[petsc-dev] Understanding Vecscatter with Kokkos Vecs

[Patrick Sanan]

I'm trying to understand how VecScatters work with GPU-native Kokkos Vecs. 

Specifically, I'm interested in what will happen in code like in src/vec/vec/tests/ex22.c, 

	ierr = VecScatterCreate(x,is1,y,is2,&ctx);CHKERRQ(ierr);

(from https://gitlab.com/petsc/petsc/-/blob/master/src/vec/vec/tests/ex22.c#L44 <https://gitlab.com/petsc/petsc/-/blob/master/src/vec/vec/tests/ex22.c#L44>)

Here, x and y can be set to type KOKKOS using -vec_type kokkos at the command line. But is1 and is2 are (I think), always
CPU/host data. Assuming that the scatter itself can happen on the GPU, the indices must make it to the device somehow - are they copied there when the scatter is created? Is there a way to create the scatter using indices already on the GPU (Maybe using SF more directly)?

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