The number of processors shall I use
 

Hi,

I run my .def by parallel processors of the company. May I ask how to decide the number of processors shall I use to speed up my simulation?

Thanks!

Hi Anna,

when using CFX the recommendations I got from the CFX support for the number of CPU's are:
1 CPU for approximately 250.000 Cells. So if your mesh consists of e.g. 1.000.000 Cells then you would use (at least) 4 CPU's.
This is what Ansys told me about optimum repartition and scaling. Although I usually use 8 CPU's for 1 Mio. Cells.

It also depends on your HPC licences, If you have HPC packs you are going to use the licence whether you run 2, 4 or 8 processors, so you might as well use 8. Two HPC pack licences would do 32 processors, so choosing 9 would be a pretty bad idea. Its different for standard HPC licences though, they are per core.

It also depends on your CPU and memory channels, 4 cores per processor seems to be where my computer tops out (Sandy Bridge E). Using more yields no gain at all and would be a waste of expensive HPC licences.

Where did you find these recommendations? I didn't find them in the CFX tutorial. In order to acceleration the simulation ,shouldn't we use more processors than you proposed?

Usually the amount of processors you use is limited by the number of licenses you have, and how you are sharing them with other people using the system at the time. If you paid for the license then you might as well use them. It is pretty rare that adding a parallel license slows the simulation down (but is possible in extreme cases).

So I would not worry about the guidelines, and just use as many processors as you can practically use.

I'm using the license of university. So the license number is not a contraint. But i have limited number of processors. I have quite a lot of jobs to run. So I need to set a reasonable number of processor to each job to not only accelerate the single simulation but also run as much job at the same time as possible. So I need a good way to relate the number of processors to be used to the grids number (just don't waste any processors).

If you have 5 big simulations to run and you have 5 machines to run it on, you will find running 5 separate serial simulations is faster and more reliable than running the simulations 5-way parallel one after the other. Of course this assumes the machines are big enough to run the simulation by themselves and lots of other issues.

But the point of this comment is that running in parallel results in speed and reliability losses. It produces results for a single solution more quickly, but if you have lots of simulations to run it might be faster and more reliable to run serial (or multi-processor with a low processor count).

AnnaTian:
"Where did you find these recommendations? I didn't find them in the CFX tutorial. In order to acceleration the simulation ,shouldn't we use more processors than you proposed? "

These Informations were shared during ANSYS Training courses. In the Documentation you will not fid them, although there are some ppt Sheets available somewhere showing the scaling abilities of CFX.