On Mon, Jan 9, 2012 at 10:37 AM, Alexander Grayver <agrayver at gfz-potsdam.de>wrote:
> ** > I see this is due to preconditioner, but could you explain shortly what > particularly causes this problem? It's not obvious to me. > The size of the block being factorized by ILU(0) decreases as p increases. Thus, it is a weaker preconditioner. I recommend Yousef Saad's book which explains all this clearly. Matt > Thanks. > > On 09.01.2012 17:36, Matthew Knepley wrote: > > On Mon, Jan 9, 2012 at 10:31 AM, Alexander Grayver < > agrayver at gfz-potsdam.de> wrote: > >> I have tested default GMRES+ILU(0) solver for problem of size ~200000 and >> I observed that both convergence rate and accuracy degrade when I increase >> number of processes. >> E.g., number of iterations almost doubles when going from 1 to 4 >> processors. >> >> I noticed that ILU(0) is somehow represented by sequential matrices >> internally and feel that this might be a reason? >> > > The default is Block Jacobi+ILU(0) and GMRES(30) which will definitely > degrade as the number of > processes is increased. You would need to use an optimal preconditioner > like Multigrid if you want > a constant number of iterates. > > Matt > > >> Regards, >> Alexander >> > > > > -- > 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 > > > -- 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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120109/2aec69d6/attachment.htm>
