[petsc-users] help needed for -snes_mf_operator

[Matthew Knepley]

On Wed, Apr 25, 2012 at 1:42 PM, Dominik Szczerba <dominik at itis.ethz.ch>wrote:

> > Start by comparing residuals, with an without. Is SNES 0 the same? Is
> KSP 0
> > the same? KSP 1?
>
> BCGS, with the option:
>
>  0 SNES Function norm 1.747536282885e+01
>  0 KSP unpreconditioned resid norm 1.747536282885e+01 true resid norm
> 1.747536282885e+01 ||r(i)||/||b|| 1.000000000000e+00
>  1 KSP unpreconditioned resid norm 6.112827145626e-01 true resid norm
> 1.751036369876e+01 ||r(i)||/||b|| 1.002002869425e+00
>
> BCGS, without:
>
>  0 SNES Function norm 1.747536282885e+01
>  0 KSP unpreconditioned resid norm 1.747536282885e+01 true resid norm
> 1.747536282885e+01 ||r(i)||/||b|| 1.000000000000e+00
>  1 KSP unpreconditioned resid norm 2.930616102426e-01 true resid norm
> 2.930616102426e-01 ||r(i)||/||b|| 1.676998715922e-02
>
> GMRES, with the option:
>
>  0 SNES Function norm 1.747536282885e+01
>  0 KSP unpreconditioned resid norm 1.747536282885e+01 true resid norm
> 1.747536282885e+01 ||r(i)||/||b|| 1.000000000000e+00
>  1 KSP unpreconditioned resid norm 6.122286256494e-01 true resid norm
> 1.747536251439e+01 ||r(i)||/||b|| 9.999999820055e-01
>

Notice that there is no increase in the residual as is required. Notice
also that
there is no decrease. This must mean that the preconditioner is rank
deficient,
and I think it probably means that it is 0 since I find it hard to believe
that P x
produces no component in the b direction.

   Matt


> GMRES, without:
>
>  0 SNES Function norm 1.747536282885e+01
>  0 KSP unpreconditioned resid norm 1.747536282885e+01 true resid norm
> 1.747536282885e+01 ||r(i)||/||b|| 1.000000000000e+00
>  1 KSP unpreconditioned resid norm 6.122286294217e-01 true resid norm
> 6.122286294218e-01 ||r(i)||/||b|| 3.503381505824e-02
>
> Dominik
>



-- 
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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