[petsc-users] MatShell & PCShell

[Barry Smith]

   The check is simply 

if (A->rmap->n == PETSC_DECIDE || A->cmap->n == PETSC_DECIDE) SETERRQ(PETSC_COMM_SELF,PETSC_ERR_ARG_WRONG,"Must give local row and column count for matrix");

it is basically a "make sure the user doesn't shoot himself in the foot" check. I can see arguments both ways but since PETSc is suppose to be as flexible as possible and give as much choice to the user as possible I am going to take that check out of petsc-dev (you can just remove this one line from your copy of src/mat/impls/shell/shell.c ) and give the choice to the user.

   Barry


On Jun 25, 2010, at 1:39 AM, Klaij, Christiaan wrote:

> I thought the spirit of ex15f was to only specify global dimensions and let PETSc take care of the partitioning at runtime. So I was expecting
> 
> call MatCreateShell(PETSC_COMM_WORLD,PETSC_DECIDE,PETSC_DECIDE, m*n,m*n,PETSC_NULL_INTEGER,AA,ierr)
> 
> which is illegal. But thanks to your explanation I understand why. Of course it's much beter to check what PETSc decided and use it for my own MatMult. Thanks for clearing this up!
> 
> Chris
> 
> 
> Date: Thu, 24 Jun 2010 13:00:54 +0200
> From: Jed Brown <jed at 59A2.org>
> Subject: Re: [petsc-users] MatShell & PCShell
> To: "Klaij\, Christiaan" <C.Klaij at marin.nl>,
>        "petsc-users\@mcs.anl.gov" <petsc-users at mcs.anl.gov>
> Message-ID: <87vd98pue1.fsf at 59A2.org>
> Content-Type: text/plain; charset=us-ascii
> 
> On Thu, 24 Jun 2010 08:08:37 +0000, "Klaij, Christiaan" <C.Klaij at marin.nl> wrote:
>> Thanks!  I modified ex15f so that it uses a new MatShell AA with
>> associated multiplication Ax. Only problem is that MatCreateShell
>> expects the local matrix dimensions which is somewhat contrary to the
>> spirit of ex15f, but at least I got it working for one proc.
> 
> How is this contrary to the spirit?  It needs to be determined for
> compatibility with the vector you will multiply against.  Perhaps it
> should determine this automatically, but it's easy to provide, and
> allows PETSc to check for this compatibility before calling your
> function.  If it was determined automatically, then you would have to
> check what PETSc decided so you can set up your internal data structures
> (which you usually need in your implementation of MatMult)
> appropriately.  I think it's more common to create these structures
> first, then call MatCreateShell, in which case you know the local sizes.
> 
> Jed
> 
> 
> dr. ir.ChristiaanKlaij
> CFD Researcher
> Research & Development
> E mailto:C.Klaij at marin.nl
> T +31 317 49 33 44M
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