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

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Old   December 10, 2010, 05:58
Default Parallel runs unstabel and composed stable, what is goinig on?
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Claus Meister
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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
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Old   December 10, 2010, 06:09
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Bernhard
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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?
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Old   December 10, 2010, 06:33
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Claus Meister
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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?
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Old   December 10, 2010, 07:12
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Denis Semyonov
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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.
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Old   December 14, 2010, 06:59
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Claus Meister
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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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