Job arrays
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Parallelizing jobs with Job Arrays
There can be scenarios, eg in simulations or benchmarking, where a job script needs to run many times with only different parameter set each time. If done manually, keeping track of the parameter values and corresponding jobIds is cumbersome. Job Arrays are a convenient mechanism for submitting and managing such jobs.
A job array is created by adding the --array=<indexes> directive to an sbatch script (or in the command line), where <indexes> can be either a comma separated list of integers, or a range with optional step size, eg, 1-10:2. The minimum index value is 0, and the maximum is a Slurm configuration parameter (MaxArraySize - 1).
Within a job array, all jobs have the same SLURM_ARRAY_JOB_ID, but each job will have its own environment variable SLURM_ARRAY_TASK_ID that corresponds to the array index value. Additionally, all jobs in the array inherit the same compute resources requirements. In the following examples, arrays of size 2 are created, but with different indexes:
$ sbatch --array=1,4 jobscript.sbatch # Indexes specified as a list, and have values 1 and 4
Submitted batch job 8580151
$
$ squeue -u SomeNetID # Replace SomeNetId with your NetID
JOBID PARTITION NAME USER ST TIME NODES NODELIST(REASON)
8580151_1 general jobscrip SomeNetID R 0:01 1 grs4
8580151_4 general jobscrip SomeNetID R 0:01 1 awi18
$ sbatch --array=1-2 jobscript.sbatch # Range specified with default step size = 1. Index have values 1 and 2
Submitted batch job 8580149
$
$ squeue -u SomeNetID # Replace SomeNetId with your NetID
JOBID PARTITION NAME USER ST TIME NODES NODELIST(REASON)
8580149_1 general jobscrip SomeNetID R 0:21 1 grs4
8580149_2 general jobscrip SomeNetID R 0:21 1 awi18
Note
To limit the maximum number of simultaneously running jobs in an array use the% separator, eg--array=1-15%3 to run only 3 tasks at a time.JobId and environment variables
As shown in the previous section, Parallelizing jobs with job arrays, jobs within an array are assigned special slurm variables. These variables can be exploited for various computational objectives. Among these, SLURM_ARRAY_TASK_ID is the index of an individual task within the array, and SLURM_ARRAY_JOB_ID is the slurm jobId of the entire array job.
In the simplest case, you can use the ${SLURM_ARRAY_TASK_ID} directly in a script to assign parameter values. For example, to run a workflow across a set of images image_1.png … image_5.png, you can simply create an array using the sbatch directive --array=1-5, and then, within your sbatch script, use image_${SLURM_ARRAY_TASK_ID}.png to indicate the corresponding image.
In more complex scenarios, eg, when the parameters of interest are not mappable to indexes (of a job array), you can use a config file to map the parameters to the job array indexes. For example, let’s assume the following parameters:
$ cat jobarray.config
i Flower Color Origin
1 Rose Red Worldwide
2 Jasmine White Asia
3 Tulip Various Persia&Turkey
4 Orchid Various Worldwide
5 Lily Various Worldwide
Now, you can use these parameters inside a job script as follows:
$ cat jobarray.sbatch
#!/bin/bash
#SBATCH --job-name=JobArrayExample
#SBATCH --ntasks=1
#SBATCH --cpus-per-task=1
#SBATCH --array=1-5 # Arry with 5 tasks
#SBATCH --output=slurm-%A_%a.out # Set name of output log. %A is SLURM_ARRAY_JOB_ID and %a is SLURM_ARRAY_TASK_ID
#SBATCH --error=slurm-%A_%a.err # Set name of error log. %A is SLURM_ARRAY_JOB_ID and %a is SLURM_ARRAY_TASK_ID
config=jobarray.config # Path to config file
# Obtain parameters from config file:
flower=$(awk -v ArrayTaskID=$SLURM_ARRAY_TASK_ID '$1==ArrayTaskID {print $2}' $config)
color=$(awk -v ArrayTaskID=$SLURM_ARRAY_TASK_ID '$1==ArrayTaskID {print $3}' $config)
origin=$(awk -v ArrayTaskID=$SLURM_ARRAY_TASK_ID '$1==ArrayTaskID {print $4}' $config)
# Use the parameters, eg, print the index and parameter values to a file:
echo "Array task: ${SLURM_ARRAY_TASK_ID}, Flower: ${flower}, color: ${color}, origin: ${origin}" >> output.txt
$
$ sbatch jobArray.sbatch
Submitted batch job 8580317
$ squeue -u SomeNetID # Replace SomeNetId with your NetID
JOBID PARTITION NAME USER ST TIME NODES NODELIST(REASON)
8580317_[1-5] general JobArray SomeNetID PD 0:00 1 (Priority)
In this example, slurm created 5 jobs in a job array, each using the same settings (the name JobArrayExample, the general partition, short QoS, 00:01:00 time, 1 task with 1 CPU and 1G memory, and an output and error file with both array job Id and task id). Each task looks up certain parameter values from a config file leveraging its index via the awk command.
Note
The command:
flower=$(awk -v ArrayTaskID=$SLURM_ARRAY_TASK_ID '$1==ArrayTaskID {print $2}' $config)
assigns a value to the variable flower by reading a configuration file ($config), and printing the value in the second column ({print $2}) where the first column matches the value of the ArrayTaskID variable ($1==ArrayTaskID). The ArrayTaskID is an awk variable set to the value of the SLURM environment variable SLURM_ARRAY_TASK_ID.
For more on the awk utility, see this awk tutorial.
Jobs within a task array are run in parallel, and hence, there’s no guarantee about their order of execution. This is evident looking at the output file from this example:
$ cat output.txt
Array task: 2, Flower: Jasmine, color: White, origin: Asia
Array task: 3, Flower: Tulip, color: Various, origin: Persia&Turkey
Array task: 1, Flower: Rose, color: Red, origin: Worldwide
Array task: 5, Flower: Lily, color: Various, origin: Worldwide
Array task: 4, Flower: Orchid, color: Various, origin: Worldwide
Other slurm variables that are set inside a job array are shown in the following table, with values based on the preceding example:
| Slurm Environment Variable | Description | Value in example |
|---|---|---|
SLURM_ARRAY_JOB_ID | The first job ID of the array. | 8580317 |
SLURM_ARRAY_TASK_ID | The job array index value. | A value in range 1-5 |
SLURM_ARRAY_TASK_COUNT | The number of tasks in the job array. | 5 |
SLURM_ARRAY_TASK_MAX | The highest job array index value. | 5 |
SLURM_ARRAY_TASK_MIN | The lowest job array index value | 1 |
Slurm commands and job arrays
The squeue command reports all submitted jobs. By default, squeue reports all of the tasks associated with a job array in one line and uses a regular expression to indicate the SLURM_ARRAY_TASK_ID values. To explicitly print one job array element per line, use the --array or -r flag. The following examples highlight the difference, using the same jobarray.sbatch file from the JobId and environment variables section:
$ sbatch jobarray.sbatch
Submitted batch job 8593299
$
$ squeue -u SomeNetID # Replace SomeNetId with your NetID
JOBID PARTITION NAME USER ST TIME NODES NODELIST(REASON)
8593299_[1-5] general JobArray SomeNetID PD 0:00 1 (Priority)
$
$ squeue -r -u SomeNetID # Replace SomeNetId with your NetID
JOBID PARTITION NAME USER ST TIME NODES NODELIST(REASON)
8593299_1 general JobArray SomeNetID PD 0:00 1 (Priority)
8593299_2 general JobArray SomeNetID PD 0:00 1 (Priority)
8593299_3 general JobArray SomeNetID PD 0:00 1 (Priority)
8593299_4 general JobArray SomeNetID PD 0:00 1 (Priority)
8593299_5 general JobArray SomeNetID PD 0:00 1 (Priority)
scancel, on the other hand, can be used to cancel an entire job array by specifying its SLURM_ARRAY_JOB_ID. Alternatively, to cancel a specific task (or tasks), both its SLURM_ARRAY_JOB_ID and SLURM_ARRAY_TASK_ID must be specified, possibly with a regular expression, as shown in the following examples:
$ sbatch jobarray.sbatch
$ squeue -u SomeNetID # Replace SomeNetId with your NetID
JOBID PARTITION NAME USER ST TIME NODES NODELIST(REASON)
8593321_[1-5] general JobArray SomeNetID PD 0:00 1 (Priority)
$
$ scancel 8593321_4 # Cancel task with index 4 in the array
$ squeue -u SomeNetID # Replace SomeNetId with your NetID
JOBID PARTITION NAME USER ST TIME NODES NODELIST(REASON)
8593321_[1-3,5] general JobArray SomeNetID PD 0:00 1 (Priority)
$
$ scancel 8593321_[1-3] # Cancel tasks in index range 1-3 in the array
$ squeue -u SomeNetID # Replace SomeNetId with your NetID
JOBID PARTITION NAME USER ST TIME NODES NODELIST(REASON)
8593321_5 general JobArray SomeNetID PD 0:00 1 (Priority)
$
$ scancel 8593321 # Cancel all tasks in the array
$ squeue -u SomeNetID # Replace SomeNetId with your NetID
JOBID PARTITION NAME USER ST TIME NODES NODELIST(REASON)
$
Note
For more information on job arrays, refer to Slurm Job Array SupportTroubleshooting Common Issues
Please see the Frequently asked questions on Scheduler problems and Job resources
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