DA question

[Amit.Itagi at seagate.com]

Hi,

I spent some more time understanding DA's, and how DA's should serve my
purpose. Since in the time domain calculation, I will have to scatter from
the global vector to the local vector and vice-versa at every iteration
step, I have some follow-up questions.

1) Does the scattering involve copying the part stored on the local node as
well (i.e. part of the local vector other than the ghost values), or is the
local part just accessed by reference ? In the first scenario, this would
involve allocating twice the storage for the local part. Also, does the
scattering of the local part give a big hit in terms of CPU time ?

2) In the manual, it says "In most cases, several different vectors can
share the same communication information (or, in other words, can share a
given DA)" and "PETSc currently provides no container for multiple arrays
sharing the same distributed array communication; note, however, that the
dof parameter handles many cases of interest". I am a bit confused. Suppose
I have two arrays having the same layout on the regular grid, can I store
the first array data on one vector, and the second array data on the second
vector (and have a DA with dof=1, instead of a DA with dof=2), and be able
to scatter and update the first vector without scattering/updating the
second vector ?

Thanks

Rgds,
Amit

owner-petsc-users at mcs.anl.gov wrote on 04/09/2008 04:09:59 PM:

> Hi Amit,
>
> Why do you need two staggered grids? I do EM finite difference frequency
> domain modeling on a staggered grid using just one DA. Works perfectly
fine.
> There are some grid points that are not used, but you just set them to
zero
> and put a 1 on the diagonal of the coefficient matrix.
>
>
> Randy
>
>
> Amit.Itagi at seagate.com wrote:
> > Hi Berend,
> >
> > A detailed explanation of the finite difference scheme is given here :
> >
> > http://en.wikipedia.org/wiki/Finite-difference_time-domain_method
> >
> >
> > Thanks
> >
> > Rgds,
> > Amit
> >
> >
> >
> >

> >              Berend van Wachem

> >              <berend at chalmers.

> >              se>
To
> >              Sent by:                  petsc-users at mcs.anl.gov

> >              owner-petsc-users
cc
> >              @mcs.anl.gov

> >              No Phone Info
Subject
> >              Available                 Re: DA question

> >

> >

> >              04/09/2008 02:59

> >              PM

> >

> >

> >              Please respond to

> >              petsc-users at mcs.a

> >                   nl.gov

> >

> >

> >
> >
> >
> >
> > Dear Amit,
> >
> > Could you explain how the two grids are attached?
> > I am using multiple DA's for multiple structured grids glued together.
> > I've done the gluing with setting up various IS objects. From the
> > multiple DA's, one global variable vector is formed. Is that what you
> > are looking for?
> >
> > Best regards,
> >
> > Berend.
> >
> >
> > Amit.Itagi at seagate.com wrote:
> >> Hi,
> >>
> >> Is it possible to use DA to perform finite differences on two
staggered
> >> regular grids (as in the electromagnetic finite difference time domain
> >> method) ? Surrounding nodes from one grid are used to update the value
in
> >> the dual grid. In addition, local manipulations need to be done on the
> >> nodal values.
> >>
> >> Thanks
> >>
> >> Rgds,
> >> Amit
> >>
> >
> >
> >
>