Running in parallel mysteries

andreas · March 14, 2013, 03:40

Dear foamers,

I want to run a fine mesh (1200*400) on OF and for that I need to run in parallel. I run a first time with some 18cpu and the simulation went to 0.1sec after 3 days. I took the same folder and ran it from the 0.1sec with 36cpu since 4 days, it barely reached the 1.1sec…

I then ran some test cases with 1, 2, 4, 8cpu, during 5minutes for a 150*50 mesh. Here are the results:

1cpu: Time = 0.0569324
2cpu: Time = 0.0680509
4cpu: Time = 0.0638118
8cpu: Time = 0.00469057
16cpu: Time = 0.00384501

Quite strange isn't?

To run in parallel, I simply change the "numberOfSubdomains" in the decomposeParDict file and change the n(x y z) in the simpleCoeffs knowing that x*y*z must be equal to the numberOfSubdomains.

That's it, do I have to change any other parameter?

Thank you for your help, I'm quite new in cfd…
Andreas

olivierG · March 14, 2013, 06:30

hello,

A common mistake is to forget the "-parallel" option with the solver, which can explain why it's slower with more core ....

regards,
olivier

March 14, 2013, 03:40	Running in parallel mysteries	#1
andreas New Member Join Date: Oct 2012 Posts: 14 Rep Power: 13	Dear foamers, I want to run a fine mesh (1200400) on OF and for that I need to run in parallel. I run a first time with some 18cpu and the simulation went to 0.1sec after 3 days. I took the same folder and ran it from the 0.1sec with 36cpu since 4 days, it barely reached the 1.1sec… I then ran some test cases with 1, 2, 4, 8cpu, during 5minutes for a 15050 mesh. Here are the results: 1cpu: Time = 0.0569324 2cpu: Time = 0.0680509 4cpu: Time = 0.0638118 8cpu: Time = 0.00469057 16cpu: Time = 0.00384501 Quite strange isn't? To run in parallel, I simply change the "numberOfSubdomains" in the decomposeParDict file and change the n(x y z) in the simpleCoeffs knowing that xyz must be equal to the numberOfSubdomains. That's it, do I have to change any other parameter? Thank you for your help, I'm quite new in cfd… Andreas

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March 14, 2013, 06:30		#2
olivierG Senior Member Olivier Join Date: Jun 2009 Location: France, grenoble Posts: 272 Rep Power: 17	hello, A common mistake is to forget the "-parallel" option with the solver, which can explain why it's slower with more core .... regards, olivier