Question about number of processors shall I use to speed up the calculation

lnk · October 3, 2012, 09:08

Hi,

I send the simulation jobs to my company's processors for calculation. I mainly use CFX-solver and FineTM. I don't know anything about parallel computing yet. But I heard about more processors may even give lower calculation speed.

May I ask how to decide the number of processors shall I use?

ski · October 3, 2012, 09:55

You'll need to ensure the the communication between processors is not causing a bottleneck. If you use too many processors, then your code will spend all its time communicating rather than solving.

A conservative estimate may be to use at least 100,000 cells per a cpu core, so if you have a 1M cell mesh, use no more than 10 CPUs. This will most certainly be enough to keep the CPU busy. That said, you will probably find acceptable performance as low as 20,000 cells/core if you have high end hardware connected with infiband. The optimum number depends a lot on the hardware and also how well the code scales, so there is no single answer, but the above figures should give you some idea.

October 3, 2012, 09:08	Question about number of processors shall I use to speed up the calculation	#1
lnk Senior Member lnk Join Date: Feb 2011 Location: Switzerland Posts: 118 Rep Power: 15	Hi, I send the simulation jobs to my company's processors for calculation. I mainly use CFX-solver and FineTM. I don't know anything about parallel computing yet. But I heard about more processors may even give lower calculation speed. May I ask how to decide the number of processors shall I use?

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October 3, 2012, 09:55		#2
ski New Member AS Join Date: Jul 2009 Posts: 16 Rep Power: 17	You'll need to ensure the the communication between processors is not causing a bottleneck. If you use too many processors, then your code will spend all its time communicating rather than solving. A conservative estimate may be to use at least 100,000 cells per a cpu core, so if you have a 1M cell mesh, use no more than 10 CPUs. This will most certainly be enough to keep the CPU busy. That said, you will probably find acceptable performance as low as 20,000 cells/core if you have high end hardware connected with infiband. The optimum number depends a lot on the hardware and also how well the code scales, so there is no single answer, but the above figures should give you some idea.