Thanks, Barry. I'll look into it.
Original message From: Barry Smith Date:
5/2/23 6:10 PM (GMT-05:00) To: "Takahashi, Tadanaga" Cc:
PETSc Subject: Re: [petsc-users] Help using FAS as an
initial guess You might consider
https://petsc.org/release/manualpages/SNES/SNESSetGridSequence/ it does exactly
what I think you want to do. FAS is a bit more subtle than that. The "coarse
grid problem" that FAS builds and solves are dependent on the current fine grid
solution so you need an "approximate" fine grid solution already in order to
create a FAS coarse problem. Of course FAS can, and often should, be used also
with grid sequencing to boot-strap the fine grid solutions. BarryOn May 2,
2023, at 5:40 PM, Takahashi, Tadanaga wrote:Hi, I want to know
how to configure the FAS so that it solves a problem on a coarse grid of size
4h, interpolate the solution, and then stop. Here is the context: I am using
Newton LS to solve a problem on square domain discretized with DMDA meshed with
step size h. I have a subroutine to compute the initial guess. I want this
subroutine to first do a Newton solve on a coarse grid of size 4h. Then it
interpolates the solution to the main mesh. I think this is achievable by using
one iteration of FAS. Below is the gist of what my subroutine looks like:
PetscErrorCode InitialState(DM da, Vec u) { // SNES snes;
SNESCreate(PETSC_COMM_WORLD,);
SNESSetDM(snes,da);SNESSetType(snes,SNESFAS); // solve with
multigridSNESSetTolerances(snes,PETSC_DEFAULT,PETSC_DEFAULT,PETSC_DEFAULT,1,PETSC_DEFAULT);
// just one iterationVecSet(u,0.0); // start with zeros
SNESSolve(snes,NULL,u); // cheap solve SNESGetSolution(snes,); // extract
solution }For some reason, my initial guess is too accurate. The initial guess
produced by this subroutine looks exactly like the final solution. My guess is
that it's doing more than what I want it to do. I'm still new to FAS. How can I
tell FAS to do just one crude solve and an interpolation?
