[petsc-users] block matrix without MatCreateNest

[Klaij, Christiaan]

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...

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<mailto:C.Klaij at marin.nl> | www.marin.nl<http://www.marin.nl>

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