[petsc-users] GAMG processor reduction

[Dave May]

Exactly!
My tests were performed on Kraken (Cray XT5) and a Cray XE6 and I was using
iterative solvers on the coarse grid.



On 22 November 2013 00:10, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> Dave May <dave.mayhem23 at gmail.com> writes:
>
> > I argue it does matter as I've seen runs on 32k cores where a huge amount
> > of time is spent in those global reductions. I can provide an
> > implementation which uses a sub comm (PCSemiRedundant) if someone thinks
> > doing reductions on less cores is beneficial.
>
> It doesn't matter much on Blue Gene, but is a big deal on older Crays.
> Aires seems to be in between.  The default GAMG configuration doesn't do
> any reductions in the coarse grid, so the issue is moot.  If an
> iterative coarse solver was used, I think we would be more motivated to
> put the coarse problem on a subcomm.
>
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