Vijay, You should definitely follow Barry's suggestion and monitor the true residual (using -ksp_monitor_true_residual). Jed, I know this may sound odd given FGMRES uses right preconditioning, but I've seen that when the system is badly scaled, the preconditioned residual and the true residual reported by -ksp_monitor_true_residual can drift from one another. In such situations with the reported numbers are initially the same, but after a number of iterations, the preconditioned residual may continue to decrease but the true residual actually stagnates.
Cheers, Dave On 21 December 2010 12:08, Jed Brown <jed at 59a2.org> wrote: > On Tue, Dec 21, 2010 at 21:04, Barry Smith <bsmith at mcs.anl.gov> wrote: >> >> This is a sign that the preconditioner is seriously messed up and should >> not be used in its current form. ?It can happen if the matrix is nearly >> singular and for example you use an incomplete factorization for a >> preconditioner that just screws up the scaling like totally. Run with >> -ksp_monitor_true_residual and you'll see that the solver is not really >> solving the problem even though it thinks it is converging fine. > > FGMRES only does right preconditioning so it should be showing the true > residual.
