[Moreza7]

Hello,

Can anyone simply explain that what does PT-Scotch exatcly do? I've read some text, but they are confusing.
Does it increase the simulation speed?
Which one is better? PT-Scotch or Par-Metis?

Regards.

[LuckyTran]

Scotch and Metis are algorithms for partitioning a grid so that you can run it in parallel. The difference is the technique used to partition the grid. Unless you are familiar with graph partitioning, it will always be confusing. All in all, the simulation speed is roughly the same after partitioning whether you use Scotch or Metis. There is a slight difference in the time it takes the algorithm to partition the grid. But since the time to make a parallel grid is small (or at least should be small) compared to the simulation time, it's not a big deal. Both are general purpose algorithms so if you really want to know which one (if) is better than the other, you need to do your own benchmarking for your problem.


Metis needs a distribution license from the IP owner so it is not always available to everyone.

[Moreza7]

Originally Posted by LuckyTran

Scotch and Metis are algorithms for partitioning a grid so that you can run it in parallel. The difference is the technique used to partition the grid. Unless you are familiar with graph partitioning, it will always be confusing. All in all, the simulation speed is roughly the same after partitioning whether you use Scotch or Metis. There is a slight difference in the time it takes the algorithm to partition the grid. But since the time to make a parallel grid is small (or at least should be small) compared to the simulation time, it's not a big deal. Both are general purpose algorithms so if you really want to know which one (if) is better than the other, you need to do your own benchmarking for your problem.


Metis needs a distribution license from the IP owner so it is not always available to everyone.

Thanks for your reply.
Do they only work in preproccesing stage? Or they also are needed in processing stage?
I mean can a mesh be partitiond on a PC by metis, and then this partitioned mesh gets imported to another PC for running the simulation which does not have metis installed?

[AtoHM]

As LuckyTran mentioned, the algorithms are used to spatially split the mesh into n parts, where n is the number of cpus used to perform the calculation. The mesh is only split for the calculation, as far as I know this split mesh is not given to the user usually. I believe in CFX there is a switch to export it or at least the split locations, so you can troubleshoot problems e.g. if your split location coincides with a phase interface for multiphase flows (it can cause divergence).
Speaking for myself, I never saw such a split mesh, as I didn't have any problems yet caused by the partitioning.

[flotus1]

Originally Posted by Moreza7

Do they only work in preproccesing stage? Or they also are needed in processing stage?

Semantics... most commercial CFD solvers have the domain decomposition integrated. I.e. they can read a single mesh, and then run the decomposition internally.
But technically, domain decomposition belongs to the pre-processing category.

Originally Posted by Moreza7

I mean can a mesh be partitiond on a PC by metis, and then this partitioned mesh gets imported to another PC for running the simulation which does not have metis installed?

Yes, this is possible.