Global Coarsening GMG

[tjhei]

This issue keeps track of the ongoing work to implement and improve the "global coarsening" GMG solver.

The global coarsening GMG has been introduced in https://dl.acm.org/doi/10.1145/3580314 and included an ASPECT prototype and shown to be 2-3x faster than the current GMG implementation for meshes which make use of adaptive refinement. For globally refined meshes, both methods behave the same way.

TODO list:

• The first working version. Implement global coarsening GMG #6403
• tangential / free slip Allow tangential boundary conditions for GMG global coarsening. #6752
• other boundary conditions:

  aspect/source/simulator/solver/stokes_matrix_free_global_coarsening.cc

  Line 1644 in 6351112

  Assert(this->get_boundary_velocity_manager().get_active_plugins().size() == 0,

• "no averaging" support GMG-GC: implement "no averaging" #6707
• free surface support GMG-GC: implement free surface #6706
• periodic boundaries
• refactor code to maximize code reuse between local smoothing and global coarsening
• unify CellData filling between local smoothing and global coarsening
• advertise in the documentation
• add tests / benchmarks
• scaling tests