[petsc-dev] Removing VecGetArrayPrivate3

[Jed Brown]

On Thu, Nov 25, 2010 at 05:08, Barry Smith <bsmith at mcs.anl.gov> wrote:

> > 2. Not necessarily make copies of everything within FieldSplit (I think
> all users of FieldSplit complain that it costs so much memory at present).
>
>    They think it takes too much, but if we had real profiling of where the
> memory is used they would see that it is actually trivial.


Calling MatGetSubMatrix to break an assembled matrix into pieces triples the
amount of memory for column indices and values, plus has the non-scalable
MPI_Allgather.  You routinely complain about other packages that don't
properly support 64-bit indices.  If you use FieldSplit with a globally
assembled matrix, that will never be a problem unless you have 16 GB per
process to dedicate solely to storing redundant indices.  It also loses
block structure and symmetry, why even bother with (S)BAIJ if you can't use
it for coupled problems?

Jed
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