[petsc-dev] I think TS should have a TSSetMassMatrix()

[Barry Smith]

> On Jul 24, 2017, at 5:03 AM, Stefano Zampini <stefano.zampini at gmail.com> wrote:
> 
> 
> 
> 2017-07-24 11:26 GMT+03:00 Lisandro Dalcin <dalcinl at gmail.com>:
> On 23 July 2017 at 21:42, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> >    Then explicit methods could automatically use a solve to apply the mass matrix inverse for each "right hand side" application and implicit methods could automatically "drop in" the mass matrix with the shift when a right hand side Jacobian is given also. Instead of requiring the user to handle this themselves. Are there any gotcha's that I am missing or is this relatively straightforward?
> >
> 
> 
> IMHO I think that it is not worth the complication. I value the generality of the current API, that already is somewhat cumbersome to maintain.
> Users providing the RHS function + mass matrix already know that they should do M^-1 * rhsvec to compute the rhs function.

   Then they get wrong or confusing information with TSAdjoint, not a good thing.


> Same for the RHS jacobian.
> 
>  
> 
> >    Why wouldn't we have this interface?
> >
> 
> 
> What if the user provides MassMatrix + IFunction/IJacobian + RHSFunction/RHSJacobian? Are you going to handle all the possible scenarios? Error? dump a warning with PetscInfo? Users that will need such a new API usually don't read PetscInfo output

   Error

> 
>  
> As usual, the problem is on defining the interface and the trade on
> performance and generality. But I'm definitely +1 on this.
> 
> >     Almost for sure MFEM must have this type of API.
> >
> 
> 
> No. AFAIK, sundials has it, not MFEM.
> Natively in MFEM, they just suppors explicit

   Sure but this can still be an explicit method (though it has  a solve in it).

   But I thought they were solving mass matrix systems all over the place? One of the CEED benchmarks is solving a mass matrix system.


> or SDIRK. They also have interface to TS and Sundials.
> 
> See https://github.com/mfem/mfem/blob/master/examples/ex9.cpp#L329 for native explicit ode solvers in MFEM.
> See here for the virtual method that allows using SDIRK https://github.com/mfem/mfem/blob/master/examples/ex10.cpp#L91
> Here for sundials https://github.com/mfem/mfem/tree/master/examples/sundials
> 
> With that said, if you want to use TS through MFEM, here you have an example https://github.com/mfem/mfem/blob/master/examples/petsc/ex9p.cpp, which is meant to be illustrative of the capabilities of the IFunction/RHSFunction (and jacobians) API in PETSc, not efficiency.
> 
>  
> Stefano will love it for sure...
> 
> 
> --
> Lisandro Dalcin
> ============
> Research Scientist
> Computer, Electrical and Mathematical Sciences & Engineering (CEMSE)
> Extreme Computing Research Center (ECRC)
> King Abdullah University of Science and Technology (KAUST)
> http://ecrc.kaust.edu.sa/
> 
> 4700 King Abdullah University of Science and Technology
> al-Khawarizmi Bldg (Bldg 1), Office # 0109
> Thuwal 23955-6900, Kingdom of Saudi Arabia
> http://www.kaust.edu.sa
> 
> Office Phone: +966 12 808-0459
> 
> 
> 
> -- 
> Stefano