[petsc-users] Question about matrix permutation

Matthew Knepley knepley at gmail.com
Fri Jan 29 11:39:35 CST 2010

If what you want to do is apply a block preconditioner to a matrix arising
from a regular
grid, DA can already do this automatically. This uses the PCFieldSplit, and
can be
completely controlled from the command line. It is designed to do these
sorts of things.

I do not understand your last question.


On Fri, Jan 29, 2010 at 11:29 AM, Michel Cancelliere <fernandez858 at gmail.com
> wrote:

> Thank you for your promptly response,  i will use unsorted inex set as
> you suggest (because my submatrices are very large (4 submatrices N/2
> x N/2 from the original one Nx N),
> I realize that i cannot use block matrices because the reordering
> split within the blocks and the resulting matrices are not block
> matrices anymore. In terms of performance how much can affect this to
> the code, in the sense if i apply for example an ILU preconditioner to
> a block matrix (baij) and solve the same system but this time using an
> aij matrix ? not sure if it is clear
> Thanks
> Michel
> On 1/29/10, Jed Brown <jed at 59a2.org> wrote:
> > On Fri, 29 Jan 2010 09:43:05 -0600 (CST), abhyshr at mcs.anl.gov wrote:
> >> Instead of calling MatGetSubMatrix to get each submatrix you can use
> >> MatGetSubMatrices to extract several submatrices at once. See
> >>
> http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/Mat/MatGetSubMatrices.html#MatGetSubMatrices
> .
> >
> > I don't think Michel was asking to extract multiple matrices.  In
> > serial, MatGetSubMatrices is implemented by calling MatGetSubMatrix so
> > there is no weaker assumption on index set sortedness.
> >
> > Jed
> >

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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