[petsc-users] block matrix without MatCreateNest

[Matthew Knepley]

On Tue, Aug 2, 2016 at 2:25 AM, Klaij, Christiaan <C.Klaij at marin.nl> wrote:

> Thanks for your help! Going from individual blocks to a whole
> matrix makes perfect sense if the blocks are readily available or
> needed as fully functional matrices. Don't change that! Maybe add
> the opposite?
>
> I'm surprised it's broken though: on this mailing list several
> petsc developers have stated on several occasions (and not just
> to me) things like "you should never have a matnest", "you should
> have a mat then change the type at runtime", "snes ex70 is not
> the intended use" and so on.
>
> I fully appreciate the benefit of having a format-independent
> assembly and switching mat type from aij to nest depending on the
> preconditioner. And given the manual and the statements on this
> list, I thought this would be standard practice and therefore
> thoroughly tested. But now I get the impression it has never
> worked...
>
Yes, that way has never worked. Nest is only a memory optimization, and with
implicit problems I am never running at the limit of memory (or I use more
procs).
The people I know who needed it had explicitly coded it in rather than
trying to
use it from options. It should not take long to get this fixed.

  Thanks,

     Matt

> Chris
>
>
> > From: Matthew Knepley <knepley at gmail.com>
> > Sent: Tuesday, August 02, 2016 12:28 AM
> > To: Klaij, Christiaan
> > Cc: petsc-users at mcs.anl.gov; Jed Brown
> > Subject: Re: [petsc-users] block matrix without MatCreateNest
> >
> > On Mon, Aug 1, 2016 at 9:36 AM, Klaij, Christiaan <C.Klaij at marin.nl>
> wrote:
> >
> >     Matt,
> >
> >
> >     1) great!
> >
> >
> >     2) ??? that's precisely why I paste the output of "cat mattry.F90"
> in the emails, so you have a small example that produces the errors I
> mention. Now I'm also attaching it to this email.
> >
> > Okay, I have gone through it. You are correct that it is completely
> broken.
> >
> > The way that MatNest currently works is that it trys to use L2G mappings
> from individual blocks
> > and then builds a composite L2G map for the whole matrix. This is
> obviously incompatible with
> > the primary use case, and should be changed to break up the full L2G
> into one for each block.
> >
> > Jed, can you fix this? I am not sure I know enough about how Nest works.
> >
> >    Matt
> >
> >     Thanks,
> >
> >     Chris
>
>
>
> dr. ir. Christiaan Klaij | CFD Researcher | Research & Development
> MARIN | T +31 317 49 33 44 | C.Klaij at marin.nl | www.marin.nl
>
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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 lead.
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