DMMG solver convergence with single precision?

[Matthew Knepley]

On Tue, Dec 2, 2008 at 7:52 PM, Chris Cooper <chrisc at al.com.au> wrote:

> Hi,
>
> Just wondering if there are any techniques for improving DMMG convergence
> on a linear problem when operating in single precision?
>
> eg using ksp example 34 in Petsc 2.3.3 with multigrid levels changed to
> 6...
>
> double precision:
>  0 KSP Residual norm 4.451733404502e-02
>  1 KSP Residual norm 2.801676364873e-04
>  2 KSP Residual norm 3.215426166316e-06
>  3 KSP Residual norm 5.446277176355e-08
> Residual norm 5.44628e-08
> Error norm 0.000356798
> Error norm 9.21724e-05
> Error norm 1.34208e-07
>
> single precision:
>  0 KSP Residual norm 4.450287297368e-02
>  1 KSP Residual norm 2.799208450597e-04
>  2 KSP Residual norm 2.232483529951e-05
>  3 KSP Residual norm 1.575733222126e-05
>  4 KSP Residual norm 1.286925271415e-05
> ...
> 27 KSP Residual norm 4.369261660031e-06
> 28 KSP Residual norm 4.287577212381e-06
> 29 KSP Residual norm 4.210308816255e-06
> 30 KSP Residual norm 2.235170904896e-05
> 31 KSP Residual norm 1.709007477757e-07


This bothers me. Shouldn't the Residual norm be the same as the last
residual, as it is above?

  Matt

Residual norm 8.45584e-07
> Error norm 0.000370784
> Error norm 9.54664e-05
> Error norm 1.38935e-07
>
> thanks,
> Chris Cooper
> Animal Logic
>
> Animal Logic
> http://www.animallogic.com
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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
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