On Mar 15, 2014, at 6:21 PM, christophe petit <christophe.peti...@gmail.com> wrote:
> Ok, so from what you say, on a "execution system" point view, the ring > communication is well achieved (i.e respecting the good order with, in last > position, rank0 which receives from rank 6) but the stdout doesn't reflect > what really happened, does it ? Well, it reflects what you printed, but not the order in which things happened. > > Is there a way to make stdout respect the expected order ? In your program, have each rank!=0 proc recv the message from the previous rank, print the message, sleep(1), and then send. > > Thanks > > > 2014-03-16 0:42 GMT+01:00 Ralph Castain <r...@open-mpi.org>: > The explanation is simple: there is no rule about ordering of stdout. So even > though your rank0 may receive its MPI message last, its stdout may well be > printed before one generated on a remote node. Reason is that rank 0 may well > be local to mpirun, and thus the stdout can be handled immediately. However, > your rank6 may well be on a remote node, and that daemon has to forward the > stdout to mpirun for printing. > > Like I said - no guarantee about ordering of stdout. > > > On Mar 15, 2014, at 2:43 PM, christophe petit <christophe.peti...@gmail.com> > wrote: > >> Hello, >> >> I followed a simple MPI example to do a ring communication. >> >> Here's the figure that illustrates this example with 7 processes : >> >> http://i.imgur.com/Wrd6acv.png >> >> Here the code : >> >> -------------------------------------------------------------------------------------------------------------------------- >> program ring >> >> implicit none >> include 'mpif.h' >> >> integer, dimension( MPI_STATUS_SIZE ) :: status >> integer, parameter :: tag=100 >> integer :: nb_procs, rank, value, & >> num_proc_previous,num_proc_next,code >> >> call MPI_INIT (code) >> call MPI_COMM_SIZE ( MPI_COMM_WORLD ,nb_procs,code) >> call MPI_COMM_RANK ( MPI_COMM_WORLD ,rank,code) >> >> num_proc_next=mod(rank+1,nb_procs) >> num_proc_previous=mod(nb_procs+rank-1,nb_procs) >> >> if (rank == 0) then >> call MPI_SEND (1000,1, MPI_INTEGER ,num_proc_next,tag, & >> MPI_COMM_WORLD ,code) >> call MPI_RECV (value,1, MPI_INTEGER ,num_proc_previous,tag, & >> MPI_COMM_WORLD ,status,code) >> else >> call MPI_RECV (value,1, MPI_INTEGER ,num_proc_previous,tag, & >> MPI_COMM_WORLD ,status,code) >> call MPI_SEND (rank+1000,1, MPI_INTEGER ,num_proc_next,tag, & >> MPI_COMM_WORLD ,code) >> end if >> print *,'Me, process ',rank,', I have received ',value,' from process >> ',num_proc_previous >> >> call MPI_FINALIZE (code) >> end program ring >> >> -------------------------------------------------------------------------------------------------------------------------- >> >> At the execution, I expect to always have : >> >> Me, process 1 , I have received 1000 from process >> 0 >> Me, process 2 , I have received 1001 from process >> 1 >> Me, process 3 , I have received 1002 from process >> 2 >> Me, process 4 , I have received 1003 from process >> 3 >> Me, process 5 , I have received 1004 from process >> 4 >> Me, process 6 , I have received 1005 from process >> 5 >> Me, process 0 , I have received 1006 from process >> 6 >> >> But sometimes, I have the reception of process 0 from process 6 which is not >> the last reception, like this : >> >> Me, process 1 , I have received 1000 from process >> 0 >> Me, process 2 , I have received 1001 from process >> 1 >> Me, process 3 , I have received 1002 from process >> 2 >> Me, process 4 , I have received 1003 from process >> 3 >> Me, process 5 , I have received 1004 from process >> 4 >> Me, process 0 , I have received 1006 from process >> 6 >> Me, process 6 , I have received 1005 from process >> 5 >> >> where reception of process 0 from process 6 happens before the reception of >> process 6 from process 5 >> >> or like on this result : >> >> Me, process 1 , I have received 1000 from process >> 0 >> Me, process 2 , I have received 1001 from process >> 1 >> Me, process 3 , I have received 1002 from process >> 2 >> Me, process 4 , I have received 1003 from process >> 3 >> Me, process 0 , I have received 1006 from process >> 6 >> Me, process 5 , I have received 1004 from process >> 4 >> Me, process 6 , I have received 1005 from process >> 5 >> >> where process 0 receives between the reception of process 4 and 5. >> >> How can we explain this strange result ? I thought that standard use of >> MPI_SEND and MPI_RECV were blocking by default and, >> with this result, it seems to be not blocking. >> >> I tested this example on Debian 7.0 with open-mpi package. >> >> Thanks for your help >> >> >> _______________________________________________ >> 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 > > _______________________________________________ > users mailing list > us...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/users
