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Strange behaviour while running in parallel

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Old   September 14, 2012, 09:52
Default Strange behaviour while running in parallel
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Samuele Z
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Dear Foamers,

something very strange happens when I run a process in parallel. I do have a server with 4 CPUs and I have made a test.

I tried to run my process with 1, 2, 3 and 4 processors.

What I noticed is that with 2 processors I get a good result (speaking about time). With 3, the result is almost the same than with 2.
And with 4 it is even worse.

That's very curious. I would like to understand why this happens and how I can solve this.

Thanks a lot for help,

Samuele
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Old   September 18, 2012, 15:20
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Simon Arne
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Hey there,

Have you checked the settings of your decomposePar -Dictionary?
You can define in which axis the mesh is splitted and how many times (or any variation of like 2*x, 3*y).
If you choose this badly, it may occur that the number of cells processed by each processor is not similiar and then I encountered processing speed disadvantages as well.
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Old   September 18, 2012, 15:33
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Bruno Santos
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Greetings to all!

The other possibility is the one explained here: Large test case for running OpenFoam in parallel post #20

(FYI: I found out from here: http://www.cfd-online.com/Forums/blo...-parallel.html)

Best regards,
Bruno
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Old   September 19, 2012, 14:23
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Niklas Nordin
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that is pretty typical behaviour when the network is bad and the communication overhead becomes too large.
The bandwidth in the cpu, between the cores choke.
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