[petsc-dev] Multigrid is confusing

[Barry Smith]

  How about a default nonlinear Krylov accelerator? fgrmes is not much more expensive than gmres under causal circumstances.

   Barry

On May 22, 2012, at 7:54 PM, Jed Brown wrote:

> ~/petsc/src/snes/examples/tutorials$ ./ex5 -da_refine 4 -ksp_monitor_true_residual -pc_type mg -snes_max_it 1 -ksp_rtol 1e-8
>     0 KSP preconditioned resid norm 5.024049081009e+00 true resid norm 1.141253746897e+00 ||r(i)||/||b|| 1.000000000000e+00
>     1 KSP preconditioned resid norm 2.430101772479e-01 true resid norm 3.124735997267e-02 ||r(i)||/||b|| 2.737985312875e-02
>     2 KSP preconditioned resid norm 1.925127800529e-03 true resid norm 1.276301220139e-03 ||r(i)||/||b|| 1.118332556287e-03
>     3 KSP preconditioned resid norm 3.150352958424e-05 true resid norm 2.212937904665e-03 ||r(i)||/||b|| 1.939041085896e-03
>     4 KSP preconditioned resid norm 5.059562742413e-07 true resid norm 2.180293054518e-03 ||r(i)||/||b|| 1.910436710895e-03
>     5 KSP preconditioned resid norm 1.095269606918e-08 true resid norm 2.180508594793e-03 ||r(i)||/||b|| 1.910625573605e-03
> 
> I think we need to make default -pc_type mg linear. We talked about this before, but now we are getting ready for the release and that's bound to cause some turmoil.
> 
> Thoughts?