Hi all, we detected the behavior with IntelMPI 2019 (which is based on MPICH 3.3). Reproducing it with MPICH-3.3 was not yet successful. But I fear, that our built of MPICH just not uses the necessary code path / build flags. When calling MPI_Iprobe with the same communicator for ~2^31 times (which can take 10-30 minutes), the execution stops with: Abort(201962501) on node 0 (rank 0 in comm 0): Fatal error in PMPI_Iprobe: Invalid communicator, error stack: PMPI_Iprobe(123): MPI_Iprobe(src=MPI_ANY_SOURCE, tag=MPI_ANY_TAG, MPI_COMM_WORLD, flag=0x7ffd925056c0, status=0x7ffd92505694) failed PMPI_Iprobe(90).: Invalid communicator From my understanding of the referenced MPICH code lines, I guess, that the ref-count for MPI_COMM_WORLD overflows, which triggers this error message. Best Joachim -- Dipl.-Inf. Joachim Protze IT Center Group: High Performance Computing Division: Computational Science and Engineering RWTH Aachen University Seffenter Weg 23 D 52074 Aachen (Germany) Tel: +49 241 80- 24765 Fax: +49 241 80-624765 [email protected] www.itc.rwth-aachen.de