On Sun, Mar 13, 2016 at 2:02 PM, Matthew Larkin <lar...@yahoo.com> wrote:
> Hello, > > My understanding is Open MPI can utilize shared and/or distributed memory > architecture (parallel programming model). OpenMP is soley for shared > memory systems. > > The MPI-3 standard provides both explicitly shared and distributed memory semantics. See MPI_Win_allocate_shared for the shared memory feature. In addition to the explicit semantic, all reasonable MPI implementations exploit shared-memory internally, which is why Send-Recv within a node is usually higher bandwidth than between nodes. > I believe Open MPI incorporates OpenMP from some of the other archives I > glanced at. > > Some implementations use OS threads (e.g. POSIX threads) internally but not for the type of concurrency that OpenMP provides. OpenMP is usually a bad choice for use inside of an MPI library because it does not generally compose well with other threading models. > Is this a true statement? If so, is there any need to create a hybrid > model that uses both OpenMP and Open MPI? > > Various people, including me, have argued that MPI+OpenMP hybrid programming is not necessary and even harmful: http://www.orau.gov/hpcor2015/whitepapers/Exascale_Computing_without_Threads-Barry_Smith.pdf http://www.cs.utexas.edu/users/flame/BLISRetreat2014/slides/hammond-blis-2014.pdf http://scisoftdays.org/pdf/2016_slides/hammond.pdf However, this does not mean that flat MPI is close to optimal. The statement here is only that MPI+MPI is more effective than MPI+OpenMP when the programmer devotes equivalent effort to both (and handles SIMD via some mechanism) Best, Jeff > Thanks! > > _______________________________________________ > users mailing list > us...@open-mpi.org > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users > Link to this post: > http://www.open-mpi.org/community/lists/users/2016/03/28696.php > -- Jeff Hammond jeff.scie...@gmail.com http://jeffhammond.github.io/
