[petsc-users] GPU implementation of serial smoothers

[Barry Smith]

> On Jan 10, 2023, at 2:19 PM, Mark Lohry <mlohry at gmail.com> wrote:
> 
>> Is DILU a point-block method? We have -pc_type pbjacobi (and vpbjacobi if the node size is not uniform). The are good choices for scale-resolving CFD on GPUs.
> 
> I was hoping you'd know :)  pbjacobi is underperforming ilu by a pretty wide margin on some of the systems i'm looking at.
> 
>> We don't have colored smoothers currently in PETSc.
> 
> So what happens under the hood when I run -mg_levels_pc_type sor on GPU? Are you actually decomposing the matrix into lower and computing updates with matrix multiplications? Or is it just the standard serial algorithm with thread safety ignored?

  It is running the regular SOR on the CPU and needs to copy up the vector and copy down the result.
> 
> On Tue, Jan 10, 2023 at 1:52 PM Barry Smith <bsmith at petsc.dev <mailto:bsmith at petsc.dev>> wrote:
>> 
>>   We don't have colored smoothers currently in PETSc.
>> 
>> > On Jan 10, 2023, at 12:56 PM, Jed Brown <jed at jedbrown.org <mailto:jed at jedbrown.org>> wrote:
>> > 
>> > Is DILU a point-block method? We have -pc_type pbjacobi (and vpbjacobi if the node size is not uniform). The are good choices for scale-resolving CFD on GPUs.
>> > 
>> > Mark Lohry <mlohry at gmail.com <mailto:mlohry at gmail.com>> writes:
>> > 
>> >> I'm running GAMG with CUDA, and I'm wondering how the nominally serial
>> >> smoother algorithms are implemented on GPU? Specifically SOR/GS and ILU(0)
>> >> -- in e.g. AMGx these are applied by first creating a coloring, and the
>> >> smoother passes are done color by color. Is this how it's done in petsc AMG?
>> >> 
>> >> Tangential, AMGx and OpenFOAM offer something called "DILU", diagonal ILU.
>> >> Is there an equivalent in petsc?
>> >> 
>> >> Thanks,
>> >> Mark
>> 

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