Rule of Thumb for Selecting cores
 

Hi All

I am doing a Flow simulation using CFX and LES .
Number of elements for my mesh is around 18E6 (18million) so I wanted to know if anyone have a suggestion on selecting the number of cores to conduct this simulation, as the available licence on my cluster is very limited. So is there any kind of rule of thumb or from your experience.

I have seen very good scaling with CFX, even when I used a lot of cores for a very small problem. I've used 10 cores for a few thousand cells, and it was much faster than serial. With 18Mcells, I doubt you could use too many cores.

Erik is right, CFX scales well to zillions of partitions. The limitation is likely to be:
1) The amount of parallel licenses you have
2) Large parallel simulations require careful cluster design including high speed interconnects and many subtle motherboard issues.

Dear Erik and Glenn

So you mean to say that I can use as much as I want depending on the number of licenses I get from the University ? So utilizing the maximum licenses available which means using the maximum possible cores right !!

If you have lots of parallel licenses then you are probably going to be restricted by the quality of the compute cluster you have access to. I would test a sweep of partition counts on a benchmark simulation to find the best speed for your cluster. Try 1,2,4,8,16,32, etc until you run out of licenses or the performance starts dropping off.

In other words, do not assume that adding more partitions will always run faster.

If I can make a comment here, with R17 and R18 ANSYS has made huge steps forwards to improve the scalability, so if you use most recent releases with HPC your simulation will be faster. I think this depends from the architecture of the machine where you are running and from your type of problem, but at the top of my head there should be an optimal number of cells for process, which it should be something around 15k per core (tests done on AWS).

Hi All

Thanks a lot for your valuable comments !!