Hyperthreading and core cost

Question

Hi all,

I have come across a technical point that could benefit from expert discussion.

Computational Fluid Dynamics (CFD, starccm+ by Siemens) software does not use hyperthreading. It actually runs about 40% slower with hyperthreading on. This is because the CFD algorithms are very efficient and use the CPUs almost 100%.

When I have a limit of say 500 vCPUs I have to turn off hyperthreading and run on only 250 vCPUs. I am of course paying for 500, so effectively I am being charged double for efficient code.

I had sort of accepted this, except it looks like the Siemens cloud GUI solution (new and uses AWS in the background - a direct compare of capability) charges only for the actual cores used. Early suggestions (very early) are that costs per core may be about half of the AWS cores (best computing ones c6i).

This is likely because of hyperthreading.

I have asked if it is possible to look at the costing of AWS compute cores with regards to hyperthreading – maybe there can be a CFD option on core cost?

Look forward to comments and thoughts!

Kind Regards

Stephen

Answer

When you disable hyperthreading you're no longer using vCPUs, you're using CPUs. If you think about it from the perspective that you're paying for CPUs then you're paying for the same CPUs whether or not you're using hyperthreading. In your example, if you're running 500 vCPUs you're paying for 250 CPUs with 2 threads per CPU. If you turn off hyperthreading, you're still paying for the same 250 CPUs with 1 thread per CPU

Answer

Hi Rodney,
Thanks for the tips and links. For the comment:

"You can configure hyperthreading off when launching, there is no additional cost or cost reduction"

The issue is when you switch off hyperthreading you lose half your vCPUs, but you have payed for all of them. I am sort of paying double because my particular code is very efficient and cany use hyperthreading.

It is a particular CFD way of looking at it:)

Kind Regards

Stephen

Answer

This is a known best practice for HPC: https://docs.aws.amazon.com/wellarchitected/latest/high-performance-computing-lens/compute.html

You can configure hyperthreading off when launching, there is no additional cost or cost reduction: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instance-optimize-cpu.html

Our open source Cluster Management Tool, [Parallel Cluster](https://docs.aws.amazon.com/parallelcluster/latest/ug/what-is-aws-parallelcluster.html) explicitly uses this option: https://docs.aws.amazon.com/parallelcluster/latest/ug/cluster-definition.html#disable-hyperthreading

Hyperthreading and core cost

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