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

[Smith, Barry F. via petsc-dev]

> On Mar 11, 2019, at 1:28 PM, Isaac, Tobin G <tis...@cc.gatech.edu> wrote:
> 
> 
> 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?

   I'm fine with it always returning the composite matrix unless you have a 
specific use case that would require MAT_INITIAL_MATRIX_GET_VALUES

   Barry

> 
> 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@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 <knep...@gmail.com> wrote:
>>>> On Sat, Mar 9, 2019 at 4:42 PM Isaac, Tobin G via petsc-dev <
>>>> petsc-dev@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
>>>>> 
>>>>> 
>>>>> 
>>