[petsc-dev] use of hash table vs array in various places in PETSc

[Jed Brown]

On Tue, Sep 20, 2011 at 01:05, Barry Smith <bsmith at mcs.anl.gov> wrote:

> It may very well be limiting with MatGetSubMatrix() for VI solvers.


As I recall, the gather is used to identify which global columns out of the
off-diagonal block are needed. Well, the process owning that part of the
index set knows, so why not use the scatter for the off-diagonal block to
mark the needed columns?

This seems quite straightforward when the submatrix is taking a local subset
(not moving rows to different processes). This also takes care of the most
common use case in fieldsplit.


This would be even nicer if we could move indices over the same channels as
scalars. This comes up so frequently that I think we should build a
type-independent scatter, something like

PetscScatterCreate(PetscLayout from,IS isfrom,PetscLayout to,IS
isto,PetscScatter*);

PetscScatterBegin(PetscScatter,void *from,MPI_Datatype,void
*to,MPI_Datatype,MPI_Op,ScatterMode);


The VecScatter API could be preserved, but the implementation would become a
thin wrapper over this thing.
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