I finally managed to track down some issues in mpi4py's test suite using Open MPI 1.8+. The code below should be enough to reproduce the problem. Run it under valgrind to make sense of my following diagnostics.
In this code I'm creating a 2D, periodic Cartesian topology out of COMM_SELF. In this case, the process in COMM_SELF has 4 logical in/out links to itself. So we have size=1 but indegree=outdegree=4. However, in ompi/mca/coll/basic/coll_basic_module.c, "size * 2" request are being allocated to manage communication: if (OMPI_COMM_IS_INTER(comm)) { size = ompi_comm_remote_size(comm); } else { size = ompi_comm_size(comm); } basic_module->mccb_num_reqs = size * 2; basic_module->mccb_reqs = (ompi_request_t**) malloc(sizeof(ompi_request_t *) * basic_module->mccb_num_reqs); I guess you have to also special-case for topologies and allocate indegree+outdegree requests (not sure about this number, just guessing). #include <stdio.h> #include <mpi.h> int main(int argc, char *argv[]) { MPI_Comm comm; int ndims = 2, dims[2] = {1,1}, periods[2] = {1,1}; int sendbuf = 7, recvbuf[5] = {0,0,0,0,0}; MPI_Init(&argc, &argv); MPI_Cart_create(MPI_COMM_SELF, ndims, dims, periods, 0, &comm); MPI_Neighbor_allgather(&sendbuf, 1, MPI_INT, recvbuf, 1, MPI_INT, comm); {int i; for (i=0;i<5;i++) printf("%d ",recvbuf[i]); printf("\n");} MPI_Finalize(); return 0; } -- Lisandro Dalcin ============ Research Scientist Computer, Electrical and Mathematical Sciences & Engineering (CEMSE) Numerical Porous Media Center (NumPor) King Abdullah University of Science and Technology (KAUST) http://numpor.kaust.edu.sa/ 4700 King Abdullah University of Science and Technology al-Khawarizmi Bldg (Bldg 1), Office # 4332 Thuwal 23955-6900, Kingdom of Saudi Arabia http://www.kaust.edu.sa Office Phone: +966 12 808-0459