[petsc-users] Vector with ghost values using DMDA

[Jed Brown]

Praveen C <cpraveen at gmail.com> writes:

> So I have to create a global vector AND a local vector using
> DMCreateLocalVector.
>
> Then I do DMGlobalToLocalBegin/End. Does this not lead to too much copying
> ?

It's typically more efficient -- the solver gets to work with contiguous
vectors and doesn't have unused "ghost" storage in every vector of a
Krylov space, for example.

> I see there is VecCreateGhost but no such thing for DMDA ?

That would necessitate non-contiguous indexing which would kill
performance.  Also note that the ghost points in a VecGhost would exist
but be unused in every vector of a Krylov space, every stage of a
Runge-Kutta method, etc.  Much better to have one or two local vectors
that you use while evaluating residuals.

> PS: Would be nice if the reply-to was set to mailing list. I frequently
> forget to do Reply All.

Then make reply-all your default instead of asking to break (individual)
reply for everyone else.

  http://www.unicom.com/pw/reply-to-harmful.html

  http://woozle.org/~neale/papers/reply-to-still-harmful.html

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