On 30/01/14 17:45, Jed Brown wrote: > Boris Kaus <k...@uni-mainz.de> writes: > >> While doing multigrid it is possible to explicitly set restriction and >> prolongation/interpolation operators in PETSC using PCMGSetRestriction >> and PCMGSetInterpolation. In many cases, the restriction (R) and >> prolongation (P) operators are simply the transpose of each other >> (R=P’). >> >> Yet, in certain cases, for example variable viscosity Stokes with a >> staggered finite difference discretization, it is advantageous if they >> are different (see http://arxiv.org/pdf/1308.4605.pdf). >> >> Accordding to the Wesseling textbook, Galerkin coarsening of the fine >> grid matrix is defined as: >> >> Acoarse = R*A*P >> >> Yet, as far as I can tell, PETSc seems to implement this as: >> >> Acoarse = R*A*R’ > Certainly, the code just had an outdated assumption. I think I have > added support for this (it does R*A*P when R!=P), but I don't have a > test case readily available. Could you try branch > 'jed/pcmg-galerkin-rap' (if you were using v3.4/maint) or 'next' if > you've been tracking petsc-dev?
This question was similar to my recent unanswered one: http://lists.mcs.anl.gov/pipermail/petsc-users/2014-January/020382.html I just tried jed/pcmg-galerkin-rap, and it seems to work fineI verified that the coarse system matrix is R*A*P, for a two-level PCMG. I'm first calling PCMGSetInterpolation() with a matrix P, and then PCMGSetInterpolation() with a matrix R. Previously R was ignored, and I got A_c = P^T*A*P With jed/pcmg-galerkin-rap, I'm getting: A_c = R*A*P So now I can experiment with using R or P^T, which is very nice! I did not have an opportunity to test what happens if I don't set any restriction, but I'm hoping it will then use P^T, as before. Best regards Torquil Sørensen