Strange behaviour while running in parallel

samiam1000 · September 14, 2012, 10:52

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

simpomann · September 18, 2012, 16:20

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.

wyldckat · September 18, 2012, 16:33

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

niklas · September 19, 2012, 15:23

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.

September 14, 2012, 10:52	Strange behaviour while running in parallel	#1
samiam1000 Senior Member Samuele Z Join Date: Oct 2009 Location: Mozzate - Co - Italy Posts: 520 Rep Power: 19	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

September 18, 2012, 16:33		#3
wyldckat Retired Super Moderator Bruno Santos Join Date: Mar 2009 Location: Lisbon, Portugal Posts: 10,981 Blog Entries: 45 Rep Power: 128	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 __________________ OpenFOAM: FAQ \| Getting started Forum: How to get help, to post code/output and forum guide Read this before sending me PM

September 19, 2012, 15:23		#4
niklas Super Moderator Niklas Nordin Join Date: Mar 2009 Location: Stockholm, Sweden Posts: 693 Rep Power: 29	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. wyldckat and samiam1000 like this.

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September 18, 2012, 16:20		#2
simpomann Member Simon Arne Join Date: May 2012 Posts: 42 Rep Power: 14	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 2x, 3y). 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.