[petsc-dev] DMGetMatrix --> DMGetMatrices?

[Jed Brown]

On Fri, Feb 10, 2012 at 19:33, Dmitry Karpeev <karpeev at mcs.anl.gov> wrote:

> How can there be a single callback, when the two DMs could be of different
> types?
> I guess it could be done using the double-dispatch mechanism:
> DMComputeOperators(DM dmJ, DM dmP, Mat *J, Mat *P) would dispatch based on
> the types of the two DMs,
> and, if no such method exists, defaulting internally to calling
> DMComputeMatrix(DM dm, Mat *A) for both J and P.
>

Or call the user's function which just assembles into the P matrix.


> I still think that DMComputeMatrix() should be different from
> DMCreateMatrix() -- the latter just preallocates --
> eliminating the need for DMSetPreallocateOnly().
>

How many times do we have to explain that DMCreateMatrix() *NEVER* fills in
useful values? It has two capabilities: preallocate (just specify number of
nonzeros per row) and also setting the column indices. It does both by
default because that is better for performance profiling and better to
catch setting values outside of the allocated sparsity pattern at the first
incident instead of whenever a row overflows. Under no circumstances does
it assemble physics and under no circumstances does DMComputeJacobian()
allocate a matrix.


>
> If the two DMs are of the same type and, say, share a mesh, then the dual
> dispatch can take advantage of
> a corresponding implementation.
>

The user should always be able to get the callback to do both.
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