Counting CPUs

Throughout this documentation we’ll refer to CPUs, but what exactly do we mean by a CPU? And if we then state that a compute node has, for example, 32 of them, it’s important to be clear on what that means too.

All modern computers contain a CPU - the central processing unit (or processor) - that ultimately allows it to do stuff. A compute cluster is no different, and the nodes within it also contain CPUs; the more of them we have, the more CPUs we have, and therefore the more tasks or jobs the cluster can process simultaneously, gaining incredible speed ups versus running them on a single computer.

But most CPUs have multiple processor cores, and these can also be multi-threaded to create additional virtual or logical CPUs (often called hyperthreading). So if we have a CPU with 16 cores, and each core can run 2 threads, then we can process 32 threads - and hence jobs - at a time (16 x 2). So, while not technically accurate, this multiplication of cores*threads is how we count how many CPUs we have. It’s what Slurm, our job scheduler uses (see Slurm - Overview), and it also mirrors what system tools like htop will show you when listing CPUs.

Finally, all of our compute nodes contain multiple CPUs (usually either 2 or 4) so a node’s final CPU count will be CPUs*cores*threads.


When specifically talking about hardware - such as on the System Overview page - then we’ll use the correct physical CPU count of a node, while also listing each CPU’s cores/thread count too.