[petsc-dev] MATCOMPOSITE as fallback for matrix-matrix multiplication

[Isaac, Tobin G]

Okay, but do you have an opinion on the original issue: should we
always return a matrix from various matrix-matrix routines, even if it
has limited functionality?

On Mon, Mar 11, 2019 at 06:09:55PM +0000, Smith, Barry F. wrote:
> 
>    Sounds like overkill on the complexity meter.
> 
>    Barry
> 
> 
> > On Mar 11, 2019, at 6:15 AM, Isaac, Tobin G via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
> > 
> > One thought: we could extend MatReuse with MAT_INITIAL_MATRIX_GET_VALUES to indicate that a matrix that implements MatGetValues() is desired, to allow some user intent.
> > 
> > 
> > On March 9, 2019 7:44:06 PM EST, Matthew Knepley <knepley at gmail.com> wrote:
> >> On Sat, Mar 9, 2019 at 4:42 PM Isaac, Tobin G via petsc-dev <
> >> petsc-dev at mcs.anl.gov> wrote:
> >> 
> >>> 
> >>> Let's say I want to do Newton-Krylov on some DAG of computations.
> >>> Building the Jacobian is programmatic from the chain rule, requiring
> >>> just a lot of matrix-matrix multiplications in the correct sequence.
> >>> 
> >>> If it really is more efficient for two Jacobians to be contracted
> >> into
> >>> a matrix with random access, that's great, but if it's not, or if
> >>> the appropriate implementation of MatMatMult_X_Y does not exist,
> >>> having a MATCOMPOSITE is better than nothing.
> >>> 
> >>> So why not make MATCOMPOSITE something that our matrix-matrix
> >>> multiplication routines return when a specialization isn't found?
> >>> It moves the "Not Implemented" warning from MatMatMult() to
> >>> whenever I try to do something other than multiply with the output,
> >>> but in some situations that's desirable.
> >>> 
> >>> I'd be interested to hear what people think.
> >>> 
> >> 
> >> I think this is a good idea. The most important thing is to maintain
> >> transparency, so that the output
> >> makes this clear in -ksp_view and we are easily able to see what
> >> someone
> >> has produced.
> >> 
> >>  Matt
> >> 
> >> 
> >>> Thanks,
> >>>  Toby
> >>> 
> >>> 
> >>> 
>