[petsc-users] KSPBuildResidual and KSPType compatibility

[Matthew Knepley]

On Mon, Dec 6, 2021 at 1:07 PM Fischer, Greg A. via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> With -pc_type none, my value matches the ksp value, but it no longer
> converges.
>

So now it seems like the check with unpreconditioned residuals was not
right somehow.

  THanks,

    Matt


> -----Original Message-----
> From: Jed Brown <jed at jedbrown.org>
> Sent: Monday, December 6, 2021 12:31 PM
> To: Fischer, Greg A. <fischega at westinghouse.com>; Fischer, Greg A. via
> petsc-users <petsc-users at mcs.anl.gov>; petsc-users at mcs.anl.gov
> Cc: Fischer, Greg A. <fischega at westinghouse.com>
> Subject: RE: [petsc-users] KSPBuildResidual and KSPType compatibility
>
> [External Email]
>
> Please try with -pc_type none.
>
> There is always a small difference due to using the recurrence, but it
> should be small so long as the Krylov basis is close to orthogonal.
>
> I'd note that if you're using this expensive convergence test, then IBCGS
> probably isn't helping over BCGS (the I part is to amortize some vector and
> reduction costs). It'd be worth comparing when you use normal BCGS.
>
> "Fischer, Greg A." <fischega at westinghouse.com> writes:
>
> > I tried your suggestion, but the values are still quite different.
> >
> > -----Original Message-----
> > From: Jed Brown <jed at jedbrown.org>
> > Sent: Monday, December 6, 2021 12:01 PM
> > To: Fischer, Greg A. via petsc-users <petsc-users at mcs.anl.gov>;
> petsc-users at mcs.anl.gov
> > Cc: Fischer, Greg A. <fischega at westinghouse.com>
> > Subject: Re: [petsc-users] KSPBuildResidual and KSPType compatibility
> >
> > [External Email]
> >
> > "Fischer, Greg A. via petsc-users" <petsc-users at mcs.anl.gov> writes:
> >
> >> Hello petsc-users,
> >>
> >> I would like to check convergence against the infinity norm, so I
> defined my own convergence test routine with KSPSetConvergenceTest. (I
> understand that it may be computationally expensive.)
> >>
> >> I would like to do this with the "ibcgs" method. When I use
> KSPBuildResidual and calculate the NORM_2 against the output vector, I get
> a value that differs from the 2-norm that gets passed into the convergence
> test function. However, when I switch to the "gcr" method, the value I
> calculate matches the function input value.
> >
> > IBCGS uses the preconditioned norm by default while GCR uses the
> unpreconditioned norm. You can use -ksp_norm_type unpreconditioned or
> KSPSetNormType() to make IBCGS use unpreconditioned.
> >
> >> Is the KSPBuildResidual function only compatible with a subset of the
> KSPType methods? If I want to evaluate convergence against the infinity
> norm, do I need to set KSPSetInitialGuessNonzero and continually re-start
> the solver with a lower tolerance values until I get a satisfactory value
> of the infinity norm?
> >>
> >> Thanks,
> >> Greg
> >>
> >>
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-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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