> I tried > to use the same trick as in step-22, where an InnerPreconditioner was > defined to switch between SparseDirectUMFPACK in 2D and SparseILU in 3D. > However I found that SparseILU won't work even when in small 2D cases. To > verify this I tried to replace the SparseDirectUMFPACK in step-57 with > SparseILU, same thing happened. Without additional off-diagonals the GMRES > solver won't converge; with additional off-diagonals, the program runs > unbearably slow. Why is the performance of SparseILU so much worse than > SparseDirectUMFPACK? Am I using it the right way?
Do you have an inner inner solve or do you just use a single SparseILU application? ILU is not great in 3d. This is even true if you consider the velocity block of the Stokes problem (which is Laplace-like). See the tables in https://urldefense.proofpoint.com/v2/url?u=https-3A__www.dealii.org_8.5.0_doxygen_deal.II_step-5F56.html-23Results&d=DwIBaQ&c=Ngd-ta5yRYsqeUsEDgxhcqsYYY1Xs5ogLxWPA_2Wlc4&r=4k7iKXbjGC8LfYxVJJXiaYVu6FRWmEjX38S7JmlS9Vw&m=--nG2SHIoX884s0M9k-XskKi6u9iVsUaE8GaIsVH_Yw&s=PXVWQ3PvRgczuV6a0GPJS7sH7uTPr9z0O-LNq7vuRtA&e= for example. You can clearly see that ILU is h-dependent. So ILU should "work" but I am not surprised that it degrades for larger problem sizes. Your system is even harder to solve because of Grad-Div and the non-symmetric convective term. > At this point I totally understand parallelization is necessary for larger > problems. But I still want to enable my toy program to run problems slightly > out the scope of SparseDirectUMFPACK. After that I will try parallelize it. I am afraid this will be difficult. While not easy to do, step-56 allows you to solve larger problems using geometric multigrid. Extending this to Navier-Stokes is not easy either (and we are getting close to state-of-the-art research). Best, Timo -- 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 dealii+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
