[petsc-users] Assembly of local Mat's to a parallel Mat

[Hui Zhang]

On Apr 12, 2013, at 7:10 PM, Jed Brown wrote:

> Hui Zhang <mike.hui.zhang at hotmail.com> writes:
> 
>> On Apr 12, 2013, at 5:22 PM, Jed Brown wrote:
>> 
>> I can understand this method.  A further question: since Ai itself is shared 
>> by many processors,  MatSetValues to A should be called by only one of the 
>> processor sharing Ai.  Is it right?
> 
> If your Ai are already shared, why can't you just start by creating the
> big block-diagonal system containing all the Ai along the diagonal?
> Then MatGetSubMatrix() will give you the part if you really need to do
> something separate with it.

Thanks!  My difficulty lie in that Ai is a result of matrix computations in a 
sub-communcator (of the communicator of A).  So I do not know a priori 
the non-zero structure of Ai.  Following the instructions of you, I think 
I need to first MatGetRow of Ai, then MatSetValues to A. Is there a better way?

> 
>>> or you can create a block diagonal parallel
>>> matrix A_i constructed by joining together all the diagonal blocks.
>> 
>> Which function can do this?  I only found diagonal block Mat with each block 
>> of the same size and dense.  Thanks!
> 
> There is no function to do it in-place, but you can just create the big
> matrix and loop through the small matrix inserting rows.  It's usually
> better to start with the big matrix.
>