$ mpiexec -n 2 -f hostfile ./mpi/mpich-3.3.1/examples/cpi
Process 1 of 2 is on csews2
Process 0 of 2 is on csews1
Fatal error in PMPI_Reduce: Unknown error class, error stack:
PMPI_Reduce(523)................: MPI_Reduce(sbuf=0x7ffeab580c10, rbuf=0x7ffeab580c18, count=1, datatype=MPI_DOUBLE, op=MPI_SUM, root=0, comm=MPI_COMM_WORLD) failed
PMPI_Reduce(509)................:
MPIR_Reduce_impl(316)...........:
MPIR_Reduce_intra_auto(231).....:
MPIR_Reduce_intra_binomial(125).:
MPIDI_CH3U_Recvq_FDU_or_AEP(629): Communication error with rank 1

Can anyone explain this error and how to overcome it.I have installed just MPICH and not HYDRA package.

By the way this same command works fine with hello program the one where each node says their identity.