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-   -   Parallel runs unstabel and composed stable, what is goinig on? (http://www.cfd-online.com/Forums/openfoam/82973-parallel-runs-unstabel-composed-stable-what-goinig.html)

idrama December 10, 2010 05:58

Parallel runs unstabel and composed stable, what is goinig on?
 
Hello Foamers,

has anybody ever made the following experience: a parallelized running the simulation breaks down. After reconstruction and letting the simulation run as a single process the error is disappeared and the simulation iterate over the problem time step. Afterwards, I decompose the case again and moves on. So, what happened? I though if I let the sim run in parallel or not, it doesn't matters the results or the behavior.

Cheers in advance

Bernhard December 10, 2010 06:09

Can you maybe post a case as simple as possible to reconstruct this behaviour? Some experienced users might be able to help you then. Maybe you can give some error messages?

idrama December 10, 2010 06:33

Hello Bernhard,

unfortunately, I do not have a simple case and I cannot make up a simulation which exhibits a similar behavior. I discovered it accidentally. Error code: I don't have more as the usual error in OpenFOAM when a blow up has taken place.
I think it could has something to do with the matrices which are computed on different cpus. In composed fashion, the might computed more easily. Have you ever made such experience?

sundaero December 10, 2010 07:12

I've had this kind of problem when was trying to use too many processors or some experimental domain decomposition (like decomposition of a simple domain along only one direction). To solve this problem it was just enough to decrease a number of the processors (not necessary to go to a single-processor mode).
As the speedup in that cases was anyway too low and there was no yield of using more processors I just forgot about that.

idrama December 14, 2010 06:59

That is usually what I do too; decreasing the number of processors, passing the critical position and then increasing the number of processor.


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