[petsc-users] MatCreateMPIAIJ with zero size on some ranks

[Jed Brown]

On Wed, Apr 25, 2012 at 06:42, Thomas Witkowski <
thomas.witkowski at tu-dresden.de> wrote:

> Also some coarse space method. When not all ranks contribute to the coarse
> space, a matrix defined on the coarse space leads to local empty matrices
> on these ranks. So the question is if this leads to technical troubles in
> PETSc. The share of nodes participating in the coarse space may vary.


There is no "technical trouble".

Communicator contraction doesn't really affect the MatMult. Depending on
the hardware, it may affect the VecDot and VecNorm (contracting the
communicator is likely worse on Blue Gene/P because certain reduction
algorithms become unavailable---unless the contraction factor is massive
and the remaining participants are nearby on the torus, it's probably good
on many fat tree networks).

Redistributing the data for some locality (independent of whether you also
contract the communicator) is more useful for MatMult, but you have to
weight the benefits relative to the cost of moving the data, and it may not
pay off.
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