[petsc-users] GAMG and linear elasticity

[Jed Brown]

Tabrez Ali <stali at geology.wisc.edu> writes:

> Hello
>
> What is the proper way to use GAMG on a vanilla 3D linear elasticity 
> problem. Should I use
>
> -pc_type gamg -pc_gamg_type agg -pc_gamg_agg_nsmooths 1

Yeah, and only the first of these is needed because the others are
default with -pc_type gamg.

> -pc_type fieldsplit -pc_fieldsplit_block_size 3 -fieldsplit_pc_type gamg 
> -fieldsplit_pc_gamg_type agg -fieldsplit_pc_gamg_agg_nsmooths 1
>
> Do these options even make sense? With the second set of options the % 
> increase in number of iterations with increasing problem size is lower 
> than the first but not optimal.

And it's probably more expensive because it has to do inner solves.
Also, if you have less compressible regions, it will get much worse.

> Also, ksp/ksp/examples/ex56 performs much better in that the number of 
> iterations remain more or less constant unlike what I see with my own 
> problem. What am I doing wrong?

You probably forgot to set the near null space.  You can use
MatSetNearNullSpace (and maybe MatNullSpaceCreateRigidBody) or the more
hacky (IMO) PCSetCoordinates.  It's important to have translational
*and* rotational modes in the near null space that GAMG uses to build a
coarse space.
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