MatGetSubMatrix question

Matthew Knepley knepley at
Tue May 26 12:22:51 CDT 2009

On Tue, May 26, 2009 at 12:11 PM, Lisandro Dalcin <dalcinl at> wrote:

> On Mon, May 25, 2009 at 10:20 AM, Jed Brown <jed at> wrote:
> >
> > Is it really necessary that iscol be a sequential index set that covers
> > all columns, with csize selecting the local columns?  I realize this
> > might be natural/easy for AIJ formats, but it feels clumsy (not to
> > mention not scalable due to the ISAllGather).
> >
> Your are right, this is not scalable, but...
> > I'm guessing this is a relic from when IS was not a real parallel
> > object.  Am I missing some deep reason for this interface, or could
> > iscol be parallel and csize be dropped?
> >
> unless things have changed very recently (after 3.0.0 release), IS is
> far from being a real parallel object.

I agree. I am trying to think of a scenario is which you would want to
a distributed IS, which would then gather those columns to each process
without gathering the integers. Theoretically, we might want this, but is it
worth any time to implement?


> --
> Lisandro Dalcín
> ---------------
> Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
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What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
-- Norbert Wiener
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