[petsc-users] How to efficiently fill in, in parallel, a PETSc matrix from a COO sparse matrix?

[Mark Adams]

ex2 looks the same as the code at the beginning of the thread, which looks
fine to me, yet fails.
(the only thing I can think of is that &v.at(i) is not doing what one wants)

Diego: I would start with this ex2.c, add your view statement, verify;
incrementally change ex2 to your syntax and see where it breaks.

Mark

On Wed, Jun 21, 2023 at 9:50 AM Matthew Knepley <knepley at gmail.com> wrote:

> On Wed, Jun 21, 2023 at 9:22 AM Diego Magela Lemos <diegomagela at usp.br>
> wrote:
>
>> Please, could you provide a minimal working example (or link) of how to
>> do this?
>>
>
> You can see here
>
>   https://petsc.org/main/src/ksp/ksp/tutorials/ex2.c.html
>
> that each process only sets values for the rows it owns.
>
>   Thanks,
>
>      Matt
>
>
>> Thank you.
>>
>> Em ter., 20 de jun. de 2023 às 15:08, Matthew Knepley <knepley at gmail.com>
>> escreveu:
>>
>>> On Tue, Jun 20, 2023 at 2:02 PM Diego Magela Lemos <diegomagela at usp.br>
>>> wrote:
>>>
>>>> So... what do I need to do, please?
>>>> Why am I getting wrong results when solving the linear system if the
>>>> matrix is filled in with MatSetPreallocationCOO and MatSetValuesCOO?
>>>>
>>>
>>> It appears that you have _all_ processes submit _all_ triples (i, j, v).
>>> Each triple can only be submitted by a single process. You can fix this in
>>> many ways. For example, an easy but suboptimal way is just to have process
>>> 0 submit them all, and all other processes submit nothing.
>>>
>>>   Thanks,
>>>
>>>   Matt
>>>
>>>
>>>> Em ter., 20 de jun. de 2023 às 14:56, Jed Brown <jed at jedbrown.org>
>>>> escreveu:
>>>>
>>>>> Matthew Knepley <knepley at gmail.com> writes:
>>>>>
>>>>> >> The matrix entries are multiplied by 2, that is, the number of
>>>>> processes
>>>>> >> used to execute the code.
>>>>> >>
>>>>> >
>>>>> > No. This was mostly intended for GPUs, where there is 1 process. If
>>>>> you
>>>>> > want to use multiple MPI processes, then each process can only
>>>>> introduce
>>>>> > some disjoint subset of the values. This is also how MatSetValues()
>>>>> works,
>>>>> > but it might not be as obvious.
>>>>>
>>>>> They need not be disjoint, just sum to the expected values. This
>>>>> interface is very convenient for FE and FV methods. MatSetValues with
>>>>> ADD_VALUES has similar semantics without the intermediate storage, but it
>>>>> forces you to submit one element matrix at a time. Classic parallelism
>>>>> granularity versus memory use tradeoff with MatSetValuesCOO being a clear
>>>>> win on GPUs and more nuanced for CPUs.
>>>>>
>>>>
>>>
>>> --
>>> 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
>>>
>>> https://www.cse.buffalo.edu/~knepley/
>>> <http://www.cse.buffalo.edu/~knepley/>
>>>
>>
>
> --
> 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
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
>
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