[petsc-users] [EXTERNAL] GPU implementation of serial smoothers

[Zhang, Chonglin]

I am using the following in my Poisson solver running on GPU, which were suggested by Barry and Mark (Dr. Mark Adams).
  -ksp_type cg
  -pc_type gamg
  -mg_levels_ksp_type chebyshev
  -mg_levels_pc_type jacobi

On Jan 10, 2023, at 3:31 PM, Mark Lohry <mlohry at gmail.com> wrote:

So what are people using for GAMG configs on GPU? I was hoping petsc today would be performance competitive with AMGx but it sounds like that's not the case?

On Tue, Jan 10, 2023 at 3:03 PM Jed Brown <jed at jedbrown.org<mailto:jed at jedbrown.org>> wrote:
Mark Lohry <mlohry at gmail.com<mailto:mlohry at gmail.com>> writes:

> I definitely need multigrid. I was under the impression that GAMG was
> relatively cuda-complete, is that not the case? What functionality works
> fully on GPU and what doesn't, without any host transfers (aside from
> what's needed for MPI)?
>
> If I use -ksp-pc_type gamg -mg_levels_pc_type pbjacobi -mg_levels_ksp_type
> richardson is that fully on device, but -mg_levels_pc_type ilu or
> -mg_levels_pc_type sor require transfers?

You can do `-mg_levels_pc_type ilu`, but it'll be extremely slow (like 20x slower than an operator apply). One can use Krylov smoothers, though that's more synchronization. Automatic construction of operator-dependent multistage smoothers for linear multigrid (because Chebyshev only works for problems that have eigenvalues near the real axis) is something I've wanted to develop for at least a decade, but time is always short. I might put some effort into p-MG with such smoothers this year as we add DDES to our scale-resolving compressible solver.

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