Thanks Matt, that was my guess actually. On 09.01.2012 17:46, Matthew Knepley wrote: > On Mon, Jan 9, 2012 at 10:37 AM, Alexander Grayver > <agrayver at gfz-potsdam.de <mailto: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 <mailto: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
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