[petsc-dev] ISRestoreNonlocalIS?

Jed Brown jed at 59A2.org
Thu Nov 18 16:41:10 CST 2010

On Thu, Nov 18, 2010 at 23:12, Dmitry Karpeev <karpeev at mcs.anl.gov> wrote:

> I'm not disagreeing, but I also don't see a general way of figuring
> out the off-rank
> columns for a submatrix that is destined to live on a subcomm.

VecScatter figures out who is sending to who without allgathering everyone's
indices.  I realize it's a slightly different problem and I don't have a
complete solution right now.  And iscol often has some structure (even if
just block).

Another point is that since we anticipate a submatrix is extracted on a
> subcomm,
> the size of the submatrix and the subcomm might not grow in the weak
> scaling case.

That depends whether the subdomains are different materials or just
solver-friendly subdomains (e.g. to reduce iterations in ASM and make
overlap less expensive in 3D).  Consider getting submatrices for Fluid and

> The names are definitely an stop-gap fix: for consistency we'd have to
> change
> ISGetIndices -->  ISGetLocalIndices, ISGetTotalIndices -->
> ISGetIndices, VecGetArray --> VecGetLocalArray, etc.
> Maybe this will be done one day :-)

I don't know, I like to endorse the domain decomposition philosophy and make
the simple things (and in particular, storage) local by default, with
parallel semantics where appropriate.  In particular, you should have to
spell out a word with non-scalable connotations in order to call a blatantly
non-scalable function.

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