[petsc-users] -ksp_rtol problem

[Qiyue Lu]

Sorry, I made a mistake in privous email.
in my output file, the *KSP residual norm* not the rtol is 1.0e-4 at the
1st iteration. My question is how this residual norm calculated? Is there
any initial guess there? thanks


On Mon, Nov 11, 2013 at 11:55 AM, Qiyue Lu <qiyuelu1 at gmail.com> wrote:

> Dear All:
>
> I am working on a linear A*x=b system.
> When I use *-ksp_rtol 1.0e-5 *option in the command line, I am not very
> sure how the relative tolerance is defined.
>
> In my understanding:
>
> At the beginning, r= b-A*x_i, so rtol = ||r|| / ||b|| with 2-norm has a
> value. with the calculation, rtol gets smaller, when the rtol_new /
> rtol_original < 1.0e-5, the calculation will stop.
>
> Is my understanding correct?
>
> If so, how to decide the rtol at the first iteration? If initial solution
> values are all zeros, then rtol should be 1.0. In my output file, it's a
> value related to the matrix and can be at 1.0e-4  scale. Is there any
> initial guess solution from preconditioner used there?
>
> Could you help to clarify this definition? Thanks
>
>
> Qiyue Lu
>
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