On Jul 8, 2016 4:04 PM, "Juan Francisco Martínez" < juan.francisco.marti...@est.fib.upc.edu> wrote: > > George, > > Thanks for your rapid answer. > > I just ask for "simple synchronized reduction implementation" because I > am using a simple (and therefore really rapid) mpi communications > simulator that models all collectives as synchronized collectives and I > appreciate a huge differences between the real and the simulated > execution because the reductions. > > After note that in reallity there is no case where the mpi_reductions > syncrhonizes, then maybe could be a good idea to try to model an > approximation to the real behaviour.
Then maybe you want to take a look into the sync collective module, which does exactly what you want /need, i.e. automatically adds a barrier every X collective per communicator. I'm away from my computer and I can't check if we still have this module in the repo, if not you should be able to resurrect it from an older version. > > There is any place where I can found documentation about the different > algorithms that are implemented for mpi_reduction? Not that I know of. Ompi_info should be good enough no? George > > - Fran > > On Fri, 2016-07-08 at 15:40 +0200, George Bosilca wrote: > > > > On Jul 8, 2016 3:16 PM, "Juan Francisco Martínez" < > > juan.francisco.marti...@est.fib.upc.edu> wrote: > > > > > > Hi everybody! > > > > > > First of all I want to congratulate all of you because the quality > > of > > > the community, I have solved a lot of doubts just reading the > > mailing > > > list. > > > > > > However I have a question that I can not solve... Until now I > > though > > > that all the collective operations have an implicit sincronization, > > but > > > I can see that this is not true at all (because optimizations?). > > Then, > > > after searching a little bit on the web I saw that there are > > several > > > implementations of the reduction in openmpi, in fact there are 6 > > > possible algorithm (at least on OMPI 1.6) that you can use by mean > > of > > > the mca parameters... > > > > > > I thought that one of them behaves as a synchronization but after > > > execute a test with each one, no one behaves as it. Then my > > question > > > is, there is any possibility, by tuning ompi, the reduce operation > > > syncrhonize all the ranks that are involved at the end of the > > > operation? > > The straightforward answer is that the reduction provides a logical > > synchronization only between the root of the reduction and each one > > of the participants individually. > > As you already noticed this is not the case from a practical > > perspective because different underlying algorithms can be used, and > > they use different communication patterns. Thus, you cannot, and you > > should not, make a parallel between a reduction and a > > synchronization. > > If you really need the synchronization behavior why don't you use > > allreduce instead? Or at least a bcast of a single byte after the > > reduction (it also works with a barrier but as already have 1/2 of > > the synchronization, aka. all-to-root, this will be an overkill). > > > > > > Also I would like to know if there is any mechanism to know at > > runtime > > > which algorithm is being used. > > Again, there is no simple answer. Even if the tuned collective > > module could expose the algorithm, how do you know that a particular > > collective will be using the tuned module? We order the collective > > modules by priority, and the decision of which module will handle > > each collective is dynamic, based on the many factors. > > George > > > > > > Thanks for all! > > > - Fran > > > _______________________________________________ > > > users mailing list > > > us...@open-mpi.org > > > Subscription: https://www.open-mpi.org/mailman/listinfo.cgi/users > > > Link to this post: > > http://www.open-mpi.org/community/lists/users/2016/07/29606.php > > _______________________________________________ > > users mailing list > > us...@open-mpi.org > > Subscription: https://www.open-mpi.org/mailman/listinfo.cgi/users > > Link to this post: > > http://www.open-mpi.org/community/lists/users/2016/07/29607.php > _______________________________________________ > users mailing list > us...@open-mpi.org > Subscription: https://www.open-mpi.org/mailman/listinfo.cgi/users > Link to this post: http://www.open-mpi.org/community/lists/users/2016/07/29609.php
