Thank you to all of you for your answers (I was off up to now). Actually my question was't well posed. I stated it more clearly in this post, with the answer:
https://stackoverflow.com/questions/50130688/mpi-cartesian-grid-cumulate-a-scalar-value-through-the-procs-of-a-given-axis-o?noredirect=1#comment87286983_50130688 Thanks again. 2018-05-02 13:56 GMT+02:00 Peter Kjellström <c...@nsc.liu.se>: > On Wed, 2 May 2018 11:15:09 +0200 > Pierre Gubernatis <pierre.guberna...@gmail.com> wrote: > > > Hello all... > > > > I am using a *cartesian grid* of processors which represents a spatial > > domain (a cubic geometrical domain split into several smaller > > cubes...), and I have communicators to address the procs, as for > > example a comm along each of the 3 axes I,J,K, or along a plane > > IK,JK,IJ, etc..). > > > > *I need to cumulate a scalar value (SCAL) through the procs which > > belong to a given axis* (let's say the K axis, defined by I=J=0). > > > > Precisely, the origin proc 0-0-0 has a given value for SCAL (say > > SCAL000). I need to update the 'following' proc (0-0-1) by doing SCAL > > = SCAL + SCAL000, and I need to *propagate* this updating along the K > > axis. At the end, the last proc of the axis should have the total sum > > of SCAL over the axis. (and of course, at a given rank k along the > > axis, the SCAL value = sum over 0,1, K of SCAL) > > > > Please, do you see a way to do this ? I have tried many things (with > > MPI_SENDRECV and by looping over the procs of the axis, but I get > > deadlocks that prove I don't handle this correctly...) > > Thank you in any case. > > Why did you try SENDRECV? As far as I understand your description above > data only flows one direction (along K)? > > There is no MPI collective to support the kind of reduction you > describe but it should not be hard to do using normal SEND and RECV. > Something like (simplified psuedo code): > > if (not_first_along_K) > MPI_RECV(SCAL_tmp, previous) > SCAL += SCAL_tmp > > if (not_last_along_K) > MPI_SEND(SCAL, next) > > /Peter K >
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