Interactive work on the compute nodes

Note

• It is possible to run R directly on the login (including ThinLinc) nodes.

  • only be done for short and small jobs

  • otherwise become slow for all users.

• If you want to work interactively with your code or data, you should

  • start an interactive session.

• If you rather will run a script which won’t use any interactive user input while running, you should

  • start a batch job, see previous session.

Questions

• How do I proceed to work interactively on a compute node

Objectives

• Show how to reach the calculation nodes on UPPMAX and HPC2N

• Test some commands on the calculation nodes

Compute allocations in this workshop

• Rackham: uppmax2025-2-272

• Kebnekaise: hpc2n2025-062

• Cosmos: lu2025-7-24

• Tetralith: naiss2025-22-262

• Dardel: naiss2025-22-262

Overview of the UPPMAX systems

Overview of the HPC2N system

Overview of the LUNARC system

Overview of the NSC system

Overview of the PDC system

General

In order to run interactively, you need to have compute nodes allocated to run on, and this is done through the batch system.

Warning

(HPC2N) Do note that it is not real interactivity as you probably mean it, as you will have to run it as a R script instead of by starting R and giving commands inside it. The reason for this is that you are not actually logged into the compute node and only sees the output of the commands you run.

R “interactively” on the compute nodes

To run interactively, you need to allocate resources on the cluster first. You can use the command salloc/intereactive to allow interactive use of resources allocated to your job. When the resources are allocated, you need to preface commands with srun in order to run on the allocated nodes instead of the login node.

• First, you make a request for resources with interactive/salloc, like this:

UPPMAX (interactive)HPC2N (salloc)LUNARCNSCPDC

$ interactive -n <tasks> --time=HHH:MM:SS -A uppmax2025-2-272

where <tasks> is the number of tasks (or cores, for default 1 task per core), time is given in hours, minutes, and seconds (maximum T168 hours), and then you give the id for your project

Your request enters the job queue just like any other job, and interactive/salloc will tell you that it is waiting for the requested resources. When salloc tells you that your job has been allocated resources, you can interactively run programs on those resources with srun. The commands you run with srun will then be executed on the resources your job has been allocated.

If you do not preface with srun on HPC2N and PDC, the command is run on the login node!

You can now run R scripts on the allocated resources directly instead of waiting for your batch job to return a result. This is an advantage if you want to test your R script or perhaps figure out which parameters are best.

Warning

Let us use ThinLinc

• ThinLinc app: <user>@rackham-gui.uppmax.uu.se

• ThinLinc in web browser: https://rackham-gui.uppmax.uu.se This requires 2FA!

Using terminal

• Remember to have X11 installed!

• On Mac

  • install XQuartz

• On Windows

  • Use MobaXterm or

  • install XMING and use with Putty or PowerShell

Example Type along

Type-Along

Requesting 4 cores for 10 minutes, then running R

Examples are here shown for UPPMAX and HPC2N, but as you could see above, the others use the same commands.

UPPMAX (similar to NSC and LUNARC)HPC2N (similar to PDC)

[bjornc@rackham2 ~]$ interactive -A naiss2024-22-107 -p devcore -n 4 -t 10:00
You receive the high interactive priority.
There are free cores, so your job is expected to start at once.

Please, use no more than 6.4 GB of RAM.

Waiting for job 29556505 to start...
Starting job now -- you waited for 1 second.

Let us check that we actually run on the compute node:

[bjornc@r483 ~]$ srun hostname
r483.uppmax.uu.se
r483.uppmax.uu.se
r483.uppmax.uu.se
r483.uppmax.uu.se

We are! Notice that we got a response from all four cores we have allocated.

Running a script

Warning

• You need to reload all modules you used on the login node!!!

The script

Adding two numbers from user input (serial_sum.R)

• You will find it in the exercise directory exercises/r/ so go there with cd.

• Otherwise, use your favourite editor and add the text below and save as serial_sum.R.

# This program will add two numbers that are provided by the user
args = commandArgs(trailingOnly = TRUE)
res = as.numeric(args[1]) + as.numeric(args[2])
print(paste("The sum of the two numbers is", res))

Running the script

• Note that the commands are the same for both HPC2N and UPPMAX!

Running a R script in the allocation we made further up. Notice that since we asked for 4 cores, the script is run 4 times, since it is a serial script

$ srun Rscript serial_sum.R 3 4
[1] "The sum of the two numbers is 7"
[1] "The sum of the two numbers is 7"
[1] "The sum of the two numbers is 7"
[1] "The sum of the two numbers is 7"

Without the srun command, R won’t understand that it can use several cores. Therefore the program is run only once.

$ Rscript serial_sum.R 3 4
[1] "The sum of the two numbers is 7"

Running R with workers

• First start R and check available workers with future.

• Create a R script called script-workers.R` with the following content:

library(future)
availableWorkers()
availableCores()

• Execute the code with srun -n 1 -c 4 Rscript script-workers.R

Exit

When you have finished using the allocation, either wait for it to end, or close it with exit

Don’t do it now!

• We shall test RStudio first in the next session!

  UPPMAXHPC2N

  [bjornc@r483 ~]$ exit
  
  exit
  [screen is terminating]
  Connection to r483 closed.
  
  [bjornc@rackham2 ~]$
  

Keypoints

• Start an interactive session on a calculation node by a SLURM allocation

  • At HPC2N, PDC: salloc …

  • At UPPMAX, LUNARC, NSC: interactive …

• Follow the same procedure as usual by loading the R module and possible prerequisites.