I can do exactly what I want using SNESLineSearchPrecheck and -snes_linesearch_type basic ... I just can't use any more exotic line searches
On Thu, Nov 30, 2023 at 1:22 PM Alexander Lindsay <[email protected]> wrote: > If someone passes me just L, where L represents the "global" degrees of > freedom, in this case they represent unknowns on the trace of the mesh, > this is insufficient information for me to evaluate my function. Because in > truth my degrees of freedom are the sum of the trace unknowns (the unknowns > in the global solution vector) and the eliminated unknowns which are > entirely local to each element. So I will say my dofs are L + U. > > I start with some initial guess L0 and U0. I perform a finite element > assembly procedure on each element which gives me things like K_LL, K_UL, > K_LU, K_UU, F_U, and F_L. I can do some math: > > K_LL = -K_LU * K_UU^-1 * K_UL + K_LL > F_L = -K_LU * K_UU^-1 * F_U + F_L > > And then I feed K_LL and F_L into the global system matrix and vector > respectively. I do something (like a linear solve) which gives me an > increment to L, I'll call it dL. I loop back through and do a finite > element assembly again using **L0 and U0** (or one could in theory save off > the previous assemblies) to once again obtain the same K_LL, K_UL, K_LU, > K_UU, F_U, F_L. And now I can compute the increment for U, dU, according to > > dU = K_UU^-1 * (-F_U - K_UL * dL) > > Armed now with both dL and dU, I am ready to perform a new residual > evaluation with (L0 + dL, U0 + dU) = (L1, U1). > > The key part is that I cannot get U1 (or more generally an arbitrary U) > just given L1 (or more generally an arbitrary L). In order to get U1, I > must know both L0 and dL (and U0 of course). This is because at its core U > is not some auxiliary vector; it represents true degrees of freedom. > > On Thu, Nov 30, 2023 at 12:32 PM Barry Smith <[email protected]> wrote: > >> >> Why is this all not part of the function evaluation? >> >> >> > On Nov 30, 2023, at 3:25 PM, Alexander Lindsay < >> [email protected]> wrote: >> > >> > Hi I'm looking at the sources, and I believe the answer is no, but is >> there a dedicated callback that is akin to SNESLineSearchPrecheck but is >> called before *each* function evaluation in a line search method? I am >> using a Hybridized Discontinuous Galerkin method in which most of the >> degrees of freedom are eliminated from the global system. However, an >> accurate function evaluation requires that an update to the "global" dofs >> also trigger an update to the eliminated dofs. >> > >> > I can almost certainly do this manually but I believe it would be more >> prone to error than a dedicated callback. >> >>
