[petsc-users] KSP step not accelerating despite good preconditioning in SNES

[Matthew Knepley]

On Mon, Sep 28, 2015 at 2:53 AM, Timothée Nicolas <
timothee.nicolas at gmail.com> wrote:

> Hi all,
>
> I have something strange in my application and I don't know what could
> cause this. I am trying to do an implicit MHD problem and I thought I
> finally figured out the preconditioner step, but somehow I don't get the
> expected result, not even close.
>
> For the preconditioning, I am using an approximate Schur complement, which
> requires two relatively easy KSP inversions at each preconditioner
> application. I apply this algorithm directly to the result function at the
> end of the routine FormFunction. I have checked that the approximation to
> the inversion of the Jacobian is good, in the sense that when I multiply
> the preconditioned vector by the *total* Jacobian matrix, I indeed
> recover almost the initial unpreconditioned vector. Also, I know that my
> Jacobian matrix is correct, because (i) I have checked manually that F(X +
> dX) ~ F(X) + J * dX and (ii) when I don't use -snes_mf and use the provided
> Jacobian matrix the result is pretty much equivalent to using -snes_mf.
>
> In my understanding, this means that what I effectively feed to SNES at
> the end of my FormFunction routine is a good approximation to J^(-1) F. As
> a result, I naturally expect that the number of KSP iterations necessary to
> achieve one SNES iteration be drastically reduced. However, I observe
> virtually no change whatsoever in the number of iterations.
>
> Any thoughts about what I could be missing ? Maybe I forgot to set a SNES
> or KSP option somewhere ? I can send pieces of code if needs be.
>

It sounds like you are putting this in the wrong place. If you have the
action of a good preconditioner for the Jacobian, then you
should use a PCSHELL and pass it to the KSP. It does not belong in the
FormFunction.

  Thanks,

     Matt


> Best
>
> Timothee
>
>
>


-- 
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
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