[firedrake] More detailled breakdown of PETSc timings / higher order geometric MG

[Eike Mueller]

sorry, those times were with an unoptimised PETSc.

> Data I have currently:
> In the matrix-free solver, one velocity mass matrix inverse costs 2.27s, and I need two per iteration just for the forward/backward substitution. On the other hand, one GMRES iteration of the PETSc solver (which includes everything: applying the mixed operator, solving the pressure system, inverting the velocity mass matrices) takes 3.87s, so something is not right there.

If I use optimised PETSc, I get ~0.8s for one velocity mass matrix solve in the matrix-free solver (and a total time per iteration of 3.2s). The time per iteration in the PETSc solver with AMG preconditioner is 0.8s.

Thanks,

Eike