This is a discussion-point for MPI 2.1, Ballot 4. This is a follow up to: Questions about Graph_create, Cart_crate, and Cart_coords in http://www.cs.uiuc.edu/homes/wgropp/projects/parallel/MPI/mpi-errata/index.h... with mail discussion in http://www.cs.uiuc.edu/homes/wgropp/projects/parallel/MPI/mpi-errata/discuss... I'm starting a new 2nd stream for the zero-dim discussion. ------------------------------- This report shows significant differences of MPI implementations in the (extreme) case of zero-dimensional topologies! Therefore clarifications may be necessary. (Although hopefully never an application may produce zero-dimensional Cartesian topologies!) If there are no objections, I would produce clarifications for MPI-2.1 according to the behavior of mpich2 and MPI from IBM (except the rank -767705708, see below). MPI-1.1 Sect. 6.5.6, page 187, routine MPI_CART_SUB defines on lines 38-42: If a cartesian topology has been created with MPI_CART_CREATE, the function MPI_CART_SUB can be used to partition the communicator group into subgroups that form lower-dimensional cartesian subgrids, and to build for each subgroup a communicator with the associated subgrid cartesian topology. (This function is closely related to MPI_COMM_SPLIT.) The text clearly says, that the new communicator must (1) have a Cartesian topology associated (2) be lower-dimensional in the case of a subgrid There is no restriction on the input array remain_dims. Therefore all MPI implementations (that I tested) allow that all entries in remain_dims are "false". I tested several MPI libraries with creating a subgrid with MPI_Cart_sub(remain_dims=0) from a 1-dim Cartesian topology. I tested the subgrid communicator with MPI_Topo_test. In the case of MPI_CART, I used MPI_Cartdim_get to retrieve ndims, and MPI_Cart_get for further details: - mpich2: MPI_CART, ndims=0, 1.0.3 MPI_Cart_get works, but keeps all OUT arguments unchanged - IBM: MPI_CART, ndims=0, (on SP) MPI_Cart_get works, but keeps all OUT arguments unchanged - OpenMPI: MPI_CART, ndims=1, 1.2.4 MPI_Cart_get works and returns dims=1, periods=0, coords=0 independent from process or periods in the original comm. (may be wrong because (2) is not fulfilled) - NEC MP/EX: not MPI_CART (may be wrong because (1) is not fulfilled) - Voltaire: not MPI_CART (mpich1) (may be wrong because (1) is not fulfilled) With the implementations that return a correct zero-dim Cartesian topology, I tested further usage of this zero-dim communicator: - MPI_Comm_size returns 1 and MPI_Comm_rank returns 0 because this communicator is like MPI_COMM_SELF, but with Cartesian topology associated. - MPI_Cart_rank(IN ZeroDimComm, IN coords=0, OUT rank) Rationale. A zero-dim communicator has zero coords, i.e., this routine should not examine the coords input argument. --> mpich2: rank = 0 is returned (may be because this is the only existing rank in this communicator, this value may make sense, independent of the coord, that should not be analyzed) --> IBM: rank = -767705708 is returned (strange value, not MPI_PROC_NULL, not MPI_UNDEFINED) - MPI_Cart_coords(IN ZeroDimComm, IN rank=0, OUT coords) Rationale. A zero-dim communicator has zero coords, i.e., this routine should not return anything in the coords output argument. --> mpich2 and IBM: coords is not modified (as expected) - MPI_Cart_sub(IN ZeroDimComm, IN remain_dims=0, OUT subsubcomm) Rationale. A zero-dim communicator has zero dimensions, i.e., this routine should not examine remain_dims and the returned communicator should be again a zero-dim Cartesian communicator. --> mpich2 and IBM: subsubcomm is a zero-dim Cartesian communicator (as expected) - MPI_Cart_shift(IN ZeroDimComm, IN direction=0, IN disp=1, OUT src, OUT dest) Rationale. This call is erroneous because in a zero-dim communicator, the direction=0 does not exist. mpich2 and IBM: They detect the error and abort. (In OpenMPI, all these calls work as expected on a 1-dimensional topology on a "MPI_COMM_SELF") The last test is not addressed by MPI-1.1: Is it possible to build a zero-dim Cartesian topology directly by calling MPI_Cart_create: MPI_Cart_create(IN MPI_COMM_SELF, IN ndims=0, IN dims=1, IN Periods, IN reorder, OUT ZeroDimComm) Results: All tested MPI implementations return an error and abort. (Same on MPI_COMM_WORLD) The latter question was raised by Jesper Traeff. Best regards Rolf Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner_at_[hidden] High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner Nobelstr. 19, D-70550 Stuttgart, Germany . (Office: Allmandring 30) *