[Users] bug in Dissipation thorn?
schnetter at cct.lsu.edu
Fri Feb 22 11:13:42 CST 2013
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
> >> at each run( of course I used same executable, par file, HPC and so
> > 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
Erik Schnetter <schnetter at cct.lsu.edu>
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