Hi Martin,
> 3 янв. 2017 г., в 22:31, Martin Kronbichler <[email protected]> > написал(а): > > Hi Denis, > > >> Can you please elaborate? >> >> My naïve understanding would be: >> 1. before doing interpolation, all ghost values are syncrhonized between the >> MPI processors >> 2. solution transfer would, roughly, take local values on each element, >> apply transformation matrices corresponding to each child and ship, if >> needed, interpolated values to new MPI owners of cells. >> 3. Each MPI would go through its cells and set values of vectors at locally >> active DoFs which, of course, means that at the interface between the two >> paritions different MPI processes may be trying to write to the same global >> DoF entry in the vector. >> >> In the above, i fail to see what could influence the order of operations and >> result in round-off errors between different MPI cores. > > The description of the three steps above is correct. The problem appears in > step 2 where the solution gets interpolated separately on the two cells > sharing a face. For an interpolatory finite element, the values on a shared > face should coincide and we know mathematically that they indeed interpolate > to the same values. However, the additions in the (different) transfer > matrices from the two sides might perform the additions in a different order. > Thus, roundoff could accumulate differently, in particular if you have some > cancellation. Does that make sense? Thanks for clarification, that makes perfect sense indeed. Regards, Denis -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en --- You received this message because you are subscribed to the Google Groups "deal.II User Group" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
