Serial Jobs

Videos

Videos of this topic may be available from one of our kickstart course playlists: 2023, 2022 Summer, 2022 February, 2021 Summer, 2021 February.

Abstract

• Batch scripts let you run work non-interactively, which is important for scaling. You create a batch script, which runs in the background. You come back later and see the results.

• Example batch script, submit with sbatch the_script.sh:

  #!/bin/bash -l
  #SBATCH --time=01:00:00
  #SBATCH --mem=4G
  
  # Run your code here
  python my_script.py
  

• See the quick reference for complete list of options.

This tutorial covers the basics of serial jobs. With what you are learning so far, you can control a small amount of power of the cluster.

Prerequisites

• Cluster general background knowledge

• Connecting to Triton

Why batch scripts?

You learned, in Slurm: the queuing system, how all Triton users must do their computation by submitting jobs to the Slurm batch system to ensure efficient resource sharing. This lets you run many things at once without having to watch each one separately - the true power of the cluster.

A batch script is simply a shell script (remember Using the cluster from a shell?), where you put your resource requests and job steps.

Your first job script

A job script is simply a shell script (Bash). And so the first line in the script should be the shebang directive (#!) followed by the full path to the executable binary of the shell’s interpreter, which is Bash in our case. What then follow are the resource requests, and then the job steps.

Let’s take a look at the following script

#!/bin/bash
#SBATCH --time=00:05:00
#SBATCH --mem=100M
#SBATCH --output=pi.out

echo "Hello $USER! You are on node $HOSTNAME.  The time is $(date)."

# For the next line to work, you need to be in the
# hpc-examples directory.
srun python3 slurm/pi.py 10000

Let’s name it run-pi.sh (create a file using your editor of choice, e.g. nano; write the script above and save it)

The symbol # is a comment in the bash script, and Slurm understands #SBATCH as parameters, determining the resource requests. Here, we have requested a time limit of 5 minutes, along with 100 MB of RAM.

Resource requests are followed by job steps, which are the actual tasks to be done. Each srun within the a slurm script is a job step, and appears as a separate row in your history - which is useful for monitoring.

Having written the script, you need to submit the job to Slum through the sbatch command. Since the command is python slurm/pi.py, you need to be in the hpc-examples directory from our sample project:

$ cd hpc-examples       # wherever you have hpc-examples
$ sbatch run-pi.sh
Submitted batch job 52428672

Warning

You must use sbatch, not bash to submit the job since it is Slurm that understands the SBATCH directives, not Bash.

When the job enters the queue successfully, the response that the job has been submitted is printed in your terminal, along with the jobid assigned to the job.

You can check the status of you jobs using slurm q/slurm queue (or squeue -u $USER):

$ slurm q
JOBID              PARTITION NAME                  TIME       START_TIME    STATE NODELIST(REASON)
52428672           debug     run-pi.sh             0:00              N/A  PENDING (None)

Once the job is completed successfully, the state changes to COMPLETED and the output is then saved to pi.out in the current directory. You can also wildcards like %u for your username and %j for the jobid in the output file name. See the documentation of sbatch for a full list of available wildcards.

Setting resource parameters

The resources were discussed in Slurm: the queuing system, and barely need to be mentioned again here - the point is they are the same. For example, you might use --mem=5G or --time=5:00:00. Always keep the reference page close for looking these up.

Checking your jobs

Once you submit your jobs, it goes into a queue. The two most useful commands to see the status of your jobs with are slurm q/slurm queue and slurm h/slurm history (or squeue -u $USER and sacct -u $USER).

More information is in the monitoring tutorial.

Cancelling a job

You can cancel jobs with scancel JOBID. To obtain job id, use the monitoring commands.

Full reference

The reference page contains it all, or expand it below.

Slurm quick ref

Command	Description
sbatch	submit a job to queue (see standard options below)
srun	Within a running job script/environment: Run code using the allocated resources (see options below)
srun	On frontend: submit to queue, wait until done, show output. (see options below)
sinteractive	Submit job, wait, provide shell on node for interactive playing (X forwarding works, default partition interactive). Exit shell when done. (see options below)
srun --pty bash	(advanced) Another way to run interactive jobs, no X forwarding but simpler. Exit shell when done.
scancel JOBID	Cancel a job in queue
salloc	(advanced) Allocate resources from frontend node. Use srun to run using those resources, exit to close shell when done (see options below)
scontrol	View/modify job and slurm configuration

Command	Option	Description
sbatch/srun/etc	-t, --time=HH:MM:SS	time limit
	-t, --time=DD-HH	time limit, days-hours
	-p, --partition=PARTITION	job partition. Usually leave off and things are auto-detected.
	--mem-per-cpu=N	request n MB of memory per core
	--mem=N	request n MB memory per node
	-c, --cpus-per-task=N	Allocate *n* CPU’s for each task. For multithreaded jobs. (compare ``–ntasks``: ``-c`` means the number of cores for each process started.)
	-N, --nodes=N-M	allocate minimum of n, maximum of m nodes.
	-n, --ntasks=N	allocate resources for and start n tasks (one task=one process started, it is up to you to make them communicate. However the main script runs only on first node, the sub-processes run with “srun” are run this many times.)
	-J, --job-name=NAME	short job name
	-o OUTPUTFILE	print output into file output
	-e ERRORFILE	print errors into file error
	--exclusive	allocate exclusive access to nodes. For large parallel jobs.
	--constraint=FEATURE	request feature (see slurm features for the current list of configured features, or Arch under the hardware list). Multiple with --constraint="hsw|skl".
	--array=0-5,7,10-15	Run job multiple times, use variable $SLURM_ARRAY_TASK_ID to adjust parameters.
	--gres=gpu	request a GPU, or --gres=gpu:n for multiple
	--gres=spindle	request nodes that have disks, spindle:n, for a certain number of RAID0 disks
	--mail-type=TYPE	notify of events: BEGIN, END, FAIL, ALL, REQUEUE (not on triton) or ALL. MUST BE used with --mail-user= only
	--mail-user=YOUR@EMAIL	whome to send the email
srun	-N N_NODES hostname	Print allocated nodes (from within script)

Exercises

The scripts you need for the following exercises can be found in our hpc-examples, which we discussed in Using the cluster from a shell. You can clone the repository by running git clone https://github.com/AaltoSciComp/hpc-examples.git. Doing this creates you a local copy of the repository in your current working directory. This repository will be used for most of the tutorial exercises.

Serial-1: Basic batch job

Submit a batch job that just runs hostname and pi.py. Remember to give pi.py some number of iterations as an argument.

1. Set time to 1 hour and 15 minutes, memory to 500MB.

2. Change the job’s name and output file.

3. Check the output. Does the printed hostname match the one given by slurm history/sacct -u $USER?

Solution

Output from hostname should match the node in slurm history. Sbatch first assigns you a node depending on your requested resources, and then runs all commands included in the script.

Serial-2: Submitting and cancelling a job

Create a batch script which does nothing (or some pointless operation for a while), for example sleep 300 (this shell command does nothing for 300 seconds). Check the queue to see when it starts running. Then, cancel the job. What output is produced?

Solution

You can check when your job starts running with slurm q. Then you can cancel it with scancel JOBID, where JOBID can be found from slurm q output. After cancelling the job, it should still produce an output file (named either slurm-JOBID.out or whatever you defined in the

#!/bin/bash

echo "We are waiting"
sleep 300
echo "We are done waiting"
srun python3 slurm/pi.py 1000000

You can check when your job starts running with slurm q. Then you can cancel it with scancel JOBID, where JOBID can be found from slurm q output. After cancelling the job, it should still produce an output file (named either slurm-JOBID.out or whatever you defined in the sbatch file.) The output file also says the job was cancelled.

Serial-3: Modifying Slurm script while its running

Modifying scripts while a job has been submitted is a bad practice.

Add sleep 120 into the Slurm script that runs pi.py. Submit the script and while it is running, open the Slurm script with an editor of your choice and add the following line near the end of the script.

echo 'Modified'

Use slurm q to check when the job finishes and check the output. What can you interpret from this?

Remove the created line after you have finished.

Solution

In this case we modified a script after we had submitted it. These modifications do not affect the script that is in the queue as that script has already been given to Slurm.

The Slurm script is locked in place when you submit a script. Modifications to the script do not affect the script that is being run.

You should always make certain that you do not modify the Slurm script being run or you cannot replicate your run.

Serial-4: Modify script while it is running

Modifying scripts while a job has been submitted is a bad practice.

Add sleep 180 into the Slurm script that runs pi.py. Submit the script and while it is running, open the pi.py with an editor of your choice and add the following line near the start of the script.

raise Exception()

Use slurm q to check when the job finishes and check the output. What can you interpret from this?

Remove the created line after you have finished. You can also use git checkout -- pi.py (remember to give a proper relative path, depending on your current working directory!)

Solution

In this case we modified the Python code before it had begun executing (we added a line that raised an error while the sleep 180 was being executed).

The code that the Slurm script executes will be determined when the script is running. It is not locked in place when you submit a script.

You should always make certain that you do not modify the code that the Slurm script will execute while it is queued or while its being run.

Otherwise you can get errors and you cannot replicate your run.

Serial-5: Checking output

You can look at the output of files as your program is running. Let’s demonstrate.

Create a slurm script that runs the following program. This is a shell script which, every 10 seconds (for 30 iterations), prints the date:

for i in $(seq 30); do
  date
  sleep 10
done

1. Submit the job to the queue.

2. Log out from Triton. Log back in and use slurm queue/squeue -u $USER to check the job status.

3. Use cat NAME_OF_OUTPUTFILE to check at the output periodically. You can use tail -f NAME_OF_OUTPUTFILE to view the progress in real time as new lines are added (Control-C to cancel)

4. Cancel the job once you’re finished.

Solution

#!/bin/bash
#SBATCH --output test-check-output.out

for i in $(seq 30); do
  date
  sleep 10
done

We note that a new line appears about every 10 seconds. Note that the delays might happen because of buffering, as the system tries to avoid doing too many small i/o operations:

$ tail -f test-check-output.out
Wed  7 Jun 10:49:58 EEST 2023
Wed  7 Jun 10:50:08 EEST 2023
(more every 10 seconds)

Serial-6: Constrain to a certain CPU architecture

Modify the script from exercise #1 to run on only one type of CPU using the --constraint option. Hint: check Triton quick reference

Solution

Simply add #SBATCH --constraint=X to your sbatch script, or give --constraint=X to srun as additional argument. For example, to run only on Haswell cpu’s you can add --constraint=hsw, or similarily for amd milan cpus --constraint=milan. This also works identically for gpus.

Serial-7: Why you use sbatch, not bash.

(Advanced) What happens if you submit a batch script with bash instead of sbatch? Does it appear to run? Does it use all the Slurm options?

Solution

It looks like it runs, but actually is only running on the login node! If you used srun python3 slurm/pi.py 10000, then it would request a Slurm allocation, but not use any of the #SBATCH parameters, so might not request the resources you need.:

$ bash run-pi.sh
Calculating Pi via 10000 stochastic trials
{"successes": 7815, "pi_estimate": 3.126, "iterations": 10000}

(advanced) Serial-8: Interpreters other than bash

(Advanced) Create a batch script that runs in another language using a different #! line. Does it run? What are some of the advantages and problems here?

Solution

Using other language to run your sbatch script is entirely possible. For example if you are more used to writing scripts on zsh compared to bash, you could use #!/bin/zsh. You could even use something completely different from a shell. For example using #!/usr/bin/env python3 would let you write python code directly in the sbatch script. This is mostly an interesting curiosity however and is not usually practical.

(advanced) Serial-9: Job environment variables.

Either make a sbatch script that runs the command env | sort, or use srun env | sort. The env utility prints all environment variables, and sort sorts it (and | connects the output of env to the input of sort.)

This will show all of the environment variables that are set in the job. Note the ones that start with SLURM_. Notice how they reflect the job parameters. You can use these in your jobs if needed (for example, a job that will adapt to the number of allocated CPUs).

What’s next?

There are various tools one can use to do job monitoring.