Practical Examples of `sbatch` Usage With the `--array` Flag and Dynamic Indices¶

Do you find yourself tediously creating many sbatch job scripts for the same type of data set? Or do you modify the same job script? Have you ever experienced frustrating errors or typos while doing this? Would you like to save time by using one script for many similar tasks? If so then please read on for how to use sbatch jobs with the --array flag.

The --array flag transforms an sbatch job script for a single task into a collection of tasks that are all scheduled simultaneously. For programmers, the --array flag turns a job script into a parallel loop, or a loop where each iteration is run independently and in no particular order. Naturally, this means that your tasks must be independent and similar. The most common use case is running the same software with different inputs or on different data sets. To get the most out of this page, you'll want to be familiar with Submitting Jobs.

We will show how to create and use sbatch jobs with the --array flag, or sbatch --array jobs. We will use a simplified, practical example that parallels the process of a computational scientific experiment. The practical task we will solve is simplified to enhance focus on the structure of the problem, rather than the content of the problem. The structure of the problem is what makes sbatch --array jobs more or less suitable for a particular need. Specifically, whether there are many independent subtasks that all have the same structure, with similar or the same parameters.

For other examples of using Slurm and its other tools, please see Submitting Jobs and Managing Jobs.

The Task¶

Your task is to determine the statistical properties of dice rolls. To measure these properties, you'll need to simulate rolling the dice many times to obtain a lot of data. Because dice rolls are independent, the task of simulating many dice rolls can be subdivided into many independent subtasks, all with the same parameters. While we could simulate dice rolls one at a time, in sequence, we could instead use an sbatch --array job to simulate dice rolls in parallel.

Suppose you've already written some code called simulate that performs these simulations. The code transforms three integer inputs into an output sequence of positive integers. Based on the usual rules for tabletop role-playing game (TTRPG) dice rolling. The block below shows a sample input requesting ten rolls that are the sum of two six-sided die rolls plus one. In TTRPG notation we would write this as 2d6+1 rolled ten times. The output, in the case below, is a sequence of ten integers. Each element of the sequence falls in the possible range of the 2d6+1 die roll, which is [3,13].

Inputs, Outputs, How to Call

# a tuplet of integers like the following...

# name       | number of rolls | quantity | sides    | modifier
# properties | positive        | positive | positive | any integer
               10              , 2        , 6        , 1

# is transformed to sequence of positive integers
8,9,4,4,3,8,6,6,13,8

# based on a call like
simulate $SEED $INPUT_FILE $OUTPUT_FILE

The input data takes the form of a simple text file (specifically a comma-separated value or CSV file) with four integers as described above. The upstream source of this data puts each simulation in a separate file in a separate folder. This may feel contrived for a simple example, but real experimental data is often structured with one treatment per folder, so we are using it in this example. We do not necessarily know in advance how many data folders will be present when we run the code. Sure, we could count manually and then hardcode that value, but we are trying to automate our process to avoid introducing errors and to save time in the future.

All of the above constraints must fit within the framework provided by Slurm and the sbatch --array job style. Now that we have a complete list of requirements, we are ready to start forming a solution.

Building a Solution¶

We are going to need three components to effectively use sbatch array jobs given the requirements and constraints of the task.

The simulate code that transforms inputs to outputs. We are assuming this exists and will not discuss the implementation in detail here.
A job shell script file that instructs Slurm how to allocate each array job task.
A main shell script to automate the --array bounds and call the job script.

Job Script¶

The job shell script file will be very much like a typical sbatch job script. The preamble will contain the details of Slurm scheduler instructions in the form of flags. After the preamble comes the payload, where we instruct the shell what commands need to be run within each task.

Preamble¶

The preamble of an sbatch job script instructs Slurm how to queue the job and what resources to allocate. Our preamble is relatively straightforward and should look familiar if you've written job scripts before. For more detailed information on what the flags mean please see Slurm Flags.

job script preamble

#! /bin/bash

#SBATCH --job-name=simulate_dice_rolls
#SBATCH --output=%x-%A-%a.log
#SBATCH --error=%x-%A-%a.log

#SBATCH --partition=express
#SBATCH --time=00:02:00

#SBATCH --ntasks=1
#SBATCH --cpus-per-task=1
#SBATCH --mem=1G

If we had a fixed number of datasets, say ten, we could add the line #SBATCH --array=0-9 to the preamble and not require a main.sh file. In that case we would just use sbatch job.sh and be done. However, for this example, we require a dynamic upper bound on the --array flag, which isn't possible in the preamble of a script file.

Payload¶

For our example, the payload consists of multiple parts. We will need to extract the file from just one of the folders, ensuring that each folder is used exactly once. Dependencies must be loaded, and then the software must be run. The most interesting part will be extracting the files.

File Extraction¶

The general idea for file extraction, and generally use of the $SLURM_ARRAY_TASK_ID variable, is for each task to pull one unique element out of a list of possibilities. The list of possibilities can take many forms. In our case, we can use a shell array of strings constructed from a glob pattern.

Other workflows may extract lines from a manifest file, or even have a computation that transforms the job array index provided by $SLURM_ARRAY_TASK_ID into some other number or collection of numbers.

Below is the file extraction portion of job.sh, along with descriptions of each line.

Job Script Payload: File Extraction

shopt -s nullglob

input_files=(../inputs/**/dice.csv)
INPUT_FILE=${input_files[$SLURM_ARRAY_TASK_ID]}

OUTPUT_FILE=${INPUT_FILE/inputs/outputs}
OUTPUT_FILE=${OUTPUT_FILE/dice/rolls}

The line shopt -s nullglob instructs bash to allow empty arrays created with glob patterns.
The line input_files=(../inputs/**/dice.csv) uses the glob pattern ../inputs/**/dice.csv to create a shell array of paths to files in our dataset. Note that this shell array is distinct from the job --array, though they should have the same total number of elements, one for each task.

The glob pattern can be read as:
- ../: move up one folder then...
- inputs/: look in the folder inputs for...
- **/: any number of nested subfolders containing...
- dice.csv: the file dice.csv.
The parentheses around the glob pattern instructs bash to create a shell array of strings from the results of the glob pattern. If there are multiple folders, each with one dice.csv file, then the shell array will have one entry for each of them.

The shell array is then stored in the variable input_files.
The line INPUT_FILE=${input_files[$SLURM_ARRAY_TASK_ID]} extracts exactly one entry from the input_files shell array and puts it in the variable INPUT_FILE. The value of $SLURM_ARRAY_TASK_ID is set by the Slurm schedule when each task starts. If there are ten tasks, as with --array=0-9, then each task has $SLURM_ARRAY_TASK_ID set to a unique value from [0,9]. We index the shell array input_files using $SLURM_ARRAY_TASK_ID to get a single entry from the shell array. Putting it all together, each task will pull out exactly one file from the set of data folders.

Tip

Curly braces with a leading dollar sign like ${...} are used for evaluating some modification to a variable.
The line OUTPUT_FILE=${INPUT_FILE/inputs/outputs} transforms the variable INPUT_FILE so that any instance of the word inputs is replaced with the word outputs. The result is assigned to the variable OUTPUT_FILE. The net result is the output file currently will be ../outputs/folder/dice.csv.
We're not quite done setting up the output path. The line OUTPUT_FILE=${OUTPUT_FILE/dics/rolls} further replaces dice with rolls. The net result is ../outputs/folder/rolls.csv.

Lines 4 and 5 result in a parallel folder structure between inputs and outputs. It is possible to use other structures such as all outputs in the same folder, or outputs in the same folders as inputs, but we won't go into detail here on how to achieve those.

Running the Software¶

Running the software requires setting up the random number generator seed (for repeatability), loading module dependencies, creating the output directory, and finally running the simulation.

Job Script Payload: Running the Software

SEED=314159

module load ...

OUTPUT_DIRECTORY=$(dirname "$OUTPUT_FILE")
mkdir -p $OUTPUT_DIRECTORY

simulate $SEED $INPUT_FILE $OUTPUT_FILE

The line SEED=314159 sets the variable SEED with a static number for repeatability. It is set in a variable, rather than directly on the final line, so future readers of the code will understand the purpose of the number.
The line module load ... is where any necessary module load or conda activate executions would go to prepare dependencies.
The line OUTPUT_DIRECTORY=$(dirname "$OUTPUT_FILE") extracts the directory part of the output file path and assigns it to the variable OUTPUT_DIRECTORY.

Tip

Single parentheses with a leading dollar sign like $(...) are used for capturing the string output of a command in a variable.
The line mkdir -p $OUTPUT_DIRECTORY creates the directory at the path stored in the variable $OUTPUT_DIRECTORY. The flag -p means the mkdir command will not produce an error if the directory already exists.

Tip

If you know your software will create any necessary output directories, then this and the previous line are not necessary.
The line simulate $SEED $INPUT_FILE $OUTPUT_FILE runs the simulation.

Main Script¶

The main shell script will determine the upper bound $N of the --array flag, and then call sbatch --array=1-$N job.sh. It will be up to job.sh to determine how to use $SLURM_ARRAY_TASK_ID. Before we go too much further, it may be helpful to think of sbatch --array=1-$N job.sh as creating an indexed loop, from 1 to $N, and running job.sh on each of those index values. The important point is that the loop indices are run in parallel, so whatever happens in each call to job.sh must be independent. The main.sh file is the same for all languages and is shown in the code block below. The comments describe what each segment of code is doing.

main.sh

#! /bin/bash

shopt -s nullglob

input_files=(../inputs/**/dice.csv)

FILE_COUNT=${#input_files[@]}
FILE_COUNT=$(( $FILE_COUNT - 1 ))

sbatch --array=0-$FILE_COUNT job.sh

The line #! /bin/bash instructs the operating system what interpreter to use if called without an explicit interpreter, like ./main.sh. It is best practice to have this line for scripts running in bash. Other lines are possible for other interpreters.
The line shopt -s nullglob instructs bash to allow empty arrays created with glob patterns.
The line input_files=(../inputs/**/dice.csv) uses the glob pattern ../inputs/**/dice.csv to create a shell array of paths to files in our dataset. The details of this are discussed above in File Extraction.
The line FILE_COUNT=${#input_files[@]} gets all entries from the input_files array using the special index @ (for all elements), then counting them with the prefix symbol #.

Tip

Curly braces with a leading dollar sign like ${...} are used for evaluating string modifications to a variable.
The line FILE_COUNT=$(( FILE_COUNT - 1 )) subtracts one from the file count. We must do this because array variables, as used in job.sh, start counting at zero (they are zero-indexed). So instead of counting from 1 to 10, we would count from 0 to 9.

Tip

Double parentheses with a leading dollar sign like $((...)) are used for evaluating integer arithmetic to a variable.
The line sbatch --array=0-$FILE_COUNT job.sh puts the array tasks in the Slurm queue using the job.sh script. The number of tasks runs from 0 to $FILE_COUNT as compute above.

To use the script, enter the command bash main.sh at the terminal.

Putting it All Together¶

We needed three parts to make the sbatch --array job work for our task. Each of these parts has been described above in some detail.

simulate program to run a simulation.
job.sh to instruct the Slurm scheduler what to do in each parallel task.
main.sh to run everything.

Executing bash main.sh at the terminal will first compute the number of array tasks, then call sbatch --array with that number of tasks on job.sh. The scheduler will then schedule that many jobs to be run. Each job will have a unique task ID, which will be used to access unique input files and write to unique output files. All of them will be run in parallel.

The reason sbatch --array could be used on our dice rolling statistics task is because dice rolls are independent. It doesn't matter when I roll the dice or whether I roll them together or sequentially, the results will be statistically the same.

Any task that can be broken into independent subtasks with similar input parameters can be used with sbatch --array in this way. Please feel free to use the scripts provided as a template for your own sbatch --array jobs, modifying them as appropriate.

The Example Scripts¶

For reference, here are the full scripts.

job.sh

#! /bin/bash

#SBATCH --job-name=simulate_dice_rolls
#SBATCH --output=%x-%A-%a.log
#SBATCH --error=%x-%A-%a.log

#SBATCH --partition=express
#SBATCH --time=00:02:00

#SBATCH --ntasks=1
#SBATCH --cpus-per-task=1
#SBATCH --mem=1G

shopt -s nullglob

input_files=(../inputs/**/dice.csv)
INPUT_FILE=${input_files[$SLURM_ARRAY_TASK_ID]}

OUTPUT_FILE=${INPUT_FILE/inputs/outputs}
OUTPUT_FILE=${OUTPUT_FILE/dice/rolls}

SEED=314159

module load ...

OUTPUT_DIRECTORY=$(dirname "$OUTPUT_FILE")
mkdir -p $OUTPUT_DIRECTORY

simulate $SEED $INPUT_FILE $OUTPUT_FILE