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GNU Parallel workflow for many small, independent runs

The goal is to have a workflow that is

  1. simple to understand,
  2. fits well into the batch queue system, and
  3. does not stress the parallel file system.

There is a plethora of workflow tools. Whatever tool one chooses, it will unlikely match the particular workflow and underlying computing platform out of the box. Some amount of programming is needed in most cases. A very much related discussion is in Array jobs chapter of

Strengths of GNU Parallel

  • Does not require a database or persistent manager process
  • Easily scales to a large number of tasks/nodes
  • Efficient use of scheduler resources

Disadvantages of GNU Parallel

  • User is required to do careful organization of input and output files
  • Scaling up requires consideration of system I/O performance
  • Modest familiarity with bash scripting recommended
  • Only serial subtasks
  • No support for dependencies or error recovery

System limits outline

The maximum number of jobs that each user can submit per month should be kept below one thousand. Too many batch jobs will generate excess log data and slows down the job scheduler. Array jobs are basically just a shorthand, so a single array job of 100 members counts the same as 100 individual jobs from the batch queue system's perspective.

The job maximum runtime is limited by the queue parameters. The minimum time is not limited, but if the job is too short, it is just generating proportionally large scheduling overhead in the batch system.


A good target is to write batch jobs that finish somewhere between two hours and two days.

Parallel file systems work poorly when a single client (application program) tries to perform too many file operations. Such cases can be e.g. applications installed with the Conda package manager directly on the shared file system. One miniconda environment is easily over 20000 files and Anaconda distribution is much worse. Many of these files need to be opened every time a Conda application is launched. When running many, relatively short jobs, avoid running applications installed with Conda. However, if your application requires a complex environment, use applications packed into Singularity containers, which are single files from the perspective of the file system. To easily containerize a Conda environment, see the Tykky container wrapper tool

"Too many files" issues are also often encountered with workflows consisting of thousands of small runs. As a general guide, keep the number of files in a single directory well below one thousand, and organize your data into multiple directories. Also, use command csc-workspaces to monitor that the total number of files in your projects stays well below the limits. If most of the files are temporary, or there simply is too many of them, using the fast local SSD disks in the I/O nodes can solve the problem. You can pack small files into a bigger archive file with the tar command. Most importantly, if there are output files that you do not need, find out how to turn off writing those in the first place.

Please contact if your workflow needs help to fit into the limits given above.

An example case, 80000 independent runs

In general, there are three pieces of input that are needed for designing the workflow:

  1. How many runs there will be in total?
  2. How long does a single run take?
  3. How many files will be created?

The first two determine how the runs are grouped into batch jobs, and the last one determines the directory hierarchy.

Let's consider an example where we have 80000 independent, non-parallel single-core runs, each taking from 0 to 30 minutes, with a 15-minute average. In the worst case, all the runs in a batch job take the maximum amount of time, 30 minutes. We can see that a single 40-hour batch job should be enough for at least 80 runs with a single core, and 3200 runs with all 40 cores in a full compute node. Thus, all 80000 runs should fit in 25 40-hour batch jobs, each reserving one full compute node.

Let's say our application is a real disk-hog, and in addition to one input file and one output file that we wish to keep, it also creates 100 temporary files in the current directory. We can have at maximum about 400 input and output files in a single directory, and use the fast local disk in the I/O nodes for the temporary files. For 80000 runs we thus get 200 directories, each with 400 runs.


Additional consideration needs to be taken if the single runs are parallel, or there are dependencies between them, but that's another story.

Let's look at the job script for our example case:

#SBATCH --partition=small
#SBATCH --account=<project>
#SBATCH --nodes=1
#SBATCH --ntasks=1
#SBATCH --cpus-per-task=40
#SBATCH --time=40:00:00
#SBATCH --mem=160G
#SBATCH --gres=nvme:3600
#SBATCH --array=0-24

module load parallel

cd /scratch/${SLURM_JOB_ACCOUNT}/many

(( from_dir_index = SLURM_ARRAY_TASK_ID * 8 + 1 ))
(( to_dir_index = SLURM_ARRAY_TASK_ID * 8 + 8 ))

job_dirs=$(printf "%dir-%03d " $(seq $from_dir_index $to_dir_index))

find $job_dirs -name 'input-*' | \
    parallel -j $SLURM_CPUS_PER_TASK bash {}

The batch job reserves a whole node for 40 hours. One task starts in the node, which has access to all 40 CPU cores in the node. Since we reserve all the cores, we can reserve all the memory and all the local disk all the same, no need to be stringy here. The last line, #SBATCH --array=0,24, tells the batch system to execute 25 copies of this job, each job identified by a unique number in environment variable SLURM_ARRAY_TASK_ID. Depending on the queue situation, many of these jobs can run in parallel.

Next we load a module providing GNU parallel. We use this tool within the node to "schedule" all 3200 runs in a job, so that at any point in time all 40 cores are busy, but not overloaded.

Next lines calculate which directories belong to the current array job, using the SLURM_ARRAY_TASK_ID environment varible.

The main "loop" of the script is implemented with GNU parallel command parallel. With the option -j $SLURM_CPUS_PER_TASK we tell GNU parallel to keep running 40 commands (applications) in parallel. Since we need to copy files into and out from the local SSD for each run, we wrap our application in a small shell script,, which takes the input file name as an argument. The names of the input files are fed to GNU parallel through a pipe, and GNU parallel keeps on running the bash <input file> command as long as there are arguments in the pipe.

Separating the wrapper script from the batch job script makes it possible to develop and test each other separately. In general, use small test sets when developing the workflow, and do not expect to get it perfect on the first try. You can study and test a small version of the example case with

export SBATCH_ACCOUNT=<your project>
wget -c -O - | tar xz
cd many
tree /scratch/${SBATCH_ACCOUNT}/many


Running multiple separate jobs inside a larger allocation may result in idle resources. Please make sure that such a job has a lot of quick jobs to run, so that the last running job is not keeping the complete allocation alive for long. Thus, the length of a subjob should be much less than the duration of the allocation, and the number of subjobs much larger than the cores requested in one task.

You can use seff to learn how long past jobs have been.

Last update: June 27, 2022