[petsc-dev] Additive Schwarz Method + ILU on GPU platforms

[Matthew Knepley]

On Mon, Apr 14, 2025 at 7:47 PM Mark Adams <mfadams at lbl.gov> wrote:

> On Mon, Apr 14, 2025 at 5:39 PM Angus, Justin Ray via petsc-dev <
> petsc-dev at mcs.anl.gov> wrote:
>
>> Hello,
>>
>>
>>
>> A project I work on uses GMRES via PETSc. In particular, we have had good
>> successes using the Additive Schwarz Method + ILU preconditioner setup
>> using a CPU-based code. I found online where it is stated that “Parts of
>> most preconditioners run directly on the GPU” (
>> https://urldefense.us/v3/__https://petsc.org/release/faq/__;!!G_uCfscf7eWS!fm7OZhr_b9AIu2cEVb2FddEGQTnpwos8zXtgJpvMdCoX2pY6yVCfLsM6iFOQGNEkMjTrRFzTiFnKWR9rBWF8$ 
>> <https://urldefense.us/v3/__https://petsc.org/release/faq/__;!!G_uCfscf7eWS!bw6qeKcY7MKSvlEgcogdKR7fpjZSOFvka6zfDprUZ_sJHdE-YZmRD6UTqWQW3_uGVBII4P-AG0zaGTLbI67_fQ$>).
>> Is ASM + ILU also available for GPU platforms?
>>
>
> Yes, but it will be hard to get speedup from CPUs.
> It is easy to test though. Jacobi or pbjacobi if you have a vector
> equation is probably faster.
>

I assume you mean ILU(0), which is what PETSc supports. Computing it would
not be fast, but applying it is a MatMult() which is not bad. If you take
lots of iterates, then it will speedup like the bandwidth ratio. You could
even compute it on the CPU and apply it on the GPU, which would be good for
an initial test.

  Thanks,

     Matt


> Mark
>
>
>>
>>
>> -Justin
>>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

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