Introduction to Slurm¶
Triton is a large system that combines many different individual computer nodes. Hundreds of people are using Triton simultaneously. Thus resources (CPU time, memory, etc.) need to be shared among everyone.
This resource sharing is done by a software called a job scheduler or workload manager. Triton’s workload manager is slurm. Triton users submit jobs which are then scheduled and allocated resources by the workload manager.
There are two ways you can submit your jobs to slurm queue system:
either interactively using
srun or by submitting a script
This tutorial walks you through running your jobs interactively. And in the next tutorial we will go through the more common and advanced way of submitting jobs, batch scripts.
An analogy: the HPC Diner
You’re eating out at the HPC Diner. What happens when you arrive?
- A host greets you and takes your party size and estimated dining time.
- You are given a number and asked to wait a bit.
- The host looks at who is currently waiting.
- If you are two people, you might squeeze in soon.
- If you are a lot of people, the host will try to slowly free up enough tables to join to eat together.
- If you are a really large party, you might need an advance reservation (or have to wait a really long time).
- They want everyone to get a fair share of their food. Thus, people that have visited more often are asked to wait slightly longer for their table, as a balancing mechanic.
Your first interactive job¶
Let’s say you want to run the following command:
$ python3 -c 'import os; print("hi from", os.uname().nodename)'
You can submit this program to Triton using
srun. All input/output still goes to your terminal
(but note that graphical applications don’t work this way - see
$ srun --mem=100M --time=1:00:00 python3 -c 'import os; print("hi from", os.uname().nodename)' srun: job 52204499 queued and waiting for resources
Here, we are asking for 100 Megabytes of memory (
--mem=100M) for a
duration of an hour (
While your job - with jobid 52204499 - is waiting to be allocated resources, your shell
effectively become non-interactive.
You can open a new shell on triton and run the command
slurm q to see all the jobs
you have submitted to the queue:
$ slurm q JOBID PARTITION NAME TIME START_TIME STATE NODELIST(REASON) 52204499 short-ivb python3 0:00 N/A PENDING (None)
You can see information such as the state, which partition the requested node reside in, etc.
The fact that we had to open another shell can be impractical
if you need to run other jobs or just simply use the current shell.
Additionally, if your shell quits while waiting (your internet may disconnect),
the process cancels and you have to run the
srun command again.
Once resources are allocated to your job, you see the name of the machine in the Triton cluster your program ran on, output to your terminal:
srun: job 52204499 has been allocated resources hi from ivb17.int.triton.aalto.fi
Interactive jobs are useful for debugging purposes, to test your setup
and configurations before you put your tasks in a batch script for later execution.
Or if you need graphical applications - such as Matlab, RStudio, etc.
Additionally, if your task is small and not worth writing a batch script for,
interactive job is the way to go.
Keep in mind that you shouldn’t open 20 shells to run 20
srun jobs at once.
Please have a look at the next tutorial about serial jobs.
What if you want an actual shell to do things interactively?
Put more precisely, you want access to a node in the cluster
through an interactive bash shell that has all of the requested
For this, you just need srun’s
--pty option coupled with the shell
srun -p interactive --time=2:00:00 --mem=600 --pty bash
The command prompt will appear when the job starts. And you will have a bash shell runnnig on one of the computation nodes with at least 600 Megabytes of memory, for a duration of 2 hours, where you can run your programs in.
Remember to exit the shell when you are done! The shell will be running if you don’t and it will count towards your usage. This effectively means your priority will degrade in the future.
-p interactive requests a node in the interactive
partition which is dedicated to interactive usage (more on this
later). A partition is a group of nodes you can run on, with set
you can use
sinfo to see information such as the available partitions,
number of nodes in each, their time limits, etc.
Interactive shell with graphics¶
sinteractive is very similar to
srun, but more clever and thus
allows you to do X forwarding. It starts a screen session on the node,
then sshes to there and connects to the screen.
You can also ssh to this node again and connect to the
sinteractive --time=1:00:00 --mem=1000
Just like with
srun --pty bash, remember to exit the shell.
Since there is a separate screen session running, just closing the terminal isn’t enough.
Exit all shells in the screen session on the node (C-d or
exit), or cancel
If you are off-campus, you might want to use https://vdi.aalto.fi as a virtual desktop to connect to Triton to run graphical programs. Otherwise, programs may run very slowly.
Monitoring your usage¶
When your jobs enter the queue, you need to be able to get information on how much time, memory, etc. your jobs are using in order to know what requirements to ask for.
slurm history gives you information such as the actual memory used by your recent jobs, total CPU time, etc.
You will learn more about these commands later on.
As shown in a previous example, the command
slurm queue will tell you the currently running processes,
which is a good way to make sure you have stopped everything.
Generally, estimating the amount of time or memory you need comes down to
monitoring you slurm history and utilizing command-line tools such as
time on a few of your jobs and averaging. This is basically a trial and error process.
Setting resource parameters¶
Slurm comes with a multitude of parameters which you can specify to
ensure you will be allocated enough memory, CPU cores, time, etc.
You saw two of them in use in the above examples (
and you will learn more in the following tutorials.
Because you are sharing resource with other users, you should always estimate the amount of time, memory, etc. you need and then request them accordingly for efficiency reasons; the default memory and time limits are intentionally set low and may not be sufficient for your jobs to run/finish.
The general rule of thumb is to request the least possible, so that your stuff can run faster. That is because the less you request, the faster you are likely to be allocated resources. If you request something slightly less than a node size (note that we have different size nodes) or partition limit, you are more likely to fit into a spare spot.
For example, we have many nodes with 12 cores, and some with 20 or 24. If you request 24 cores, you have very limited options. However, you are more likely to be allocated a node if you request 10 cores. The same applies to memory: most common cutoffs are 48, 64, 128, 256GB. It’s best to use smaller values when submitting interactive jobs, and more for batch scripts.
The scripts you need for the following exercises can be in this git repository:
hpc-examples/slurm. You can clone the repository by running
git clone https://github.com/AaltoSciComp/hpc-examples.git. This repository
will be used for the rest of the tutorial exercises.
hpc-examples/slurm/memory-hog.pyuses up a lot of memory to do nothing. Let’s play with it. It’s run as follows:
python hpc-examples/slurm/memory-hog.py 50M, where the last argument is however much memory you want to eat. You can use
--helpto see the options of the program.
- Try running the program with
- Run the program with
- Increase the amount of memory the Python process tries to use (not the amount of memory Slurm allocates). How much memory can you use before the job fails?
- Look at the job history using
slurm history- can you see how much memory it actually used? - Note that Slurm only measures memory every 60 seconds or so. To make the program last longer, so that the memory used can be measured, give the
--sleepoption to the Python process, like this:
python hpc-examples/slurm/memory-hog.py 50M --sleep=60- keep it available.
- Try running the program with
hpc-examples/slurm/pi.pycalculates pi using a simple stochastic algorithm. The program takes one positional argument: the number of trials.
timeprogram allows you to time any program, e.g. you can
time python x.pyto print the amount of time it takes.
- Run the program, timing it with
time, a few times, increasing the number of trials, until it takes about 10 seconds:
time python hpc-examples/slurm/pi.py 500, then 5000, then 50000, and so on.
srunin front (
srun python ...). Use the
seff <jobid>command to see how much time the program took to run. (If you’d like to use the
timecommand, you can run
srun --mem=<m> --time=<t> time python hpc-examples/slurm/pi.py <iters>)
- Tell srun to use five CPUs (
-c 5). Does it go any faster?
- Use the
--threads=5option to the Python program to tell it to also use five threads.
... python .../pi.py --threads=5
- Look at the job history using
slurm history- can you see how much time each process used? What’s the relation between TotalCPUTime and WallTime?
- Run the program, timing it with
Check out some of these commands:
slurm job <jobid>on some running job - does anything look interesting?
scontrol show node csl1What is this? (
csl1is the name of a node on Triton - if you are not on Triton, look at the
sinfo -Ncommand and try one of those names).