[petsc-dev] DMCreateMatrix_Shell

[Stefano Zampini]

> On Feb 9, 2019, at 10:19 PM, Smith, Barry F. <bsmith at mcs.anl.gov> wrote:
> 
> 
> 
>> On Feb 9, 2019, at 11:48 AM, Stefano Zampini <stefano.zampini at gmail.com> wrote:
>> 
>> 
>> 
>>> On Feb 9, 2019, at 11:17 AM, Smith, Barry F. <bsmith at mcs.anl.gov> wrote:
>>> 
>>> 
>>> Hmm, why is this check wrong (though it may be incomplete)? Is there an example where it triggers an error when it shouldn’t?
>> 
>> Suppose in SNESSetJacobian  you don’t pass a matrix, and you don’t use a DM. DMShell will give you back an AIJ matrix by default.
>> Then, In the callback, you call MatSetType (which I presume is legal) on the matrix, with a type different than {MPI | Seq}AIJ
>> If you then call
>> SNESGetDM(snes,&dm)
>> DMCreateMatrix(dm,&A)\\ error!
> 
>   Right, this is exactly what it is suppose to do. Otherwise you request a new matrix type but it returns a matrix with the default AIJ matrix, which is wrong.
> 

My point is that, instead of raising the error, it should return the matrix of the same type I have set in the callback
I know I can call DMSetMatType in the callback, but on the other hand, isn’t DMShell supposed to obey user choices?

And I still don’t understand why an AIJ is fine and, as Matt pointed out, a CUSPARSE is not. Are we discriminating? :-)

>   Barry
> 
>> 
>>> 
>>> It is my understanding that the check is there to make sure if the user requested a particular matrix type then it won't return a different type that is unexpected for the user (the AIJ checks are just to handle MATAIJ vs MATSEQAIJ, MATMPIAIJ).
>>> 
>>> Barry
>>> 
>>> 
>>>> On Feb 8, 2019, at 5:29 AM, Stefano Zampini via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
>>>> 
>>>> I don’t understand the need of these checks (tacky and incomplete…, as per the comment) 
>>>> 
>>>> https://bitbucket.org/petsc/petsc/src/75aeaa18afd73be7f3020f94b788241b9e0ba3d7/src/dm/impls/shell/dmshell.c#lines-197
>>>> 
>>>> Why are these checks needed? I’m inclined to remove these lines, but I would like some comments before doing it.
>>> 
>> 
>