[petsc-dev] using typedef for each different memory space the variable lives in

[Jed Brown]

Barry Smith <bsmith at petsc.dev> writes:

>     Yes, it is generally for arrays, but also makes sense for local variables inside a kernel; just makes the code clearer. For example 
>
>   Kokkos::parallel_for(Kokkos::RangePolicy<ExecutionSpace>(0,nx), KOKKOS_LAMBDA (const PetscInt &i)
>     {
>       PetscKokkosReal uvleft  = eval(U,0, i, 1, .5);
>       PetscKokkosReal uvright = eval(U,0, i+1, 1, -.5);
>       PetscKokkosReal uwleft  = eval(U,1, i, 1, .5);
>       PetscKokkosReal uwright = eval(U,1, i+1, 1, -.5);
>       flux(0,i, 1) = -.5*(uwleft+uwright)+.5*alpha_LF*(uvleft-uvright);
>       flux(1,i, 1) = -.5*(uvleft+uvright)+.5*alpha_LF*(uwleft-uwright);
>     });
>
> vs
>
>   Kokkos::parallel_for(Kokkos::RangePolicy<ExecutionSpace>(0,nx), KOKKOS_LAMBDA (const PetscInt &i)
>     {
>       PetscReal uvleft  = eval(U,0, i, 1, .5);
>       PetscReal uvright = eval(U,0, i+1, 1, -.5);
>       PetscReal uwleft  = eval(U,1, i, 1, .5);
>       PetscReal uwright = eval(U,1, i+1, 1, -.5);
>       flux(0,i, 1) = -.5*(uwleft+uwright)+.5*alpha_LF*(uvleft-uvright);
>       flux(1,i, 1) = -.5*(uvleft+uvright)+.5*alpha_LF*(uwleft-uwright);
>     });

I don't see the benefit in this context.

>   I don't think there is need for conversion between PetscKokkosReal and PetscReal. One uses the normal code for moving data between the memory spaces.
>  It is different than PetscInt and PetscBLASInt. 

Fine for values, but you can't just convert between device and host pointers (without unified memory and a potential performance hit).