# porousSimpleFoam crash on 40 cores

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 February 26, 2012, 15:56 porousSimpleFoam crash on 40 cores #1 New Member   Marco Join Date: Feb 2012 Posts: 7 Rep Power: 7 Sponsored Links Hi, I'm running a simulation with porousSimpleFoam. The simulation executed on 7 cores converges after 1295 iterations, but if I try to run the same simulation on 40 cores it diverges after 96 iterations. Is there any mathematical reason for this? If yes, there is a way to calculate the max number of cores that can be used? Thanks to all. Marco

 February 26, 2012, 17:09 #2 Super Moderator   Bruno Santos Join Date: Mar 2009 Location: Lisbon, Portugal Posts: 9,978 Blog Entries: 39 Rep Power: 108 Greetings Marco, My experience on this is limited, but AFAIK, there are several hypothesis for the crash, since you didn't give any specifics about the case. Some examples: There could be communication problems if you are using the 7 cores case in a single machine, but the 40 cores in several machines. Did you check with 7 cores, one or two per machine? Analyze how the mesh was decomposed into several parts. You might find some weird situation where patches got improperly divided. Certain boundary conditions have a few issues with parallel processing. Usually the guilty ones are the cyclic patches, where their division between sub-domains might lead to problems. Bug reports as examples: Usually the rule of thumb is to have at least 50k cells per core, but this varies with core power and communication stack (number of machines vs bandwidth). Best regards, Bruno __________________ OpenFOAM: FAQ | Getting started Forum: How to get help, to post code/output and forum guide What am I doing/planning: blog/wiki Read this before sending me PM

February 26, 2012, 18:22
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Arjun
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Quote:
 Originally Posted by ciberbob1987 Hi, Is there any mathematical reason for this? If yes, there is a way to calculate the max number of cores that can be used?
numerically possible. So yes there is a mathematical reason for it.

But I am not sure if it is the case. What was your pressure correction solver.
If it was multigrid then it is possible to crash or diverge. If it was Kyrlov based solver like conjugate gradient then less likely but still depends on preconditioner used.

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