On Thu, 24 Jan 2008 21:26:09 +0800 "William Yu" <wyu_at_[hidden]> wrote:

Understood. Nice work. Thanks!

Thank you. And here is the proposal to correct all the unclear specifications on Cartesian topologies. This is a proposal for MPI 2.1, Ballot 4. ________________________________________________________________ Proposal about handling zero-dimensional Cartesian communicators that are produced with MPI_Cart_sub if all remain_dims are false. MPI-1.1 Sect.6.5.4, page 187, lines 38-42 (definition of MPI_Cart_sub) reads 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.) but should read 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. If all entries in remain_dims are false or comm is already associated with a zero-dimensional Cartesian topology then newcomm is associated with a zero-dimensional Cartesian topology. (This function is closely related to MPI_COMM_SPLIT.) Rationale for this clarification (not to be included into the MPI standard): Several MPI implementations have implemented the MPI-1 standard which requires a "lower-dimensional cartesian" subgrid and that newcomm is "associated" with the "subgrid Cartesian topology". Other MPI implementations did not or only partially. Therefore, a clarification may help that all MPI implementations implment the same interface. ________________________ MPI-1.1 Sect.6.5.4, page 183, lines 29-30 (definition of MPI_Cartdim_get and MPI_Cart_get) reads: The functions MPI_CARTDIM_GET and MPI_CART_GET return the cartesian topology information that was associated with a communicator by MPI_CART_CREATE. but should read The functions MPI_CARTDIM_GET and MPI_CART_GET return the cartesian topology information that was associated with a communicator by MPI_CART_CREATE. If comm is associated with a zero-dimensional Cartesian topology, MPI_Cartdim_get returns ndims=0 and MPI_Cart_get will keep all output arguments unchanged. Rationale for this clarification (not to be included into the MPI standard): Zero-dimensional topologies have zero coords, i.e., do not have coords and therefore also no dims, or periods. ________________________ MPI-1.1 Sect.6.5.4, page 184, lines 17-23 (definition of MPI_Cart_rank) reads For a process group with cartesian structure, the function MPI_CART_RANK translates the logical process coordinates to process ranks as they are used by the point-to-point routines. For dimension i with periods(i) = true, if the coordinate, coords(i), is out of range, that is, coords(i) < 0 or coords(i) >= dims(i), it is shifted back to the interval 0 <= coords(i) < dims(i) automatically. Out-of-range coordinates are erroneous for non-periodic dimensions. but should read For a process group with cartesian structure, the function MPI_CART_RANK translates the logical process coordinates to process ranks as they are used by the point-to-point routines. For dimension i with periods(i) = true, if the coordinate, coords(i), is out of range, that is, coords(i) < 0 or coords(i) >= dims(i), it is shifted back to the interval 0 <= coords(i) < dims(i) automatically. Out-of-range coordinates are erroneous for non-periodic dimensions. If comm is associated with a zero-dimensional Cartesian topology, coord is not significant and 0 is returned in rank. Rationale for this clarification (not to be included into the MPI standard): Zero-dimensional topologies have zero coords, i.e., do not have coords. The communicator has only one process and therefore only the rank 0 is valid. ________________________ MPI-1.1 Sect.6.5.4, page 184, lines 38-39 (definition of MPI_Cart_coords) reads The inverse mapping, rank-to-coordinates translation is provided by MPI CART COORDS. but should read The inverse mapping, rank-to-coordinates translation is provided by MPI CART COORDS. If comm is associated with a zero-dimensional Cartesian topology, coords will be unchanged. Rationale for this clarification (not to be included into the MPI standard): Zero-dimensional topologies have zero coords, i.e., do not have coords. ________________________ Alternative 1: -------------- MPI-1.1 Sect.6.5.5, page 186, after line 47 (end of definition of MPI_Cart_shift), the following paragraph is added: It is erroneous to call MPI_CART_SHIFT with comm that is associated with a zero-dimensional Cartesian topology. Rationale for this clarification (not to be included into the MPI standard): It is impossible to define a correct input for direction because there does not exist any directions in a zero-dimensional topology. (This is currently implemented by several MPI implementations.) Alternative 2: -------------- MPI-1.1 Sect.6.5.5, page 186, after line 47 (end of definition of MPI_Cart_shift), the following paragraph is added: If comm is associated with a zero-dimensional Cartesian topology, then the input arguments direction and disp are ignored and always MPI_PROC_NULL is returned in rank_source and rank_dest. Rationale for this clarification (not to be included into the MPI standard): It is impossible to define a correct input for direction because there does not exist any directions in a zero-dimensional topology. For convenience, the routine returns MPI_PROC_NULL. _______________________ MPI-1.1 Sect.6.5.1, page 179, lines 29-30 (end of definition of MPI_Cart_create) reads The call is erroneous if it specifies a grid that is larger than the group size. but should read The call is erroneous if it specifies a grid that is larger than the group size or ndims is zero or negative. Rationale for this clarification (not to be included into the MPI standard): Although it is allowed to produce zero-dimensional subgrids with MPI_CART_SUB, it makes no sense to establish a zero-dimensional grid a priori with MPI_CART_CREATE. _______________________ MPI-1.1 Sect.6.5.4, page 184, lines 30 (definition of MPI_Cart_coords) reads IN maxdims length of vector coord in the calling program (integer) but should read (missing “s” at coords) IN maxdims length of vector coords in the calling program (integer) Rationale for this clarification (not to be included into the MPI standard): Typo. _______________________ Best regards Rolf Rabenseifner PS: All background informations with reports from 5 different MPI implementations and their differences have been in my previous mail on Wed, 23 Jan 2008 18:42:43. 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)