Problem with rectangular matrices

Lisandro Dalcin dalcinl at
Tue May 26 13:46:04 CDT 2009

On Tue, May 26, 2009 at 2:31 PM, Matthew Knepley <knepley at> wrote:
> On Tue, May 26, 2009 at 12:29 PM, Jed Brown <jed at> wrote:
>> Matthew Knepley wrote:
>> > It not computing Ab. Its computing Ab, A^2b, A^3b, etc.
>> Of course, but that's all possible if A*b is computable.  Lisandro
>> demonstrated that A*x was fine, which it better be because of how x was
>> obtained.  Had he tried A*b, which is the first thing the KSP needs, the
>> problem would have been obvious.
> This is the whole point. b and x cannot have different local sizes in a
> Krylov
> method. It makes no sense.

It seems I should go to for some coffee. You are right. The actual
limitations are vec operations involving vecs with different local
sizes. Perhaps making KSP work with different parallel x-b layouts
could be enhanced to work (for example, by scatter x <-> x_aux on
entry (for init_guess!=0) and on exit), but likely this is not worth
the effort..

Lisandro Dalcín
Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
PTLC - Güemes 3450, (3000) Santa Fe, Argentina
Tel/Fax: +54-(0)342-451.1594

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