On Dec 21, 2010, at 2:08 PM, Jed Brown 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.
No because it uses a recursive formula for "computing" the residual norm it does not compute it explicitly. So in extreme circumstances the recursively compute one generates "garbage". Barry
