[petsc-dev] I do not think this is the right solution

[Matthew Knepley]

On Sun, Mar 18, 2012 at 6:37 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Sun, Mar 18, 2012 at 18:24, Matthew Knepley <knepley at gmail.com> wrote:
>
>> In the alternate model, you call MatAssemblyBegin/End() when the matrix
>> is marked as unassembled. Its easy to check this flag, so the user
>> will not be confused. Barry is correct that the flag reset is not
>> collective, so that query would have to be.
>>
>
> We should think as though we are in a world where the query costs as much
> as the assembly itself because that is a better approximation of reality
> than the alternatives.
>
>
>> I do not think "fragile" is an accurate characterization because failure
>> is immediate when trying to use a function that requires assembly, and it
>> also obvious what the fix is, to assemble the matrix.
>>
>
> It's not collective.
>
>
>>
>> You might mean that the existing model is more intuitive. I disagree
>> since it is currently impossible to do something simple like
>> chop out small values from a matrix.
>>
>
> MatGetArray()
>

Of course I fucking tried that first. It does not work.

   Matt


> Or you actually want to remove them from the nonzero pattern? In that
> case, you definitely have to assemble, which means there is a new
> partly-assembled state (locally consistent, but not globally).
>
>
>> The proposal is to augment the current model with MatGetRowWrite(). This
>> would mean
>> not only adding to the interface, but writing all the code to put values
>> back into the matrix if the row is a copy. I think it is more
>> intuitive to allow the user to SetValues() if the nonzero pattern is
>> unchanged and no values need to be communicated.
>>
>
> MatSetValues() is a very permissive interface for modifying data in-place.
>



-- 
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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