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[Christian Jakob <jakob@xxxxxxxxxxxxxxxxxxx>]

hi jerome,


I think you get indeterminism while summing something in parallel,  
have a look here:

https://yade-dem.org/doc/formulation.html?highlight=indeterminism#result-indeterminism

see what can happen running exactly the same simulation two times in parallel:

https://www.youtube.com/watch?v=ZAkfHMyG6MM


cheers,

christian


Zitat von Jerome Duriez <Jerome.Duriez@xxxxxxxxxxx>:

> Hi,
>
> I have one engine in which I am looping over interactions to compute  
> incrementally something :
>
> something = 0
> for each interaction
>     something += f(considered interaction)
>
> In fact, I wrote this code using parallel loops, like that :
>
>    Real something = 0;
>
>   #ifdef YADE_OPENMP
>   const long size=scene->interactions->size();
>   #pragma omp parallel for schedule(guided) num_threads(ompThreads>0  
> ? min(ompThreads,omp_get_max_threads()) : omp_get_max_threads())
>   for(long i=0; i<size; i++){
>     const shared_ptr<Interaction>& interaction=(*scene->interactions)[i];
>   #else
>   FOREACH(const shared_ptr<Interaction>& interaction, *scene->interactions){
>   #endif
>       something += f(considered interaction);
>    }
>
> And I got (significantly) different results, for the final value of  
> something, running this code either in parallel, or not... Surely  
> linked with a poor understanding of what's really behind this openmp  
> lines, I do not get if I do something wrong, or not (and, if yes,  
> what ?). Is this kind of tasks achievable in parallel, or not ?
>
> Thanks a lot,
>
> Jerome