On Mon, Feb 3, 2014 at 8:26 AM, Dana Christen <dana.chris...@akselos.com>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 Libmesh-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/libmesh-users
