[petsc-dev] features of GAMG

[Jed Brown]

On Sun, Dec 25, 2011 at 14:02, Mark F. Adams <mark.adams at columbia.edu>wrote:

> I've looked at ksp/ksp/examples/tutorial/ex56 and the modification that I
> make to emin and emax seem to be essential and very useful, respectively.
>  But I am seeing something funny with Cheb.
>

Are you comparing to "-mg_levels_ksp_chebychev_estimate_eigenvalues
0,0.1,0,1.1" or do you mean relative to some naive Cheby configuration?


>
> Jed:  I'm seeing very little difference in convergence rates on ex56 when
> I use a first, second or fourth order Cheb smoother.  I've seen some cases
> where the final residual was slightly larger with a higher order smoother.
>  I think there is something wrong here ...
>

Can you show me an example? I have certainly seen it make a different for
other problems.


>
> Also, I thought we had added something like -pc_mg_smooth to set the
> number of smoothing steps when you are using the same smoother for the up
> and down smoother.  I can not see a command for this.  I did '-help | grep
> smooth', and I tried a few things.  So to test this you need to manually
> fix the smoothing steps at about line 662 in gamg.c.
>

We would normally use something like -mg_levels_ksp_max_it 3. Or are you
talking about applying first-order Cheby multiple times?
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