Then the documentation needs to make it very clear what it does (which is not much) and how it is VERY different than getting the vector from the DM.


> On Jul 16, 2015, at 1:35 PM, Matthew Knepley <knepley@gmail.com> wrote:
>
> On Thu, Jul 16, 2015 at 1:31 PM, Barry Smith <bsmith@mcs.anl.gov> wrote:
>
> > On Jul 16, 2015, at 11:14 AM, Jed Brown <jed@jedbrown.org> wrote:

> >
> > Gianluca Meneghello <gianmail@gmail.com> writes:
> >
> >> Hi again,
> >>
> >> I am now trying to write a vector to an HDF5, this time using a DM
> >> structure to impose the vector layout.
> >>
> >> Creating the vector with (as in src/dm/examples/tutorials/ex10.c)
> >>
> >> DMCreateGlobalVector(da2D,&gauss);
> >>
> >> every works fine, i.e. the hdf5 file has the correct layout given by the
> >> dimension of the DM. On the other side, if I use:
> >>
> >> VecCreate(PETSC_COMM_WORLD,&gauss);
> >> VecSetSizes(gauss,PETSC_DECIDE,Nx*Ny);
> >> VecSetDM(gauss,da2D);
> >
> > VecSetDM only associates a DM with the Vec.  It does not interpose
> > itself into VecView.
> >
> > Should we change this so that creating a Vec of the correct size/block
> > size/layout and associating a DM is equivalent?  Perhaps, but it's more
> > nuanced than it might seem.
>
>   Yikes. VecSetDM() seems a very dangerous and odd thing. Should it exist? If it exists it seems it should be exactly equivalent to creating the vector from the DM.
>
> I tend to disagree. It is used to set a member variable which is not exposed:
>
> find src -name "*.c" | xargs grep VecSetDM
> src/dm/impls/composite/pack.c:  ierr = VecSetDM(*gvec, dm);CHKERRQ(ierr);
> src/dm/impls/composite/pack.c:  ierr = VecSetDM(*lvec, dm);CHKERRQ(ierr);
> src/dm/impls/da/dadist.c:    ierr = VecSetDM(*g, da);CHKERRQ(ierr);
> src/dm/impls/da/dalocal.c:    ierr = VecSetDM(*g, da);CHKERRQ(ierr);
> src/dm/impls/network/networkcreate.c:  ierr = VecSetDM(*vec,dm);CHKERRQ(ierr);
> src/dm/impls/network/networkcreate.c:  ierr = VecSetDM(*vec,dm);CHKERRQ(ierr);
> src/dm/impls/redundant/dmredundant.c:  ierr  = VecSetDM(*gvec,dm);CHKERRQ(ierr);
> src/dm/impls/redundant/dmredundant.c:  ierr  = VecSetDM(*lvec,dm);CHKERRQ(ierr);
> src/dm/impls/shell/dmshell.c:  ierr = VecSetDM(*gvec,dm);CHKERRQ(ierr);
> src/dm/impls/shell/dmshell.c:  ierr = VecSetDM(*gvec,dm);CHKERRQ(ierr);
> src/dm/impls/sliced/sliced.c:  ierr  = VecSetDM(*gvec,dm);CHKERRQ(ierr);
> src/dm/interface/dmi.c:  ierr = VecSetDM(*vec, dm);CHKERRQ(ierr);
> src/dm/interface/dmi.c:  ierr = VecSetDM(*vec, dm);CHKERRQ(ierr);
> src/dm/interface/ftn-auto/dmf.c:*__ierr = VecSetDM(
>
>  I don't like putting in a bunch of side-effects. Those come from actually being created from a DM, which is correct.
>
>    Matt
>
> >
> > Why don't you want to just call DMCreateGlobalVector()?
>
>
>
>
> --
> 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