space allocation for a parallel matrix

[Yixun Liu]

Matthew Knepley wrote:
> 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 I mean is that the process holds all the column, but only part of
the vector, how to do the multiplication?  It needs  communicate with
other processes. Right?