Our problem is too large to use SVD, but I believe it to be well-posed, since it converges with LU and I've been able to make it converge using CG+BoomerAMG using one of the example codes. We must have a problem somewhere in our code somewhere...
Surprisingly I've noticed that PETSc doesn't use the -pc_fieldsplit_0 parameter: Option left: name:-pc_fieldsplit_0 value: 0,1,2 Trying smoothed aggregation to include rigid body modes makes a lot of sense. We may try that soon. Dana On Mon, Feb 3, 2014 at 6:53 PM, John Peterson <[email protected]> wrote: > > > > On Mon, Feb 3, 2014 at 8:26 AM, Dana Christen > <[email protected]>wrote: > >> Hi, >> >> Thanks for your assistance. >> >> Switching to BiCGStab (-ksp_type bcgs) doesn't improve convergence and >> shows the same symptoms (residual with frequent oscillations). >> Using fieldsplit as Jens suggests doesn't help either; residual doesn't go >> below 1e-3 even after 1000 iterations. >> Using GMRES makes the oscillations disappear, but the residual doesn't >> converge either. >> > > OK, my other completely-anecdotally-based suggestion is that possibly the > AMG coarsening process removes enough constraints that the coarse-scale > problem has rigid body modes, thereby making the preconditioner singular? > > If we light the Jed Brown signal fire by mentioning PETSc three times, he > might appear and explain why this suggestion is BS... > > Also, since you said LU works for smaller problems, I don't suspect you > have an ill-posed problem, but have you checked to see how fast the > condition number is increasing with problem size? If your problem is small > enough, you can run with: > > -pc_type svd -pc_svd_monitor > > And see what the condition number is. > > -- > John > ------------------------------------------------------------------------------ Managing the Performance of Cloud-Based Applications Take advantage of what the Cloud has to offer - Avoid Common Pitfalls. Read the Whitepaper. http://pubads.g.doubleclick.net/gampad/clk?id=121051231&iu=/4140/ostg.clktrk _______________________________________________ Libmesh-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/libmesh-users
