[petsc-users] SNESSolve_KSPONLY not working for my linear problem

[Yaqi Wang]

While reading my post again, I realized I did not have -snes_mf or
-snes_mf_operator set with -ksp_only. This possibly explains the behavior I
observed. I was not thinking this combination. I will confirm this tomorrow.

Thanks,
Yaqi

On Wed, Oct 14, 2015 at 1:08 AM, Yaqi Wang <yaqiwang at gmail.com> wrote:

> Hello All,
>
> This is possibly a newbie question but I cannot figure out why. A SNES is
> set up in my code but I would like to use it for solving a simple linear
> problem.
>
> The sequence of setting up the SNES is as the following:
> SNESCreate()
> SNESSetFunction()
> SNESSetJacobian()
> SNESSetFromOptions()
>
> I do not list all calls but just list those I think matters. Note that my
> Jacobian is not exact, i.e. it is not consistent with my function.
>
> Then SNESSolve() is called. If I use -snes_type newtonls and
> -snes_mf_operator, SNES can solve the linear problem properly, i.e. the
> residual norm dropped bellow the tolerance I specified. I know I donot have
> to by playing with tolerances, but I tried -snes_type ksponly. I saw my
> residual function is only called at the beginning and the end of solve.
> Most likely KSP is trying to solve the linear problem with the Jacobian I
> specified, which is not exact. I cannot make the solution converge probably
> because of this.
>
> My question is why KSPONLY is not using my function for evaluating (b-Ax)
> at each linear iteration? I am using the default KSP type, which I guess
> should be GMRES. I do not think it matters anyway.
> Did I do something wrongly or I had some misunderstanding?
>
> Many thanks,
> Yaqi
>
>
>
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