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Notes on how agglomeration works in MLMG #4602

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14 changes: 9 additions & 5 deletions Docs/sphinx_documentation/source/LinearSolvers.rst
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Expand Up @@ -380,17 +380,21 @@ Available choices are
- :cpp:`MLMG::BottomSolver::petsc`: Currently for cell-centered only.

- :cpp:`LPInfo::setAgglomeration(bool)` (by default true) can be used
continue to coarsen the multigrid by copying what would have been the
bottom solver to a new :cpp:`MultiFab` with a new :cpp:`BoxArray` with
fewer, larger grids, to allow for additional coarsening.
to copy the current level of multigrid data to fewer, larger
boxes. Two advantages of using this option is that the bottom solver will become
smaller, and communication overhead is reduced.

- :cpp:`LPInfo::setAgglomerationGridSize(int)` (by default 8 in 1D, 16 in 2D, 32 in 3D)
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If it's a GPU build, it's 32 be default. If it's a CPU build, it's 32 in 1D, 16 in 2D and 8 in 3D. (The order in the original commit is incorrect.)

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@ajnonaka -- do you want to go ahead and fix this PR then we can merge?

if agglomeration is used, when the average box size becomes smaller than agg_grid_size^{DIM},
boxes will agglomerate until this is no longer the case. Note that this action is
recursive and can happen at several different levels in the multigrid hierarchy.

- :cpp:`LPInfo::setConsolidation(bool)` (by default true) can be used
continue to transfer a multigrid problem to fewer MPI ranks.
There are more setting such as :cpp:`LPInfo::setConsolidationGridSize(int)`,
:cpp:`LPInfo::setConsolidationRatio(int)`, and
:cpp:`LPInfo::setConsolidationStrategy(int)`, to give control over how this
process works.

process works. If agglomeration is used, consolidation is ignored.

:cpp:`MLMG::setThrowException(bool)` controls whether multigrid failure results
in aborting (default) or throwing an exception, whereby control will return to the calling
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