[petsc-users] Monotonic convergence in FGMRES.

[Dave May]

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.