[petsc-users] KSPMonitorSingularValue

[Klaij, Christiaan]

> If you just want to see the monitor, why not use the command line or
> PetscOptionsSetValue()?

I have three ksp's: one for the momentum eqs (GMRES) and one for
the pressure eq (CG) inside the SIMPLE preconditioner and one for
the matrix-free coupled mass-momentum system (FGMRES).  The
command line shows me results for all but right now I'm only
interested in monitoring FGMRES on the coupled system.




> This is the code for setting up the monitor, you can call the part inside
> the if statement yourself if you like.
>
>     ierr = PetscOptionsString("-ksp_monitor_singular_value","Monitor
> singular
> values","KSPMonitorSet","stdout",monfilename,PETSC_MAX_PATH_LEN,&flg);CHKERRQ(ierr);
>     if (flg) {
>       ierr = KSPSetComputeSingularValues(ksp,PETSC_TRUE);CHKERRQ(ierr);
>       ierr =
> PetscViewerASCIIOpen(((PetscObject)ksp)->comm,monfilename,&monviewer);CHKERRQ(ierr);
>       ierr =
> KSPMonitorSet(ksp,KSPMonitorSingularValue,monviewer,(PetscErrorCode
> (*)(void**))PetscViewerDestroy);CHKERRQ(ierr);
>     }

I'm still confused whether it is supposed to work with FGMRES,
the manual states only CG and GMRES. (Besides, I'm using fortran)



> How are you applying the action of the linear operator? If you use finite
> differencing, it could be inaccurate. Is this incompressible or a low-Mach
> compressible formulation? Try -ksp_monitor_true_residual, if the true
> residual drifts from the unpreconditioned residual computed by FGMRES, the
> Krylov space could be losing orthogonality. You can try
> -ksp_gmres_modifiedgramschmidt. Are you losing a lot of progress in
> restarts?

It's incompressible Navier-Stokes. No finite differencing, the
action is computed directly without approximations. It's right
preconditioning, so preconditioned and true residual should be
the same. I don't get any progress, the residual is stagnating
from the very first iteration way before any restart.

Regarding modified Gram Schmidt, I tried to set it as follows:

call KSPGMRESSetOrthogonalization(ksp,KSPGMRESModifiedGramSchmidtOrthogonalization,ierr)

But my compiler tells me:

This name does not have a type, and must have an explicit type.   [KSPGMRESMODIFIEDGRAMSCHMIDTORTHOGONALIZATIO]

(petsc-3.1-p7, fortran with "use petscksp" and #include "finclude/petsckspdef.h")


dr. ir. Christiaan Klaij
CFD Researcher
Research & Development
E mailto:C.Klaij at marin.nl
T +31 317 49 33 44

MARIN
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