I was reading through www.unixer.de/publications/img/mprobe-proposal-rev4.pdf and - just for fun - wondered if there is a workaround, not because MPI_Mprobe shouldn't be in the MPI-3 standard, but because some folks might want a workaround for backwards compatibility if they are forced to use MPI-2 somewhere. I apologize in advance if something equivalent to what I say below was discussed at the Forum and I was not present. Torsten and coworkers say the following. ============================ For example, the following code can not be executed concurrently by two threads in an MPI process, because a message could be found by the MPI Probe in both threads, while only one of the threads could successfully receive the message (the other will block): MPI_Status status; int value; MPI_Probe(MPI_ANY_SOURCE, /*tag=*/0, MPI_COMM_WORLD, &status); MPI_Recv(&value, 1, MPI_INT, status.MPI_SOURCE, /*tag=*/0, MPI_COMM_WORLD, MPI_STATUS_IGNORE); <snip> There is no known workaround that addresses all of the problems with MPI Probe and MPI Iprobe in multi-threaded MPI applications. ============================ Obviously, a fat mutex around this block solves the problem, but the time spent in the mutex will scale with the message size. I was curious the following workaround is reasonable when MPI-2 must be used. ============================ MPI_Status status; MPI_Request request; AppropriateMutex mutex; int value; ACQUIRE_MUTEX(&mutex); MPI_Probe(MPI_ANY_SOURCE, /*tag=*/0, MPI_COMM_WORLD, &status); /* ? */ MPI_Irecv(&value, 1, MPI_INT, status.MPI_SOURCE, /*tag=*/0, MPI_COMM_WORLD, &request); RELEASE_MUTEX(&mutex); MPI_Wait(&request, MPI_STATUS_IGNORE); ============================ Assuming MPI_Irecv returns quickly and there is nothing in between the two operations, then the time spent in the critical section defined by the mutex will be rather short. My solution may become inefficient if one has to allocate memory to receive the message while holding the mutex. However, instances where memory allocation is slow should be quite rare and there are workarounds when such scenarios exist. Are there nuances regarding the use of MPI that I have missed? Do the real-world use cases have too much to do in the "/* ? */" to make this viable? Thanks, Jeff -- Jeff Hammond Argonne Leadership Computing Facility University of Chicago Computation Institute [email protected] / (630) 252-5381 http://www.linkedin.com/in/jeffhammond https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond