DMMG solver convergence with single precision?

Chris Cooper chrisc at al.com.au
Tue Dec 2 21:47:51 CST 2008

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

Hi Matt,

If I use ksp_monitor_true_residual the numbers match...

30 KSP preconditioned resid norm 2.235170904896e-05 true resid norm
2.235170904896e-05 ||Ae||/||Ax|| 5.022531840950e-04
31 KSP preconditioned resid norm 1.709007477757e-07 true resid norm
8.455837132715e-07 ||Ae||/||Ax|| 1.900065399241e-05
Residual norm 8.45584e-07
Error norm 0.000370784
Error norm 9.54664e-05
Error norm 1.38935e-07

I do get better convergence on that example with -ksp_type stcg but
would appreciate any other recommendations for single precision linear
problems.

thanks,
Chris

>
>   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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