Jeff, Thank you for the reply. In other words, MPI_IN_PLACE only eliminates data movement on root, right?
David ***** Correspondence ***** > From: Jeff Squyres <jsquy...@open-mpi.org> > Reply-To: Open MPI Users <us...@open-mpi.org> > Date: Fri, 3 Mar 2006 19:18:52 -0500 > To: Open MPI Users <us...@open-mpi.org> > Subject: Re: [OMPI users] MPI_IN_PLACE > > On Mar 3, 2006, at 6:42 PM, Xiaoning (David) Yang wrote: > >> call MPI_REDUCE(mypi,pi,1,MPI_DOUBLE_PRECISION,MPI_SUM,0, >> & MPI_COMM_WORLD,ierr) >> >> Can I use MPI_IN_PLACE in the MPI_REDUCE call? If I can, how? >> Thanks for any help! > > MPI_IN_PLACE is an MPI-2 construct, and is defined in the MPI-2 > standard. Its use in MPI_REDUCE is defined in section 7.3.3: > > http://www.mpi-forum.org/docs/mpi-20-html/node150.htm#Node150 > > It says: > > "The ``in place'' option for intracommunicators is specified by > passing the value MPI_IN_PLACE to the argument sendbuf at the root. > In such case, the input data is taken at the root from the receive > buffer, where it will be replaced by the output data." > > In the simple pi example program, it doesn't make much sense to use > MPI_IN_PLACE except as an example to see how it is used (i.e., it > won't gain much in terms of efficiency because you're only dealing > with a single MPI_DOUBLE_PRECISION). But you would want to put an > "if" statement around the call to MPI_REDUCE and pass MPI_IN_PLACE as > the first argument, and mypi as the second argument for the root. > For all other processes, use the same MPI_REDUCE that you're using now. > > -- > {+} Jeff Squyres > {+} The Open MPI Project > {+} http://www.open-mpi.org/ > > > _______________________________________________ > users mailing list > us...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/users
