Strange behaviour while running in parallel
 

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

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.

Greetings to all!

The other possibility is the one explained here: http://www.cfd-online.com/Forums/ope...tml#post230087 post #20

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

Best regards,
Bruno

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.