[petsc-dev] Multigrid is confusing

[Jed Brown]

~/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?
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