[petsc-dev] MatSetValues with v=0

[Matthew Knepley]

On Wed, Apr 16, 2014 at 9:00 AM, Eric Chamberland <
Eric.Chamberland at giref.ulaval.ca> wrote:

> On 04/16/2014 09:44 AM, Jed Brown wrote:
>
>> Eric Chamberland <Eric.Chamberland at giref.ulaval.ca> writes:
>>
>>> I was writing (new) code which do the firsts MatSetValues after the
>>> MatXAIJSetPreallocation.  We have to do this because the "real"
>>> non-zeros will be added later by a mix of ADD_VALUES and
>>> INSERT_VALUES... which would prevent us to "lock" the matrix
>>> (MatSetOption(aMatricePETSc, MAT_NEW_NONZERO_LOCATION_ERR, PETSC_TRUE)
>>> because this option must be passed after the first MatAssemblyEnd()...
>>> but all the non-zeros are "triggered" only after this "mixed" assembly...
>>>
>>> In other words, we have to do a "fake" assembly with all "0" to cover
>>> all non-zeros that will be in fact added later... So we have to create
>>> many "fake" elementary matrices to pass for assembly... that is why
>>> Patrick was asking if the feature was supported... It would save us from
>>> creating the fake matrices...  Or maybe there is a better solution?...
>>>
>> Just create one large-enough buffer containing all zeros.  For example,
>>
>>    PetscScalar *values;
>>    PetscCalloc1(1000,&values);
>>
>>    for (...) {
>>      MatSetValues(...,values,INSERT_VALUES);
>>    }
>>    PetscFree(values);
>>
>> Is this okay?
>>
> Yes this can be ok.
>
> I have a new question now : what would be the fastest way to do the first
> assembly: with INSERT_VALUES or ADD_VALUES?  Right now we are doing
> ADD_VALUES but I imagine it have to hold all the "0" which will be "added"
> later... maybe "INSERT_VALUES" will forget about overlapping values??...
>  Anyway, the code I am rewriting now will hold all the non-zeros for each
> "block" of similar lines... so I don't have overlapping values... in
> sequential... but in parallel I think I will have some... Anyway, any
> pros/cons between INSERT_VALUES or ADD_VALUES for this first assembly?
>

There should not be any significant difference. I would use INSERT.

   Matt


> Thanks,
>
> Eric
>
>
>
>


-- 
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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