[petsc-users] Ordering with MatCreateNest and MPI

[Jed Brown]

FRANCAVILLA MATTEO ALESSANDRO <d019598 at polito.it> writes:

> Dear Jed,
>
> thanks for the reply. Despite my efforts, I'm still unable to use the 
> information carried by the index set. I'm able to extract the IS through 
> MatNestGetISs but I don't really know how to interprete it and to exploit it 
> to pursue my goal.

Try VecGetSubVector(y,isrows[0],&y0) and VecView y0.

> I don't seem to find a tutorial explaining how to do the following:
>
> "MatNestGetISs give you the ISs to extract or set the portions of x and y 
> corresponding to the first and second block. The index  sets are not 
> globally contiguous and can't be if you want sane parallelism."
>
> Can you please give me some references for the above operation?

If you had two processes and all of the first block was on the first
process, then there could be no load balancing.  Load balancing requires
that the index sets not be globally contiguous.
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