|September 14, 2012, 09:52||
Strange behaviour while running in parallel
Join Date: Oct 2009
Location: Mozzate - Co - Italy
Posts: 490Rep Power: 11
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,
|September 18, 2012, 15:20||
Join Date: May 2012
Posts: 42Rep Power: 6
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.
|September 18, 2012, 15:33||
Join Date: Mar 2009
Location: Lisbon, Portugal
Blog Entries: 39Rep Power: 103
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)
|September 19, 2012, 14:23||
Join Date: Mar 2009
Location: Stockholm, Sweden
Posts: 693Rep Power: 21
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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