I wrote my assignment using MPI, and tested it both locally on Open MPI (1.6.5) and on IBM Blue Gene/P (with mpi implementation provided by IBM). Everything worked fine. Turns out that our solutions are tested also in our labs where MPICH, is installed: $ mpich2version MPICH2 Version: 1.4.1p1 And when I run my solution there I get this strange error: $ mpirun -n 2 msp-par.exe 10 10 1 Internal Error: invalid error code 409e10 (Ring ids do not match) in MPIR_Reduce_impl:1087 Fatal error in PMPI_Reduce: Other MPI error, error stack: PMPI_Reduce(1270).....: MPI_Reduce(sbuf=0x7fff693a92e8, rbuf=0x7fff693a9300, count=1, dtype=USER<struct>, op=0x98000000, root=0, MPI_COMM_WORLD) failed MPIR_Reduce_impl(1087): I am literally out of ideas what is wrong! Below source code fragments (c++): struct msp_solution { int x1, y1, x2, y2; m_entry_t max_sum; msp_solution(); msp_solution(const pair<int, int> &c1, const pair<int, int> &c2, int max_sum); friend bool operator<(const msp_solution &s1, const msp_solution &s2); }; void max_msp_solution(msp_solution *in, msp_solution *inout, int, MPI_Datatype*) { *inout = max(*in, *inout); } // somewhere in code { MPI_Datatype MPI_msp_solution_t; MPI_Op max_msp_solution_op; // create MPI struct from msp_solution MPI_Datatype types[] = { MPI_INT, MPI_LONG_LONG_INT }; int block_lengths[] = { 4, 2 }; MPI_Aint base_addr, x1_addr, max_sum_addr; MPI_Get_address(&collected_solution, &base_addr); MPI_Get_address(&collected_solution.x1, &x1_addr); MPI_Get_address(&collected_solution.max_sum, &max_sum_addr); MPI_Aint displacements[] = { x1_addr - base_addr, max_sum_addr - base_addr }; MPI_Type_create_struct(2, block_lengths, displacements, types, &MPI_msp_solution_t); MPI_Type_commit(&MPI_msp_solution_t); // max reduction function MPI_Op_create((MPI_User_function *) max_msp_solution, 1, &max_msp_solution_op); ... msp_solution solution, received_solution; MPI_Comm comm, ... // comm is created using MPI_Comm_split // solution is initialized MPI_Reduce(&solution, &received_solution, 1, MPI_msp_solution_t, max_msp_solution_op , 0, MPI_COMM_WORLD); // ERROR above!!! } Is there some error in this? How can I make it run? P.S. MPI_Send and MPI_Recv on my struct MPI_msp_solution_t seems to work fine Thanks in advance! Best regards, Kuba