Apply mlmg to compute fluxes and gradient

[ZiyangHuang]

Greetings,

If my understanding is correct, I need to first call mlmg.solve(...) to obtain the solution of a linear solve, and then call mlmg.getFluxes(...) and mlmg.getGradSolution(...) to obtain the fluxes and gradient.

I am aware that there is another member function:

void amrex::MLMGT< MF >::getFluxes (  const Vector< Array< AMF *, AMREX_SPACEDIM > > &  a_flux,
                                                                 const Vector< AMF * > &  a_sol,
                                                                Location  a_loc = Location::FaceCenter
                                                              )

to compute the fluxes with input a_sol. Can I use this function to compute fluxes related to a MLLinOp without the need of calling mlmg.solve(...) beforehand?

However, I could not find a similar version of member function for getGradSolution. Does it mean that I always need to call mlmg.solve() before I compute the gradient?

Thanks!

[WeiqunZhang]

Yes, you have to call solve. There is no guarantee that getFluxes and getGradSolution could work without calling solve first. However, you could call solve with a huge tolerance that no iteration will be needed. You could also call LPInfo::setMaxCoarseningLevel(0) to reduce the setup cost of the LinOp.