[petsc-dev] DMCreateMatrix_Shell

[Smith, Barry F.]

> 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.

   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.
>> 
>