[petsc-users] Guidance on GAMG preconditioning

[Mark Adams]

>
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> The overwhleming cost of AMG is the Galerkin triple-product RAP.
>
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That is overstating it a bit.  It can be if you have a hard 3D operator and
coarsening slowly is best.

Rule of thumb is you spend 50% time is the solver and 50% in the setup,
which is often mostly RAP (in 3D, 2D is much faster).  That way you are
within 2x of optimal and it often works out that way anyway.

Mark
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