\f0\fs24 \cf0 0 SNES Function norm 4.688596679512e-01\ Linear solve converged due to CONVERGED_ITS iterations 1\
I don't trust this. Change it to allow several KSP iterations instead of one and use -ksp_monitor_true_residual to see if the linear is actually solved accurately. Also I agree with Matt's email, it is possible the Jacobian is wrong and hence even if the linear solve is accurate Newton may converge. Barry On Aug 30, 2013, at 9:35 PM, subramanya sadasiva <[email protected]> wrote: > Hi Barry, > I have attached output from the solver with the options that you suggested. > As you can see the direct solver causes the nonlinear solver to diverge - but > the code converges easily with gmres > Thanks, > Subramanya > > > > Subject: Re: [petsc-users] Behaviour of Newton Methods with Direct Solvers. > > From: [email protected] > > Date: Fri, 30 Aug 2013 21:16:33 -0500 > > CC: [email protected] > > To: [email protected] > > > > > > On Aug 30, 2013, at 9:14 PM, subramanya sadasiva <[email protected]> > > wrote: > > > > > Hi Barry, > > > The linear solvers converge in 1 iterations. But the SNES solver fails > > > when I use the direct solver. > > > > Hmm. I don't have an explanation for this. Could you send some output when > > running with -snes_monitor -ksp_monitor_true_residual for both the > > iterative case and the direct solver case? > > > > Barry > > > > > Thanks, > > > Subramanya > > > > > > > Subject: Re: [petsc-users] Behaviour of Newton Methods with Direct > > > > Solvers. > > > > From: [email protected] > > > > Date: Fri, 30 Aug 2013 21:11:36 -0500 > > > > CC: [email protected] > > > > To: [email protected] > > > > > > > > > > > > I would not expect this. Are you sure that direct solvers are actually > > > > working well? You can run -ksp_type gmres -pc_type lu to "accelerate" > > > > the direct solver. Under normal circumstances the direct solver will > > > > converge in one iteration but if, for example, the problem is singular > > > > or nearly singular several iterations might solve the linear system but > > > > one may not. > > > > > > > > Barry > > > > > > > > On Aug 30, 2013, at 9:08 PM, subramanya sadasiva <[email protected]> > > > > wrote: > > > > > > > > > Hi, > > > > > Is there a reason that the convergence of the newton methods in SNES > > > > > is much better with iterative solvers instead of with direct solvers? > > > > > This is particularly in reference with the VI solvers. I get nearly > > > > > quadratic convergence with the iterative solvers, but the code just > > > > > diverges or converges linearly when i run with -ksp_type preonly . > > > > > Thanks, > > > > > Subramanya > > > > > > > <snes_vi_with_direct_solver.rtf><snes_vi_with_gmres.txt>
