Dear Barry and Matt, thanks for your helpful response. ML works now using, e.g., -mg_coarse_redundant_pc_factor_shift_type POSITIVE_DEFINITE. However, it converges very slowly using the default REDUNDANT for the coarse solve. On 10 processors, e.g., even bjacobi plus -mg_coarse_ksp_max_it 10 works better. What solver do you recommend for the coarse solve? Maybe superlu?
Best regards, Kathrin Quoting "Barry Smith" <bsmith at mcs.anl.gov>: > > -mg_coarse_pc_factor_shift_nonzero since it is the coarse level of > the multigrid that is producing the zero pivot. > > Barry > > On Apr 13, 2010, at 8:51 AM, Matthew Knepley wrote: > >> On Tue, Apr 13, 2010 at 2:49 PM, <tribur at vision.ee.ethz.ch> wrote: >> Hi, >> >> using ML I got the error >> >> "[0]PETSC ERROR: Detected zero pivot in LU factorization" >> >> As recommended at >> http://www.mcs.anl.gov/petsc/petsc-as/documentation/troubleshooting.html , >> I tried -pc_factor_shift_nonzero but it doesn't have the desired >> effect using ML. >> >> How do I have to formulate the command line option? What does - >> [level]_pc_factor_shift_nonzero mean? What other parallel >> preconditioner could I try besides Hypre/Boomeramg or ML? >> >> This means the MG level, like 2. You can see all available options >> using -help. >> >> Matt >> >> Thanks in advance for your precious help, >> >> Kathrin >> >> >> >> >> >> -- >> What most experimenters take for granted before they begin their >> experiments is infinitely more interesting than any results to >> which their experiments lead. >> -- Norbert Wiener > >
