[Trilinos-Users] Epetra_Vector with overlap

[Bartlett, Roscoe A]

If a vector contains duplicated off-processor elements does that not mess up dot product and norm calculations?

- Ross


> -----Original Message-----
> From: trilinos-users-bounces at software.sandia.gov [mailto:trilinos-
> users-bounces at software.sandia.gov] On Behalf Of Pawlowski, Roger P
> Sent: Wednesday, July 07, 2010 5:58 PM
> To: Heroux, Michael A; Salinger, Andrew; Berndt, Markus (LANL)
> Cc: trilinos-users at software.sandia.gov
> Subject: Re: [Trilinos-Users] Epetra_Vector with overlap
> 
> Option 2 should not be fragile for NOX algorithms except in one single
> case.  The generic NOX algorithms have no notion of an overlapped
> vector or that we are even running in parallel (except for outputting
> text to cout).  It only uses unique-GIDs map/graphs.  NOX passes back
> unique GID vectors to the user interface.   The gather/scatters with
> overlapped objects occur in the user implemented interface for computeF
> and computeJacobian).  The only place where overlapping might be an
> issue is for the finite difference coloring algorithm that is specific
> to the Epetra support.
> 
> We should not be initializing nox solvers with overlapped vectors since
> the size() method that is used in scaling some stopping criteria
> tolerances will yield different accuracies when running with different
> numbers of processors.
> 
> Roger
> ________________________________________
> From: trilinos-users-bounces at software.sandia.gov [trilinos-users-
> bounces at software.sandia.gov] On Behalf Of Heroux, Michael A
> [maherou at sandia.gov]
> Sent: Wednesday, July 07, 2010 3:10 PM
> To: Salinger, Andrew; Berndt, Markus (LANL)
> Cc: trilinos-users at software.sandia.gov
> Subject: Re: [Trilinos-Users] Epetra_Vector with overlap
> 
> Andy,
> 
> I think careful use of Option 2 is OK.  It is used quite frequently for
> what
> Markus is doing.
> 
> Mike
> 
> 
> On 7/7/10 3:25 PM, "Andrew Salinger" <agsalin at sandia.gov> wrote:
> 
> >
> > Marcus,
> >
> > I see two choices:
> > In both cases you have 2 Maps: owned (unique GIDs) and overlap.
> > (1) Have two copies of the vector (which you want to avoid)
> > (2) Construct your overlap map so that the owned elements
> > are contiguous. Then your owned vector can be constructed
> > as a View inside the overlap vector.
> > In your example, if the overlap map on PE1 is
> >  4 5 6 7 8 9   or    5 6 7 8 9 4,
> > then the owned vector can be constructed with
> > the map  5 6 7 8 9 and a pointer to the element
> > with "5" in the overlap map.
> >
> > I think (1) is the Best Practices approach. Option (2)
> > is fragile, since an algorithm (say in NOX) that makes a
> > copy of an owned vector will allocate it at the shorter
> > length. If you try this trick on that vector, it will
> > try and write past the allocated length.
> >
> > Andy
> >
> > -----Original Message-----
> > From: trilinos-users-bounces at software.sandia.gov
> > [mailto:trilinos-users-bounces at software.sandia.gov] On Behalf Of
> Markus Berndt
> > Sent: Wednesday, July 07, 2010 9:46 AM
> > To: trilinos-users at software.sandia.gov
> > Subject: [Trilinos-Users] Epetra_Vector with overlap
> >
> > Is it possible to set up communication schedules for an Epetra_Vector
> with
> > overlap that can be used to simply update the overlap region in
> place? The
> > documentation for Epetra_Export and Epetra_Import indicates that one
> of the
> > maps that these use to initialize communication patterns must have
> GIDs that
> > are unique. I interpret that as one of the vectors in an Import or
> Export call
> > must have no overlap at all.
> >
> >
> > To illustrate, here is an example:
> >
> > x_ovl is an Epetra_Vector with overlap such that on PE0 it owns GIDs
> 0 1 2 3 4
> > and has a copy of 5 and on PE1 it owns GIDs 5 6 7 8 9 and has a copy
> of 4.
> >
> > How do I set up a communication pattern that
> >
> > - sends x_ovl(5) from PE1 to PE0 and
> > - sends x_ovl(4) from PE0 to PE1
> >
> > without an intermediate vector that has only the local data and no
> overlap at
> > all.
> >
> > If this is not possible, what would you consider a best practices
> approach to
> > working with vectors that have overlap that avoids unnecessary copy
> operations
> > between equivalent vectors with and without overlap?
> >
> > Thanks
> >
> > - Markus
> >
> > --
> > Markus Berndt, LANL CCS-2, Mail Stop D413, 505-665-4711
> >
> >
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> 
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