Aurélien Bouteiller wrote:
You can't assume that MPI_Send does buffering.
Yes, but I think this is what Eric meant by misinterpreting Enrico's problem. The communication pattern is to send a message, which is received remotely. There is remote computation, and then data is sent back. No buffering is needed for such a pattern. The code is "apparently" legal. There is apparently something else going on in the "real" code that is not captured in the example Enrico sent.
Further, if I understand correctly, the remote process actually receives the data! If this is true, the example is as simple as:
process 1: MPI_Send() // this call blocks process 0:MPI_Recv() // this call actually receives the data sent by MPI_Send!!!
Enrico originally explained that process 0 actually receives the data. So, MPI's internal buffering is presumably not a problem at all! An MPI_Send effectively sends data to a remote process, but simply never returns control to the user program.
Without buffering, you are in a possible deadlock situation. This pathological case is the exact motivation for the existence of MPI_Sendrecv. You can also consider Isend Recv Wait, then the Send will never block, even if the destination is not ready to receive, or MPI_Bsend that will add explicit buffering and therefore return control to you before the message transmission actually begun.Aurelien Le 15 sept. 08 à 01:08, Eric Thibodeau a écrit :Sorry about that, I had misinterpreted your original post as being the pair of send-receive. The example you give below does seem correct indeed, which means you might have to show us the code that doesn't work. Note that I am in no way a Fortran expert, I'm more versed in C. The only hint I'd give a C programmer in this case is "make sure your receiving structures are indeed large enough (ie: you send 3d but eventually receive 4d...did you allocate for 3d or 4d for receiving the converted array...).Eric Enrico Barausse wrote:sorry, I hadn't changed the subject. I'm reposting: Hi I think it's correct. what I want to to is to send a 3d array from the process 1 to process 0 =root): call MPI_Send(toroot,3,MPI_DOUBLE_PRECISION,root,n,MPI_COMM_WORLD in some other part of the code process 0 acts on the 3d array and turns it into a 4d one and sends it back to process 1, which receives it withcall MPI_RECV(tonode, 4,MPI_DOUBLE_PRECISION,root,n,MPI_COMM_WORLD,status,ierr)in practice, what I do i basically give by this simple code (which doesn't give the segmentation fault unfortunately): a=(/1,2,3,4,5/) call MPI_INIT(ierr) call MPI_COMM_RANK(MPI_COMM_WORLD, id, ierr) call MPI_COMM_SIZE(MPI_COMM_WORLD, numprocs, ierr) if(numprocs/=2) stop if(id==0) then do k=1,5 a=a+1call MPI_SEND(a,5,MPI_INTEGER, 1,k,MPI_COMM_WORLD,ierr)call MPI_RECV(b,4,MPI_INTEGER,1,k,MPI_COMM_WORLD,status,ierr) end do else do k=1,5 call MPI_RECV(a,5,MPI_INTEGER,0,k,MPI_COMM_WORLD,status,ierr) b=a(1:4)call MPI_SEND(b,4,MPI_INTEGER, 0,k,MPI_COMM_WORLD,ierr)end do end if _______________________________________________ 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-- * Dr. Aurélien Bouteiller * Sr. Research Associate at Innovative Computing Laboratory * University of Tennessee * 1122 Volunteer Boulevard, suite 350 * Knoxville, TN 37996 * 865 974 6321 _______________________________________________ users mailing list us...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/user s
