[Users] bug in Dissipation thorn?

[Erik Schnetter]

On Fri, Feb 22, 2013 at 12:01 PM, Ian Hinder <ian.hinder at aei.mpg.de> wrote:

>
> On 22 Feb 2013, at 17:43, Frank Loeffler <knarf at cct.lsu.edu> wrote:
>
> > Hi Kentaro,
> >
> > On Fri, Feb 22, 2013 at 04:48:48PM +0100, Kentaro Takami wrote:
> >> I encountered the problem in which the simulation result is randomly
> different
> >> at each run( of course I used same executable, par file, HPC and so
> on.).
> >
> > I see the same for some of my runs, but I wouldn't expect otherwise.
> >
> > In my case the simulation depends on 'global' quantities, e.g.,
> > quantities obtained by reductions. Reductions cannot be done exactly, at
> > least not efficiently. Thus, values obtained via reductions (can) have
> > always an error that is different even when running the same simulation
> > twice, at least when done in parallel. If your simulation depends on
> > this, then these (small) differences can quickly grow, especially in
> > iterative schemes, e.g., hydro.
>
> I disagree.  Reductions should be deterministic; assuming the same number
> of MPI processes, the contributions to the reduction should always be added
> in the same order.  If you change the number of MPI processes, then I agree
> that the result of reductions can change, as the order of the sum over
> points will change, and floating point addition is not associative.
>

MPI assumes that reduction operations are associative. So does OpenMP, and
Carpet. So does SimFactory it its option settings. So does Kranc when it
generates code...

-erik

-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/
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