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Hello,
I'd like to solve a Stokes-like equation with PETSc, i.e. div( mu * symgrad(u) ) = -grad(p) - grad(mu*q) div(u) = q with the spatially variable coefficients (mu, q) coming from another application, which will advect and evolve fields via the velocity field u from the Stokes solution, and throw back new (mu, q) to PETSc in a loop, everything using finite difference. In preparation for this and getting used to PETSc I wrote a simple inhomogeneous coefficient Poisson solver, i.e. div (mu*grad(u) = -grad(mu*q), u unknown, based on src/ksp/ksp/tutorials/ex32.c which converges really nicely even for mu contrasts of 10^10 using -ksp_type fgmres -pc_type mg. Since my coefficients later on can't be calculated from coordinates, I put them on a separate DM and attached it to the main DM via PetscObjectCompose and used a DMCoarsenHookAdd to coarsen the DM the coefficients live on, inspired by src/ts/tutorials/ex29.c . Adding another uncoupled DoF was simple enough and it converged according to -ksp_converged_reason, but the solution started looking very weird; roughly constant for each DoF, when it should be some function going from roughly -value to +value due to symmetry. This doesn't happen when I use a direct solver ( -ksp_type preonly -pc_type lu -pc_factor_mat_solver_type umfpack ) and reading the archives, I ought to be using -pc_type fieldsplit due to the block nature of the matrix. I did that and the solution looked sensible again. Now here comes the actual problem: Once I try adding multigrid preconditioning to the split fields I get errors probably relating to fieldsplit not "inheriting" (for lack of a better term) the associated interpolations/added DMs and hooks on the fine DM. That is, when I use the DMDA_Q0 interpolation, fieldsplit dies because it switches to DMDA_Q1 and the size ratio is wrong ( Ratio between levels: (mx - 1)/(Mx - 1) must be integer: mx 64 Mx 32 ). When I use DMDA_Q1, once the KSP tries to setup the matrix on the coarsened problem the DM no longer has the coefficient DMs which I previously had associated with it, i.e. PetscCall(PetscObjectQuery((PetscObject)da, "coefficientdm", (PetscObject *)&dm_coeff)); puts a NULL pointer in dm_coeff and PETSc dies when trying to get a named vector from that, but it works nicely without fieldsplit. Is there some way to get fieldsplit to automagically "inherit" those added parts or do I need to manually modify the DMs the fieldsplit is using? I've been using KSPSetComputeOperators since it allows for re-discretization without having to manage the levels myself, whereas some more involved examples like src/dm/impls/stag/tutorials/ex4.c build the matrices in advance when re-discretizing and set them with KSPSetOperators, which would avoid the problem as well but also means managing the levels. Any advice concerning solving my target Stokes-like equation is welcome as well. I am coming from a explicit timestepping background so reading up on saddle point problems and their efficient solution is all quite new to me. Best regards, Marco
