DA question

[Matthew Knepley]

On Tue, Apr 29, 2008 at 8:54 AM,  <Amit.Itagi at seagate.com> wrote:
> 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

No, you get a separate local vector since we reorder to give contiguous
access.

>  involve allocating twice the storage for the local part. Also, does the

Yes, however unless you run an explicit code at the limit of memory, this
really does not matter.

>  scattering of the local part give a big hit in terms of CPU time ?

Not for these cartesian topologies with small overlap. This is easy to prove.

>  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 ?

Yes. You call DAGetGlobalVector() twice, and then when you want one vector
updated, call DALocalToGlobal() or DAGlobalToLocal() with that vector.

  Matt

>  Thanks
>
>  Rgds,
>  Amit
-- 
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