern CPU compute nodes (xeon40, xeon56, and epyc96 partitions), and do not submit jobs to these partitions which only use partial nodes.

  • Partial node usage, including single-core jobs, are permitted in the xeon24 partition by submitting to 1 and up to 23 cores of a 24-core node.

  • Partial node jobs are also permitted in the partitions xeon32_4096 (BIG memory) as well as the GPU partitions sm3090 and a100.

  • Please do not use the GPU partitions a100 or sm3090 unless your group has been authorized to use GPUs.

  • The RAM memory is slightly less than the physical RAM due to operating system overheads.

  • The xeon40 partition consists of both Skylake and Cascade_Lake CPU types. While these CPUs are (almost) binary compatible, the new Cascade_Lake CPUs will have a higher performance.

  • Some partitions are overlapping so that nodes with more memory are also members of the partition with the lower amount of memory.

  • The local node scratch disk space is shared between all Slurm jobs currently running on the node, see Using compute node temporary scratch disk space below.

Compute nodes and jobs

Use sinfo to view available nodes:

sinfo

and to view the queue use squeue:

squeue

and for an individual user ($USER in this example):

squeue -u $USER

To see detailed information about a job-id use this command:

showjob <jobid>

List of pending jobs in the same order considered for scheduling by Slurm:

squeue --priority --sort=-p,i --states=PD

Hint: Set an environment variable in your .bashrc file so that the default output format contains more information:

export SQUEUE_FORMAT="%.18i %.9P %.8j %.8u %.10T %.9Q %.10M %.9l %.6D %.6C %R"

or for even more details:

export SQUEUE_FORMAT2="JobID:8,Partition:11,QOS:7,Name:10 ,UserName:9,Account:9,State:8,PriorityLong:9,ReasonList: ,TimeUsed:12,SubmitTime,TimeLimit:11,NumNodes:6,NumCPUs:5,MinMemory:6"

To change the time display format see man squeue, for example:

export SLURM_TIME_FORMAT="%a %T"

To show all jobs on the system with one line per user:

showuserjobs

Submitting batch jobs to Niflheim

The command sbatch is used to submit jobs to the batch queue. Submit your Slurm script file by:

sbatch scriptfile

See the above mentioned pages for information about writing Slurm script files, which may contain a number batch job parameters. See the sbatch page and this example:

#!/bin/bash
#SBATCH --mail-type=ALL
#SBATCH --mail-user=<Your E-mail>  # The default value is the submitting user.
#SBATCH --partition=xeon24
#SBATCH -N 2      # Minimum of 2 nodes
#SBATCH -n 48     # 24 MPI processes per node, 48 tasks in total, appropriate for xeon24 nodes
#SBATCH --time=1-02:00:00
#SBATCH --output=mpi_job_slurm_output.log
#SBATCH --error=mpi_job_slurm_errors.log

It is strongly recommended to specify both nodes and tasks numbers so that jobs will occupy entire nodes (see Compute node partitions). For selecting the correct number of nodes and tasks (cores) see the sbatch man-page items:

-N, --nodes=<minnodes[-maxnodes]>    # Request that a minimum of minnodes nodes be allocated to this job. A maximum node count may also be specified with maxnodes. If only one number is specified, this is used as both the minimum and maximum node count...
-n, --ntasks=<number>                # Number of tasks

You may validate your batch script, and return an estimate of when a job would be scheduled to run:

sbatch --test-only <scriptfile>  # No job is actually submitted.

You can select a specific node partition (see Compute node partitions) with lines in the script (or on the command line):

  • Select the 24-core nodes in the xeon24 partition:

    #SBATCH --partition=xeon24

  • Select the 24-core nodes in the xeon24 partition which also have 512 GB RAM memory:

    #SBATCH --partition=xeon24_512

If you have permission to charge jobs to another (non-default) account, jobs can be submitted like:

sbatch -A <account>

To delete a job use scancel:

scancel <jobid>

To hold or release a jobid xxx use the scontrol command:

scontrol hold xxx     Hold a job
scontrol release xxx  Release a held job

View status of jobs and nodes

You can view your jobs (running, pending, etc.) with squeue like these examples:

squeue -u $USER
squeue -u $USER -t running
squeue -u $USER -t pending

To get information about the status of the compute nodes running your jobs, use the pestat command:

pestat -u $USER

The pestat lists usage of CPU cores, RAM memory, GPUs (if used), and the current CPU load with 1 line per node. To see all the possible pestat options:

pestat -h

You may use this information to determine if your jobs are behaving correctly in terms of CPU and memory resources.

User limits on batch jobs

It may happen that some jobs will be pending due to limits imposed on the user account. The typical reasons for a job not starting are that the following limits could be exceeded:

  • AssocGrpCpuLimit: Limit on the number of CPU cores.

  • AssocGrpCPURunMinutesLimit: Limit on the number of CPU cores multiplied by the minutes of wallclock time requested.

  • AssocGrpNodeLimit: Limit on the number of compute nodes.

  • MaxJobsAccrue: Maximum number of pending jobs able to accrue age priority

For a full list of limits, see the section Limits in both Associations and QOS in the limits page.

Use the following command to display the limits currently in effect for your userid:

showuserlimits

Use showuserlimits -h to see all options. For example, to display the number of CPUs limit:

showuserlimits -l GrpTRES -s cpu

Newly created users will have some lower limits for the first 30 days in order to prevent erroneous bad usage of the system.

Fairshare usage

We have defined the following Slurm FairShare default parameters:

User type

FairShare

VIP/PhD

3%

Student

2%

Faculty

5%

Guest/external

1%

To display job FairShare priority values use:

sprio -l -u $USER

Job arrays

Job_arrays offer a mechanism for submitting and managing collections of similar jobs quickly and easily; job arrays with millions of tasks can be submitted in milliseconds (subject to configured size limits). All jobs must have the same initial options (e.g. size, time limit, etc.), however it is possible to change some of these options after the job has begun execution using the scontrol command specifying the JobID of the array or individual ArrayJobID.

Job_arrays are only supported for batch jobs and the array index values are specified using the –array or -a option of the sbatch command. The option argument can be specific array index values, a range of index values, and an optional step size as shown in the examples below.

Jobs which are part of a job array will have the environment variable SLURM_ARRAY_TASK_ID set to its array index value.

See some examples of usage in the Job_arrays page.

Using compute node temporary scratch disk space

It is very important that every user refrain from overloading the central file servers! This may happen when jobs write job temporary files to their $HOME directories on those file servers.

Users are kindly requested to configure job scripts to use the compute nodes’ /tmp folder for any temporary files in the job. This may sometimes be implemented by using this job script command:

export TMPDIR=/tmp

This $TMPDIR setting is the default value in many computer codes and may not need to be set explicitly.

Notes:

  • On the login nodes you must not use /tmp for large files! Please use in stead the local /scratch/$USER folder.

Technical details:

  • Each Slurm job automatically allocates a temporary /tmp disk space which is private to the job in question.

  • This temporary disk space lives only for the duration of the Slurm job, and is automatically deleted when the job terminates.

  • This temporary disk space is actually allocated on the compute node’s local /scratch disk, the size of which is specified above under the Compute node partitions section.

Shared scratch disk spaces

For those applications which require the medium-term use of scratch files across several different nodes or for subsequent batch jobs, we provide some scratch file spaces at:

/home/scratch3/$USER/         # CAMD, CatTheory, Energy groups
/home/scratch11/$USER/        # Construct/MEK group

REMEMBER: There is no backup of files!! Lost files cannot be recovered by any means!

Please remember to clean up scratch files regularly when they are no longer needed.

Viewing completed or failed job information

After your job has completed (or terminated), you can view job accounting data by inquiring the Slurm database. For example, to inquire about a specific job Id 1234:

sacct -j 1234 -o jobid,jobname,user,Timelimit,Elapsed,NNodes,Partition,ExitCode,nodelist

If some jobs have failed or been cancelled, you can display a list of such jobs within a given time interval using a command such as:

sacct -s FAILED,CANCELLED -X --starttime 2024-01-11T19:00 --endtime 2024-01-12T09:00 -o User,jobid,jobname%40,partition,State,ExitCode

Here the --starttime indicates the Start and --endtime indicates the End of the desired time interval. The sacct manual page documents the valid time formats.

You may inquire about many job parameters, to see a complete list run:

sacct -e

Correct usage of node types

Usage of multi-CPU nodes

The most modern compute nodes with many CPU cores should be utilized fully by the batch jobs:

epyc96 node jobs should utilize 96 CPU cores per node
xeon56 node jobs should utilize 56 CPU cores per node
xeon40 node jobs should utilize 40 CPU cores per node

If you have jobs that utilize less than 40 CPU cores per node, we request that you use the older compute nodes:

xeon24 nodes permit jobs using 1-24 CPU cores on 1 node
xeon24 node jobs should utilize 24 CPU cores per node, but only in case 2 or more nodes are requested

Please see also the list of Compute node partitions.

Job scripts that do not use CPU cores or GPUs correctly may be rejected at submit time or be cancelled by the administrators.

Usage of BIG memory nodes

We have installed 4 BIG memory nodes for special applications used by selected groups. These nodes have 4096 GB (4 TB) of RAM memory, and it is expected (required) that all jobs submitted to the xeon32_4096 partition will use at least 768 GB of RAM memory and/or use the large scratch disk space. Jobs using up to 768 GB of RAM memory should use one of the other Compute node partitions. Partial-node jobs are permitted in the xeon32_4096 partition.

The xeon32_4096 nodes are also equipped with a very large (14 TB) and very fast scratch file system. Large scratch spaces are typically required by big-memory jobs. Slurm jobs use the local scratch disk as the job’s private /tmp directory, but note that the scratch disk space is shared between all jobs on the node.

Here are some special instructions for submitting jobs to the xeon32_4096 partition:

  • Memory must always be specified in the Slurm submit script. Memory can be specified in either of two ways: --mem=xx for the total memory requirement of the job or --mem-per-cpu=xx for memory per CPU allocated in the job.

  • Any job can ask for up to 4 TB of memory even if it does not require all of the CPU cores, for example:

    #SBATCH --mem=3000GB
#SBATCH -n 4

    Here, Slurm will allocate 4 cores and 3 TB of memory. This means that another job can run on the same node utilizing at most the remaining 28 cores and 1 TB of memory.

Job scripts that do not use CPU cores correctly may be rejected at submit time or be cancelled by the administrators.

Usage of GPU compute nodes

Please do not use the GPU partitions unless your group has been authorized to use GPUs. The appropriate login_nodes (RockyLinux 8) for GPU partitions are:

The appropriate login_nodes must be used to build software for GPUs, since they have the same CPU architecture as the GPU-nodes. GPU-specific software modules will only be provided on GPU-compatible nodes.

NVIDIA’s CUDA software is available as a module on the login_nodes and compute nodes:

$ module avail CUDA/

Batch jobs submitted to the GPU nodes must request GPU resources! Jobs that only use CPUs without using GPUs are not permitted. Partial node jobs are permitted in the GPU partitions.

You must include batch job statements for specifying correct numbers of CPUs and GPUs. Since the nodes in the sm3090 partition have 10 GPUs each and 80 “virtual” CPU cores, you must submit jobs with 80/10 = 8 CPUs per GPU:

#SBATCH -n 8

For example, to submit a batch jobs to 1 GPU on 8 CPU cores of a node in the sm3090 partition:

#SBATCH --partition=sm3090
#SBATCH -N 1-1
#SBATCH -n 8
#SBATCH --gres=gpu:1

Similarly, the nodes in the a100 partition have 4 A100 GPUs each and 128 “virtual” CPU cores, so you should request 32 CPU cores per GPU. Job scripts that do not use CPU cores or GPUs correctly may be rejected at submit time or be cancelled by the administrators.

For further Slurm information see the GRES page.

Software environment modules

The classical problem of maintaining multiple versions of software packages and compilers is solved using Software_Modules.

Niflheim uses the Lmod implementation of software environment modules (we do not use the modules command which might be supplied by the OS). For creating modules we support the EasyBuild_modules build and installation framework.

The Lmod command module (and its brief equivalent form ml) is installed on all nodes.

Read the Lmod_User_Guide to learn about usage of modules. For example, to list available modules:

module avail
ml av

You can load any available module like in this example:

module load GCC
ml GCC

If you work on different CPU architectures, it may be convenient to turm off Lmod’s caching feature by:

export LMOD_IGNORE_CACHE=1

WARNING: With a software module system there is an important advice:

Do NOT modify manually the environment variable LD_LIBRARY_PATH

Loading complete toolchains

The modules framework at the Niflheim Linux supercomputer cluster includes a number of convenient toolchains built as EasyBuild_modules. We currently provide these toolchains:

  • The intel toolchain provides Intel_compilers (Parallel Studio XE), the Intel MKL Math Kernel library, and the Intel_MPI message-passing library.

    Usage and list of contents:

    module load intel
module list

  • The foss toolchain provides GCC, OpenMPI, OpenBLAS/LAPACK, ScaLAPACK(/BLACS), FFTW.

    Usage and list of contents:

    module load foss
module list

  • The iomkl toolchain provides Intel_compilers, Intel MKL, OpenMPI.

    Usage and list of contents:

    module load iomkl
module list

In the future there may be several versions of each toolchain, list them like this:

module whatis foss
module whatis iomkl

Some notes about modules

Matplotlib

Matplotlib has a term called a Matplotlib_backend and you can specify it by:

export MPLBACKEND=module://my_backend

If Matplotlib cannot start up, in some cases you have to turn the Matplotlib_backend off by:

unset MPLBACKEND

Intel VTune Profiler

We have installed module for the Intel VTune Profiler:

module load VTune

Please read the VTune_documentation.

Need additional modules?

Please send your requests for additional modules to the Niflheim support E-mail. We will see if EasyBuild_modules are already available.

Building your own modules

It is possible for you to use your personal modules in addition to those provided by the Niflheim Linux supercomputer cluster system. If you use EasyBuild_modules you can define your private module directory in your home directory and prepend it to the already defined modules:

mkdir $HOME/modules
export EASYBUILD_PREFIX=$HOME/modules
module use $EASYBUILD_PREFIX/modules/all
module load EasyBuild

and then build and install EasyBuild_modules into $HOME/modules. If you need help with this, please write to the Niflheim support E-mail.

Please note that the Niflheim Linux supercomputer cluster is a heterogeneous cluster comprising several generations of CPUs, where the newer ones have CPU instructions which don’t exist on older CPUs. Therefore code compiled on a new CPU may crash if executed on an older CPU. However, the Intel_compilers should generate multiple versions of machine code which may automatically select the correct code at run-time.

If you compile code for the “native” CPU-architecture, it is proposed that you compile separate versions for each CPU architecture. For your convenience we offer a system environment variable which you may use to select the correct CPU architecture:

[ohni@svol ~]$ echo $CPU_ARCH
skylake

The Skylake architecture corresponds to the xeon40 compute nodes, and the GCC compiler (version 4.9 and above) will recognize this architecture name:

module load GCC
gcc -march=native -Q --help=target | grep march | awk '{print $2}'
skylake

GPAW and ASE software on Niflheim

Prebuilt software modules for GPAW and ASE are available on Niflheim. List the modules by:

$ module avail GPAW/ ASE/

It is recommended to read the instructions in https://wiki.fysik.dtu.dk/gpaw/platforms/platforms.html for different ways to use GPAW and ASE on Niflheim.

Jupyter_Notebook on Niflheim

Jupyter_Notebook documents are produced by the Jupyter Notebook App, which contain both computer code (e.g. Python) and rich text elements (paragraph, equations, figures, links, etc…). Notebook documents are both human-readable documents containing the analysis description and the results (figures, tables, etc..) as well as executable documents which can be run to perform data analysis.

On Niflheim we have installed Jupyter_Notebook software modules which you can load and use:

$ module avail JupyterNotebook
-------------------------- /home/modules/modules/all ---------------------------
 JupyterNotebook/7.0.2-GCCcore-12.3.0

You have to select the correct jupyter version shown above, according to which compiler has been used to compile the other software you are using (such as GPAW).

NOTE: If you use a Python virtual environment (venv), you cannot use the IPython module, as the Jupyter_Notebook will not see the modules in the venv. Instead you have to install jupyter in your venv (pip install notebook).

Restrictions on the use of Jupyter Notebook

  • NOTICE: Jupyter Notebooks cannot be connected to directly from any other network at DTU or outside DTU.

  • The web-server on port 8888 can only be accessed from a PC on the DTU Physics cabled network (which includes demon).

  • The jupyter command starts a special web-server on the login_nodes serving a network port number 8888 (plus/minus a small number).

Using Jupyter_Notebook documents on Niflheim from DTU Physics

  1. Use SSH to login to one of the Niflheim login_nodes, preferably slid.fysik.dtu.dk.

  2. Load the relevant module, for example:

    module load JupyterNotebook

    Users of venv should not load this module!

  3. Go to the relevant folder for your notebooks, and start Jupyter_Notebook with the command:

    jupyter notebook --no-browser --ip=$HOSTNAME

    Jupyter_Notebook will respond with around ten lines of text, at the bottom is a URL. Paste that URL into a browser on your local machine.

  4. IMPORTANT: Once you are done using your notebooks, remember to shut down the Jupyter server so you do not tie up valuable ressources (mainly RAM and port numbers).

    You shut down Jupyter by either:

    1. Pressing Control-C twice in the terminal running the jupyter command, or

    2. Clicking on the Quit button on the Jupyter_Notebook overview page

      This is not the same as the Logout buttons on each notebook, which will disconnect your browser from the Jupyter_Notebook server, but actually leave Jupyter_Notebook running on the login_nodes.

Using Jupyter_Notebook documents from home or elsewhere on a Linux or macOS PC

Use these instructions when you are located outside DTU Physics, and your laptop/desktop is running Linux or macOS.

  1. Connect to the DTU_VPN network (information about DTU_VPN is on DTU Inside).

  2. Use SSH to connect to one of the Niflheim login_nodes, preferably slid.fysik.dtu.dk.

  3. Load the relevant module, for example:

    module load JupyterNotebook/7.0.2-GCCcore-12.3.0

    Users of venv should not load this module!

  4. Go to the relevant folder for your notebooks, and start Jupyter_Notebook with the command:

    jupyter notebook --no-browser

    Jupyter_Notebook will respond with around ten lines of text, at the bottom is a URL. It will contain the text localhost:NNNN where NNNN is a port number, typically 8888 or close. You need that number in the next step.

  5. From your desktop/laptop, log in to niflheim again in a new window, using this command to set up an SSH tunnel:

    ssh -L NNNN:localhost:NNNN username@xxxx.fysik.dtu.dk -N

    where:

    • xxxx is slid.fysik.dtu.dk or whatever machine you are using,

    • username is your DTU username,

    • NNNN is the port number printed by the notebook command,

    Note There will be no output from this command. To test if it is working; proceed to the next step.

  6. Open a browser, and cut-and-paste the address starting with https://localhost into your browser.

  7. IMPORTANT: Once you are done using your notebooks, remember to shut down the Jupyter server! See point 4 in the instructions in the previous section (usage from DTU Physics).

Using Jupyter_Notebook documents on Niflheim from home or elsewhere on a Windows PC

Use these instructions when you are located outside DTU Physics, and your laptop/desktop is running Microsoft Windows.

  1. Log in to a Niflheim login_nodes, preferably slid.fysik.dtu.dk. Use MobaXterm to log in directly to e.g. slid.fysik.dtu.dk, but when you create the login session (the Session tab), select Network Settings, then Jump Host. Fill in the Jump Host (and your DTU user name).

  2. Load the relevant module, for example:

    module load JupyterNotebook/7.0.2-GCCcore-12.3.0

    Users of venv should not load this module!

  3. Go to the relevant folder for your notebooks, and start Jupyter_Notebook with the command:

    jupyter notebook --no-browser --ip=$HOSTNAME

    Note the extra --ip option needed when connecting with MobaXterm. Jupyter_Notebook will respond with around ten lines of text, at the bottom is a URL. It will contain the text localhost:NNNN or 127.0.0.1:NNNN where NNNN is a port number, typically 8888 or close. You need that number in the next step.

  4. Use MobaXterm to set up an SSH tunnel (the Tunneling tab).

    • On “My computer” enter port number printed by jupyter.

    • On “SSH server”, enter the jump host hostname, and your DTU username as SSH user. Leave the port number blank.

    • On the remote server, enter slid.fysik.dtu.dk (or whatever node you are using) as the Remote server name, and the port number printed by jupyter as the port number.

    Click save, and then start the tunnel with the small “play” icon.

  5. Open a browser, and cut-and-paste the address starting with https://localhost or http://127.0.0.1 into your browser.

  6. IMPORTANT: Once you are done using your notebooks, remember to shut down the Jupyter server! See point 4 in the instructions in the previous section (usage from DTU Physics).

Containers on Niflheim

Containers for virtual operating system and software environments have become immensely popular. The most well-known Containers system is Docker, and huge numbers of Containers have been created for this environment. Containers are well suited to running one or two applications non-interactively in their own custom environments. Containers share the underlying Linux kernel of the host system, so only Linux Containers can exist on a Linux host.

However, Docker is not well suited for a shared multi-user system, let alone an HPC supercomputer system, primarily due to security issues and performance issues with parallel HPC applications. Please see the Apptainer_security page.

A Containers technology created for HPC is Apptainer (previously known as Singularity). Apptainer assumes (more or less) that each application will have its own container. Apptainer assumes that you will have a build system where you are the root user, but that you will also have a production system where you may not be the root user.

Please consult the Apptainer_documentation for further information. There is a Singularity video tutorial on the Apptainer homepage. For system administrators there are some useful pages Admin Quick Start and User Namespaces & Fakeroot.

Apptainer on Niflheim

We have installed Apptainer (current version: 1.3 from EPEL) as RPM packages.

If you have root priviledge on your personal Linux PC, you may want to make an Apptainer installation locally on the PC. Finished containers can be copied to Niflheim, and executing Apptainer containers is as a normal user without any root priviledge at all!

Please note that you must build containers within a local file system (not a shared file system like NFS where root access is prohibited).

Docker containers can be executed under Apptainer. For example, make a test run of a simple Docker container from DockerHub:

apptainer run docker://godlovedc/lolcow

You can run many recent versions of CentOS Docker containers from the CentOS library, for example a 6.9 container:

apptainer run docker://centos:centos6.9

Ubuntu Linux may be run from the Ubuntu library:

apptainer run docker://ubuntu:17.10

Application codes may also be on DockerHub, for example an OpenFOAM container can be run with:

apptainer run docker://openfoam/openfoam4-paraview50

Apptainer batch jobs

You can submit normal Slurm batch jobs to the queue running Apptainer containers just like any other executable.

An example job script running a container image lolcow.simg:

#!/bin/sh
#SBATCH --mail-type=ALL
#SBATCH --partition=xeon24
#SBATCH --time=05:00
#SBATCH --output=lolcow.%J.log
apptainer exec lolcow.simg cowsay 'How did you get out of the container?'

To run a Apptainer container in parallel on 2 nodes and 10 CPU cores with MPI use the following lines:

#SBATCH -N 2-2
#SBATCH -n 10
module load OpenMPI
mpirun -n $SLURM_NTASKS apptainer exec lolcow.simg cowsay 'How did you get out of the container?'

Visual Studio Code

The Visual Studio Code (VS_code) editor can be used on your personal desktop and make remote SSH connections to the Niflheim login_nodes.

The DTU course 02002/02003: Computer Programming has some material in the page Using VSCode.

There is a bug with remote SSH connections from VS_code which will leave processes behind on the remote server, even after you quit VS_code, see VS_code_bug_8546. The workaround is to add to your VS_code file settings.json the line:

"remote.SSH.useLocalServer": true

Enabling useLocalServer will be the default in the future, but hasn’t happened yet due to some issues on Windows SSH servers.

The Settings_editor is the UI that lets you review and modify setting values that are stored in a settings.json file. The location is documented in Settings file locations.

Pages for system administrators