[petsc-users] parallel Mat Mult a serial Vec or a serial Mat

[Hui Zhang]

On Jan 20, 2013, at 5:28 PM, Jed Brown wrote:

> On Sun, Jan 20, 2013 at 10:22 AM, Hui Zhang <mike.hui.zhang at hotmail.com> wrote:
> 
> On Jan 20, 2013, at 5:13 PM, Jed Brown wrote:
> 
> > No, MatMult and MatMatMult require all objects to be on the same communicator.
> 
> Thanks for the quick answer!  It seems a general rule that all operands and results must be
> on the same communicator.  Can I lift an object from a sub-communicator to the sup-communicator
> without changing the parallel layout of the object?
> 
> You can with a Vec (e.g., using VecPlaceArray), but you're responsible for the sharing so it's fragile.
> 
> In general, I recommend creating the object on the largest communicator involved and then getting access more locally. In the first pass, just make a copy to a local subcomm unless a routine exists to do it automatically. If you profile and see that the copy is significant (remarkably rare in practice) you can sometimes optimize what is copied versus shared, or change the parent parallel data structure to make the local access faster. 

I think about the method and find it is not as convenient as allowing inter-communicator operations.
For example, VecScatter actually allows the two Vec in different communicators (or one must include
the other?).  

I have more questions.
(1) Does MatScatter supports MatMatMult? 
(2) Does MatConvert works on MATNEST?
(3) Does MatGetLocalSubMat support localization to a sub-communicator instead of a process?

Thanks!

> 
> >
> >
> > On Sun, Jan 20, 2013 at 10:12 AM, Hui Zhang <mike.hui.zhang at hotmail.com> wrote:
> > parallel Mat, multiplies a serial Vec or a serial Mat
> >
> > Is it supported directly? If yes, can the resulting Vec/Mat be serial or parallel?
> >
> >
> 
>