[petsc-users] block matrix without MatCreateNest

[Matthew Knepley]

On Thu, Aug 4, 2016 at 3:43 AM, Klaij, Christiaan <C.Klaij at marin.nl> wrote:

> OK, looking forward to the fix!
>
> Related to this, the preallocation would need to depend on the
> type that is given at runtime, say
>
> if type=XXX, call MatXXXSetPreallocation()
>
> That would work for say seqaij and mpiaij, probably even without
> the if-statement, right? And since there's no
> MatNestSetPreallocation, should one get the submats and
> preallocate those if type=nest?
>

This is really a situation where the DM interface is far superior. We could
try
to break down the AIJ preallocation into pieces, however since we have no
column information, this problem is not determined. However, in the DM case
we know everything and could properly allocate the submatrices
automatically.

   Matt


> Chris
>
> > Date: Tue, 2 Aug 2016 08:49:36 -0500
> > From: Matthew Knepley <knepley at gmail.com>
> > To: "Klaij, Christiaan" <C.Klaij at marin.nl>
> > Cc: "petsc-users at mcs.anl.gov" <petsc-users at mcs.anl.gov>
> > Subject: Re: [petsc-users] block matrix without MatCreateNest
> > Message-ID:
> >         <CAMYG4Gk9-cp8nrHnkCePnUvty-emDWSMQco0S3AJ5u8_W=fiXw at mail.
> gmail.com>
> > Content-Type: text/plain; charset="utf-8"
> >
> > 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 | mailto:C.Klaij at marin.nl | http://www.marin.nl
>
> MARIN news: http://www.marin.nl/web/News/News-items/Vice-Admiral-De-
> Waard-makes-virtual-test-voyage-on-MARINs-FSSS.htm
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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