space allocation for a parallel matrix

[Matthew Knepley]

On Tue, Feb 3, 2009 at 9:25 AM, Yixun Liu <enjoywm at cs.wm.edu> wrote:
> Hi,
> I am confusing to the allocation for a parallel matrix in PETSC manual,
>
> "If PETSC DECIDE is not used for the arguments m and n, then the user
> must ensure that they are chosen
> to be compatible with the vectors. To do this, one first considers the
> matrix-vector product y = Ax. The m
> that is used in the matrix creation routine MatCreateMPIAIJ() must match
> the local size used in the vector
> creation routine VecCreateMPI() for y. Likewise, the n used must match
> that used as the local size in
> VecCreateMPI() for x."
>
> Since the process holds all the columns, why does it only hold part of
> vector x? Is it convenient for the matrix vector multiplication?

I am not sure I understand your question. However, Mat objects do hold
all columns of a row. The local column size 'n' is used to match against
a Vec, not for storage.

  Matt

> Thanks.
>
> Yixun
-- 
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