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

[Mark F. Adams]

A type independent scatter would make my code simpler, faster, shorter.  I reuse Mat scatters for all sorts of stuff and currently have to call scatter on each variable (stuffing ints in floats). 

On Sep 20, 2011, at 12:22 PM, Matthew Knepley wrote:

> On Tue, Sep 20, 2011 at 8:43 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> 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.
> 
> Wait, a type-independent Scatter? The one I have been begging for for 7 years, and finally coded myself (see ParallelMapping.hh) suboptimally. I
> would like to see VecScatter completely refactored. We need
> 
>   - Flexible types
>   - Flexible combination function
>   - Better linkage to DM
> 
>     Matt
> 
> -- 
> 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

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