Hyperthreading

Hyperthreading is enabled on the NeSI machines, so for each physical CPU core, there are two logical CPUs. This increases the efficiency of some multithreaded jobs, but the fact that Slurm is counting in logical CPUs makes aspects of running non-hyperthreaded jobs confusing, even when hyperthreading is turned off in the job with --hint=nomultithread.

  • Non-hyperthreaded jobs which use  --mem-per-cpu requests should halve their memory requests as those are based on memory per logical CPU, not per the number of threads or tasks.  For non-MPI jobs, or for MPI jobs that request the same number of tasks on every node, we recommend to specify --mem (i.e. memory per node) instead. See How to request memory (RAM) for more information.
  • Non-MPI jobs which specify --cpus-per-task and use srun should also set --ntasks=1, otherwise the program will be run twice in parallel, halving the efficiency of the job.

The precise rules about when hyperthreading applies are as follows:

  Mahuika Māui
Jobs Never share physical cores
MPI tasks within the same job Never share physical cores Share physical cores by default. You can override this behaviour by using --hint=nomultithread in your job submission script.
Threads within the same task Share physical cores by default. You can override this behaviour by using
--hint=nomultithread in your job submission script.

How many logical CPUs will my job use or be charged for?

The possible job configurations and their results are shown in the following table. We have also included some recommendations to help you make the best choices, depending on the needs of your workflow.

Job configuration Mahuika Māui
  • Only one task
  • --cpus-per-task is not used
The job gets, and is charged for, two logical CPUs. --hint=nomultithread is irrelevant.

The job gets one logical CPU, but is charged for 80.
--hint=nomultithread is irrelevant.

This configuration is extremely uneconomical on Māui. Consider using Mahuika or the Māui ancillary nodes instead.

  • Only one task
  • --cpus-per-task=x
  • --hint=nomultithread is not used

The job gets, and is charged for, x logical CPUs, rounded up to the nearest even number.

Set x to an even number if possible.

The job gets x logical CPUs, but is charged for 80.

Set x to 80 if possible.

  • Only one task
  • --cpus-per-task=x
  • --hint=nomultithread is used
The job gets, and is charged for, 2x logical CPUs.

The job gets 2x logical CPUs, but is charged for 80.

Set x to 40 if possible.

  • More than one task on one or more nodes
  • --cpus-per-task is not used
  • --hint=nomultithread is not used

Each task gets two logical CPUs. The job is charged for two logical CPUs per task. --hint=nomultithread is irrelevant.

 

Each task gets one logical CPU. The job is charged for 80 logical CPUs per allocated node.

If possible, set the number of tasks per node to 80.

  • More than one task on one or more nodes
  • --cpus-per-task is not used
  • --hint=nomultithread is used

Each task gets two logical CPUs. The job is charged for 80 logical CPUs per allocated node.

If possible, set the number of tasks per node to 40. 

  • More than one task on one or more nodes
  • --cpus-per-task=x
  • --hint=nomultithread is not used

Each task gets x logical CPUs, rounded up to the nearest even number. The job is charged for that number of logical CPUs per task.

Set x to an even number if possible.

Each task gets x logical CPUs. The job is charged for 80 logical CPUs per allocated node.

If possible, set x and the number of tasks per node such that x × (tasks per node) = 80.

  • More than one task on one or more nodes
  • --cpus-per-task=x
  • --hint=nomultithread is used
Each task gets 2x logical CPUs. The job is charged for 2x logical CPUs per task.

Each task gets 2x logical CPUs. The job is charged for 80 logical CPUs per allocated node.

If possible, set x and the number of tasks per node such that x × (tasks per node) = 40.

 

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