[MOAB-dev] DMMoabCreateVector

[Gerd Heber]

Vijay, thanks, I finally figured that (DMMoabSetFieldNames...)
does the trick. I think much of the confusion comes from that
there doesn't seem to be any documentation of the DMMoab 
"protocol", for the lack of a better term.
The functions are all reasonably documented individually,
but there's no documentation that explains what the protocol is,
 i.e., which calls are supposed (or not! supposed) to happen
 before certain other calls. For example, is DMMoabSetBlockSize
supposed to be called before DMSetup or after, or maybe
it's not supposed to be called at all under certain 
circumstances. How do I know? Does DMMoabSetBlockSize interfere
with DMMoabSetFieldNames? maybe. Maybe not. Other than trial
and error, how can I determine that?

Don't get me wrong, I really like what I see in DMMoab!
I only wish the model scenarios were documented somewhere.

Best, G.



-----Original Message-----
From: Vijay S. Mahadevan [mailto:vijay.m at gmail.com] 
Sent: Tuesday, November 25, 2014 2:11 PM
To: Gerd Heber
Cc: MOAB dev
Subject: Re: [MOAB-dev] DMMoabCreateVector

Hi Gerd,

I think you've uncovered a bug (part in design and part in
implementation) and it wasn't technically supposed to be confusing.
When you set a tag_size that is not 1, currently there isn't support to decipher this from the tag and allocate a vector accordingly. This is an implementation bug that I will fix soon. Thanks for reporting.

But a more relevant design question is whether you are creating fields in DMMoab and if the number there is the same as the one defined here (=3). This is necessary to ensure that the matrices are also getting allocated properly based on whether you want interleaved or strided representation, which will also affect the Vec representation with a size 3*N. To get a hold of this resulting vector, you would need to call DMCreateGlobalVector [1] instead of calling the more specific DMMoabCreateVector API function.

Note that with the usage of DMMoabCreateVector, you've requested for interleaved access by default. I'm not sure how to handle the inconsistency here yet.

Vijay

[1] http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMCreateGlobalVector.html

On Tue, Nov 25, 2014 at 9:21 AM, Gerd Heber <gheber at hdfgroup.org> wrote:
> I'm a little confused about the use of DMMoabCreateVector.
> I create a tag like this:
>
> Field default_value = { 0.0, 0.0, 0.0 }; size_t u_tag_size = 
> sizeof(Field)/sizeof(PetscScalar);
> Tag u_tag;
> merr = mbiface->tag_get_handle("u", u_tag_size,
>                                      MB_TYPE_DOUBLE, u_tag,
>                                      MB_TAG_DENSE|MB_TAG_CREAT,
>                                      &default_value);
>
> Let N be the number of entities. If I then go on to create a PETSc Vec 
> via
>
> ierr = DMMoabCreateVector(dm, u_tag, owned, PETSC_TRUE, PETSC_FALSE, 
> &u);
>
> and retrieve the (global) size of the Vec
>
> ierr = VecGetSize(u, &size);
>
> I get N as the answer. Why is it N rather than 3*N?
>
> Should it be 3*N and I'm doing something stupid, or is it supposed to 
> be N, which I'd find somewhat counterintuitive. (What would be the 
> relationship between that Vec and the tag data?)
>
> Thanks, G.
>
>
>