Hi, Running one of our test programs using MPICH 3.4.3 and ch4:ofi, I notice that MPI_Wait on an MPI_Request that has been MPI_Cancelled never completes (it does when using ch3). (The documentation for MPI_Cancel states "If a communication is marked for cancellation, then a MPI_WAIT call for that communication is guaranteed to return, irrespective of the activities of other processes (i.e., MPI_WAIT behaves as a local function)") Here's a simple example: #include <mpi.h> #include <stdio.h> #include <stdlib.h> void check(int const value) { if (value != MPI_SUCCESS) { fprintf(stderr, "Failed.\n"); exit(1); } } int main(int argc, char** argv) { MPI_Request r1; int payload = 42; int result; check(MPI_Init(0,0)); check(MPI_Issend(&payload, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, &r1)); check(MPI_Test(&r1, &result, MPI_STATUS_IGNORE)); fprintf(stdout, "MPI_Test result: %d\n", result); check(MPI_Cancel(&r1)); check(MPI_Wait(&r1, MPI_STATUS_IGNORE)); MPI_Finalize(); return 0; } This stalls in MPI_Wait when executed using "mpiexec -n 1 ./a.out". Cheers, Edric.