[petsc-users] GAMG processor reduction

[Matthew Knepley]

On Thu, Nov 21, 2013 at 3:43 PM, Dave May <dave.mayhem23 at gmail.com> wrote:

> Is using the "big communicator" really the right way to go? What happens
> when I call VecNorm() when the local size on most ranks =0.. the global
> reduction still has to be performed and all ranks in the original
> communicator associated with the fine get participate.
>
> I thought the primary advantage/reason to use less ranks with small
> distributed systems was to avoid seeing the network latency when there is
> little computational work. I don't see how using the big communicator
> avoids this.
>

Its not just this. You do not want to get to the point where you have 1 or
< 1 point per process, so
you rebalance to put a reasonable number of unknowns per process and leave
others empty. You
could create a subcomm with only the nonzero procs to use in the solve. Not
sure if this is worth it.

   Matt


> Am I missing something?
>
> Cheers,
>   Dave
>
> On Thursday, 21 November 2013, Jed Brown wrote:
>
>> John Mousel <john.mousel at gmail.com> writes:
>>
>> > Thanks Jed. How does this represent itself in the KSPView output?
>>
>> I'm afraid it's not there, though you can extract the ownership ranges
>> From code.
>>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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