[petsc-dev] MatComputeExplicitOperator

[Smith, Barry F.]

> On Apr 20, 2019, at 1:30 PM, Stefano Zampini <stefano.zampini at gmail.com> wrote:
> 
> Using preallocator will require looping twice for the computation of the entries,

   No, that is totally impractical. The old code that Jed removed just preallocated the AIJ as dense and this was fine for this purpose

1) MatConvert_Shell() should call all the preallocating possibilities (treating as dense)
2) MatComputeExplicitOperator() could get a new second argument that is a matrix type (then we don't need to argue 
    if it should use DENSE or AIJ).



   Barry

> and this may be very expensive.
> In my case, every MatMult requires one forward and one backward TS solve.


> 
> We can either use AIJ or DENSE format, but I think this should be the same for the sequential or parallel runs, not as it is now.
> 
> Il giorno sab 20 apr 2019 alle ore 14:20 Matthew Knepley <knepley at gmail.com> ha scritto:
> On Fri, Apr 19, 2019 at 5:58 PM Smith, Barry F. via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
> 
> 
>    I think MPIAIJ was selected because it provided the most parallel functionality compared to MPIDENSE for which more operations were not written.
>    This may not be relevant any more. 
> 
>     Definitely the code needs to be fixed. Fixing MatConvert_Shell(); it should just assume 
> the matrices are dense and preallocate for them.
> 
> We should restore preallocation. It should be easy to do with Preallocator.
> 
>   Matt
>  
>    Barry
> 
> > On Apr 19, 2019, at 3:07 PM, Stefano Zampini via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
> > 
> > What is the rationale behind MatComputeExplicitOperator returning SEQDENSE in sequential and MPIAIJ in parallel?
> > 
> > Also, before commit https://bitbucket.org/petsc/petsc/commits/b3d09e869df0e6ebcb615ca876706bfed4fcf1cd  full preallocation of the MPIAIJ matrix was happening. Now, if we have a very dense operator that we want to sample just for testing purposes (inspecting entries, etc..) we have to pay the price of reallocating over and over.
> > 
> > What is the proper fix? 1) Use MPIDENSE? 2) Restore full preallocation for MPIAIJ? 3) Have MatComputeExplicitOperator to accept more arguments ? 
> > 
> > I'm in favor of 1
> > 
> > -- 
> > Stefano
> 
> 
> 
> -- 
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener
> 
> https://www.cse.buffalo.edu/~knepley/
> 
> 
> -- 
> Stefano