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Request Compute Resources

Request Cores and Nodes

When running jobs with Slurm, you must be explicit about requesting CPU cores and nodes. See our page on monitoring usage for tips on verifying your jobs are using the resources you expect. The three options --nodes or -N, --ntasks or -n, and --cpus-per-task or -c can be a bit confusing at first but are necessary to understand for applications that use more than one CPU.

Tip

If your application references threads or cores but makes no mention of MPI, only use --cpus-per-task to request CPUs. You cannot request more cores than there are on a single compute node where your job runs.

Multi-thread, Multi-process, and MPI

The majority of applications in the world were written to use one or more cores on a single computer. Most can only use one core, and do not benefit from being given more cores. The best way to speed these applications up is to run many separate jobs at once, using Dead Simple Queue or job arrays.

If an application is able to use multiple cores, it usually achieves this by either spawning threads and sharing memory (multi-threaded) or starting entire new processes (multi-process). Some applications are written to use the Message Passing Interface (MPI) standard to run across many compute nodes. This allows such applications to scale computation in a way not limited by the number of cores on a single node. MPI translates what Slurm calls tasks to separate workers or processes. Because each of these processes can communicate across compute nodes, Slurm does not constrain them to the same node by default. Though tasks can be distributed across nodes, Slurm will not split the CPUs allocated to individual tasks. For this reason a single task that has multiple CPUs allocated will always be on a single node. In some cases using --ntasks=4 (or -n 4) and --cpus-per-task=4 (or -c 4) achieves the same job allocation by luck, but you should only use --cpus-per-task when using non-MPI applications to guarantee that the CPUs you expect your program to use are all accessable.

Some MPI programs are also multi-threaded, so each process can use multiple CPUs. Only these applications can use --ntasks and --cpus-per-task to run faster.

MPI Applications

For more control over how Slurm lays out your job, you can add the --nodes and --ntasks-per-node flags. --nodes specifies how many nodes to allocate to your job. Slurm will allocate your requested number of cores to a minimal number of nodes on the cluster, so it is likely if you request a small number of tasks that they will all be allocated on the same node. However, to ensure they are on the same node, set --nodes=1 (obviously this is contingent on the number of CPUs on your cluster's nodes and requesting too many may result in a job that will never run). Conversely, if you would like to ensure a specific layout, such as one task per node for memory, I/O or other reasons, you can also set --ntasks-per-node=1. Note that the following must be true:

ntasks-per-node * nodes >= ntasks

Hybrid (MPI+OpenMP) Applications

For the most predictable performance for hybrid applications, you will need to use all three of the --ntasks, --cpus-per-task, and --nodes flags, where --ntasks equals the number of MPI tasks, --cpus-per-task equals the number of OMP_NUM_THREADS and --nodes is the number of nodes required to fit --ntasks * --cpus-per-task.

Request Memory (RAM)

Slurm strictly enforces the memory your job can use. If you request 5GiB of memory for your job and the total used by all processes you launch hits that limit, some of your processes may die and you will get errors. Make sure you either request the right amount of memory per core on each node in your job with --mem-per-cpu or memory per node in your job with --mem. You can request more memory than you think you might need for an example job, then make note of its actual usage to better tune future requests for similar jobs.

Request GPUs

Some of our clusters have nodes that contain GPU co-processors. Please refer to the individual cluster pages regarding node configurations that include GPUs. The Slurm options --mem and --mem-per-cpu do not request memory on GPUs, sometimes called vRAM. Memory accessible on GPUs is limited by their model, and is also listed on each cluster page.

In order for your job to be able to access gpus, you must submit your job to a partition that contains nodes with GPUs and request them as a Slurm "Generic Resource" or gres. You specify the gres configuration per-node for a job with the --gres flag and a number of GPUs. If you are agnostic about the kind of GPU your job gets, --gres=gpu:1 will allocate one of any kind of GPU per node. To specifically request, for example, a P100 for each node in your job you would use the flag --gres=gpu:p100:1. Some applications require double-precision capable GPUs. If yours does, see the next section for using "features" to request any node with compatible GPUs.

Tip

As with requesting multiple cores or multiple nodes, we strongly recommend that you test your jobs using the gpu_devel partition to make sure they can well utilize multiple GPUs before requesting them; allocating more GPUs does not speed up code that can only use one at a time.

For more documentation on using GPUs on our clusters, please see GPUs and CUDA.

Features and Constraints

You may want to run programs that require specific hardware. To ensure your job runs on specific types of nodes, use the --constraint flag.

You can use the processor codename (e.g. haswell) or processor type (e.g. E5-2660_v3) to limit your job to specific node types. You can also specify an instruction set (e.g. avx2) to require that no matter what CPU your job runs on, it must understand at least these instructions. See the individual cluster pages for the exact tags for the different node types.

# run on a node with a haswell codenamed CPU (e.g. a E5-2660 v3)
sbatch --constraint=haswell submit.sh

# only run on nodes with E5-2660 v4 CPUs
sbatch --constraint=E5-2660_v4 submit.sh

# run on any node that understands avx2 instructions
# Your job may also launch on an avx512 node
sbatch --constraint=avx2 submit.sh

We also have keyword features to help you constrain your jobs to certain categories of nodes.

  • oldest: the oldest generation of node on the cluster. Use this constraint when compiling code if you wish to ensure it can run on any standard node on the cluster.
  • nogpu: nodes without GPUs.
  • standard: nodes without GPUs or extra memory. Useful for protecting special nodes in a private partition for jobs that can use the extra capabilities.
  • singleprecision: nodes with single-precision only capable GPUs (e.g. GTX 1080s, RTX 2080s).
  • doubleprecision: nodes with double-precision capable GPUs (e.g. K80s, P100s and V100s).

Tip

Use the command scontrol show node <hostname>, replacing <hostname> with the node's name you're interested in, to see more information about the node including its features.


Last update: July 24, 2020