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

[Jed Brown]

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()

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.
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