I'm not quite sure how collective computation calls work. For example, for an MPI_REDUCE with MPI_SUM, do all the processes collect values from all the processes and calculate the sum and put result in recvbuf on root? Sounds strange.
David ***** Correspondence ***** > From: Jeff Squyres <jsquy...@open-mpi.org> > Reply-To: Open MPI Users <us...@open-mpi.org> > Date: Mon, 6 Mar 2006 13:22:23 -0500 > To: Open MPI Users <us...@open-mpi.org> > Subject: Re: [OMPI users] MPI_IN_PLACE > > Generally, yes. There are some corner cases where we have to > allocate additional buffers, but that's the main/easiest benefit to > describe. :-) > > > On Mar 6, 2006, at 11:21 AM, Xiaoning (David) Yang wrote: > >> 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 >> >> >> _______________________________________________ >> users mailing list >> us...@open-mpi.org >> http://www.open-mpi.org/mailman/listinfo.cgi/users > > > -- > {+} 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
