[petsc-users] GPU implementation of serial smoothers

[Mark Lohry]

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> wrote:

> Mark Lohry <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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