[petsc-dev] Vector operations on ghosted vectors

[John Mousel]

Let me be even more specific. I'm implementing a velocity correction scheme
in rotational form (reference given below). In the first step, forgetting
even a sub-iterative loop, a projecting pressure is solved for from

L(p(n+1)) = -div(beta*u(n) + gamma*u(n-1) + exci*u(n-2) + H(n+1) - H(n) +
f(n+1) - f(n))

Here, L and div are Laplacian and divergence operators, and H(n) and f(n)
are the non-linear and body force at time level n. In my mind, to form part
of the RHS, I can use VecAXPY on all the terms in the div(), and then
perform the divergence on that. To avoid communication, I can just use the
VecAXPY on the ghosted portion as well, which since I've copied the entire
ghosted vector during the VecCopy(u(n),u(n-1)) operation, should make sense.
Is that not a wise thing to do?

John

J. L. Guermond, P. Minev, and J. Shen. An overview of projection methods
for incompressible flows. Computer Methods in Applied Mechanics and
Engineering, 195(44-47):6011–6045, 2006.


On Fri, Mar 1, 2013 at 9:28 AM, Matthew Knepley <knepley at gmail.com> wrote:

> On Fri, Mar 1, 2013 at 10:24 AM, John Mousel <elafint.john at gmail.com>wrote:
>
>> I use the data in the vectors for things besides pure vector operations.
>> In the context of a sub-iteration scheme for an incompressible solver, I
>> want to use the values stored in the vector at the previous iteration to
>> construction the non-linear term. This requires copying not just the local
>> portion of the vector in the u(k)->u(k-1) operation, but the ghosts as
>> well. Doesn't copying the entire ghosted vector make sense in this context?
>>
>
> It seems like a strange work flow. Since none of the vector operations
> touch these, you will be getting
> stale values anyway. It would make more sense to me if you were using
> GlobalToLocal() during the iteration.
>
>    Matt
>
>
>> John
>>
>>
>> On Fri, Mar 1, 2013 at 8:09 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>>
>>> On Wed, Feb 27, 2013 at 4:56 PM, John Mousel <john.mousel at gmail.com>wrote:
>>>
>>>> Is there a possibility of adding a wrapper function around a few basic
>>>> vector operations such as VecCopy, VecAXPY, VECAXPYPZ... to operate on
>>>> ghosted vectors? I perform a lot of vector operations including the ghost
>>>> region to avoid communication.
>>>>
>>>
>>> Can you explain the context where operating with local forms in this way
>>> makes sense? Usually you would either be working locally, in which case you
>>> copy between local forms (or purely local work vectors) or you are
>>> operating globally in which case there is nothing to gain by applying
>>> operations to the local form.
>>>
>>
>>
>
>
> --
> 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
>
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