HPC cluster: select the number of CPUs and threads in SLURM sbatch

Question

The terminology used in the sbatch man page might be a bit confusing. Thus, I want to be sure I am getting the options set right. Suppose I have a task to run on a single node with N threads. Am I correct to assume that I would use --nodes=1 and --ntasks=N?

I am used to thinking about using, for example, pthreads to create N threads within a single process. Is the result of that what they refer to as "cores" or "cpus per task"? CPUs and threads are not the same things in my mind.

Answer 1

Depending on the parallelism you are using: distributed or shared memory

--ntasks=# : Number of "tasks" (use with distributed parallelism).

--ntasks-per-node=# : Number of "tasks" per node (use with distributed parallelism).

--cpus-per-task=# : Number of CPUs allocated to each task (use with shared memory parallelism).

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From this question: if every node has 24 cores, is there any difference between these commands?

sbatch --ntasks 24 [...]
sbatch --ntasks 1 --cpus-per-task 24 [...]

Answer: (by Matthew Mjelde)

Yes there is a difference between those two submissions. You are correct that usually ntasks is for mpi and cpus-per-task is for multithreading, but let’s look at your commands:

For your first example, the sbatch --ntasks 24 […] will allocate a job with 24 tasks. These tasks in this case are only 1 CPUs, but may be split across multiple nodes. So you get a total of 24 CPUs across multiple nodes.

For your second example, the sbatch --ntasks 1 --cpus-per-task 24 [...] will allocate a job with 1 task and 24 CPUs for that task. Thus you will get a total of 24 CPUs on a single node.

In other words, a task cannot be split across multiple nodes. Therefore, using --cpus-per-task will ensure it gets allocated to the same node, while using --ntasks can and may allocate it to multiple nodes.

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Another good Q&A from CÉCI's support website: Suppose you need 16 cores. Here are some use cases:

• you use mpi and do not care about where those cores are distributed: --ntasks=16
• you want to launch 16 independent processes (no communication): --ntasks=16
• you want those cores to spread across distinct nodes: --ntasks=16 and --ntasks-per-node=1 or --ntasks=16 and --nodes=16
• you want those cores to spread across distinct nodes and no interference from other jobs: --ntasks=16 --nodes=16 --exclusive
• you want 16 processes to spread across 8 nodes to have two processes per node: --ntasks=16 --ntasks-per-node=2
• you want 16 processes to stay on the same node: --ntasks=16 --ntasks-per-node=16
• you want one process that can use 16 cores for multithreading: --ntasks=1 --cpus-per-task=16
• you want 4 processes that can use 4 cores each for multithreading: --ntasks=4 --cpus-per-task=4