[petsc-users] KSP Residual for GMRES

[Dave May]

GMRES uses left preconditioning so -ksp_monitor will report the
preconditioned residual

You can use
-ksp_monitor_true_residual
if you don't know whether the preconditioner is using left or right
preconditioning.
This will display both residuals.



On 9 March 2011 17:53, Travis C. Fisher <travis.fisher at nasa.gov> wrote:
> I am running a Navier Stokes Solver that uses implicit Runge Kutta for
> timestepping. I am using SNES to perform preconditioned JFNK. As a sanity
> check, I am running in serial and using the FD generated Jacobian at the
> beginning of a time step as the preconditioner matrix for the linear solves,
> and superlu_dist as the preconditioner. This means that the linear solve
> (GMRES) for the first Newton iteration of the first stage should converge
> almost exactly. I do not observe this for large time steps. While it does
> converge to the Eisenstat-Walker convergence criterion, the KSP residual
> norm is not small. It only decreases four orders of magnitude. My question
> is whether or not this is the preconditioned residual? The solver spits out
> the same residual whether or not I use left or right preconditioning. I
> should also mention that in this case I have turned off all viscous cross
> terms and the problem is essentially 1-D, so I have taken as much care as
> possible to ensure that the FD jacobian is a proper representation of the
> system. Additionally, I observe the same problem in the inviscid case with
> both the FD jacobian and the exact jacobian that I have computed. I
> appreciate any insight you can give me into this observation.
>
> Thanks,
>
> Travis Fisher
>