SYNTAX

C Syntax

#include <mpi.h>
int MPI_Op_create(MPI_User_function *function, int commute,
        MPI_Op *op)

Fortran Syntax

INCLUDE 'mpif.h'
MPI_OP_CREATE(FUNCTION, COMMUTE, OP, IERROR)
        EXTERNAL        FUNCTION
        LOGICAL COMMUTE
        INTEGER OP, IERROR

C++ Syntax

#include <mpi.h>
void Op::Init(User function* function, bool commute)

INPUT PARAMETERS

function

User-defined function (function).

commute

True if commutative; false otherwise.

OUTPUT PARAMETERS

op

Operation (handle).

IERROR

Fortran only: Error status (integer).

DESCRIPTION

MPI_Op_create binds a user-defined global operation to an op handle that can subsequently be used in MPI_Reduce, MPI_Allreduce, MPI_Reduce_scatter, and MPI_Scan. The user-defined operation is assumed to be associative. If commute = true, then the operation should be both commutative and associative. If commute = false, then the order of operands is fixed and is defined to be in ascending, process rank order, beginning with process zero. The order of evaluation can be changed, taking advantage of the associativity of the operation. If commute = true then the order of evaluation can be changed, taking advantage of commutativity and associativity.

function is the user-defined function, which must have the following four arguments: invec, inoutvec, len, and datatype.

The ANSI-C prototype for the function is the following:

  typedef void MPI_User_function(void *invec, void *inoutvec,
                                 int *len, 
                                 MPI_Datatype *datatype);

The Fortran declaration of the user-defined function appears below.

  FUNCTION USER_FUNCTION( INVEC(*), INOUTVEC(*), LEN, TYPE) 
  <type> INVEC(LEN), INOUTVEC(LEN) 
   INTEGER LEN, TYPE 

The datatype argument is a handle to the data type that was passed into the call to MPI_Reduce. The user reduce function should be written such that the following holds: Let u[0], ..., u[len-1] be the len elements in the communication buffer described by the arguments invec, len, and datatype when the function is invoked; let v[0], ..., v[len-1] be len elements in the communication buffer described by the arguments inoutvec, len, and datatype when the function is invoked; let w[0], ..., w[len-1] be len elements in the communication buffer described by the arguments inoutvec, len, and datatype when the function returns; then w[i] = u[i] o v[i], for i=0 ,..., len-1, where o is the reduce operation that the function computes.

Informally, we can think of invec and inoutvec as arrays of len elements that function is combining. The result of the reduction over-writes values in inoutvec, hence the name. Each invocation of the function results in the pointwise evaluation of the reduce operator on len elements: i.e, the function returns in inoutvec[i] the value invec[i] o inoutvec[i], for i = 0,..., count-1, where o is the combining operation computed by the function.

By internally comparing the value of the datatype argument to known, global handles, it is possible to overload the use of a single user-defined function for several different data types.

General datatypes may be passed to the user function. However, use of datatypes that are not contiguous is likely to lead to inefficiencies.

No MPI communication function may be called inside the user function. MPI_Abort may be called inside the function in case of an error.

NOTES

Suppose one defines a library of user-defined reduce functions that are overloaded: The datatype argument is used to select the right execution path at each invocation, according to the types of the operands. The user-defined reduce function cannot "decode" the datatype argument that it is passed, and cannot identify, by itself, the correspondence between the datatype handles and the datatype they represent. This correspondence was established when the datatypes were created. Before the library is used, a library initialization preamble must be executed. This preamble code will define the datatypes that are used by the library and store handles to these datatypes in global, static variables that are shared by the user code and the library code.

Example: Example of user-defined reduce:

Compute the product of an array of complex numbers, in C.

    typedef struct { 
        double real,imag; 
    } Complex; 
 
    /* the user-defined function 
     */ 
    void myProd( Complex *in, Complex *inout, int *len, 
                 MPI_Datatype *dptr ) 
    { 
        int i; 
        Complex c; 
 
    for (i=0; i< *len; ++i) { 
            c.real = inout->real*in->real - 
                       inout->imag*in->imag; 
            c.imag = inout->real*in->imag + 
                       inout->imag*in->real; 
            *inout = c; 
            in++; inout++; 
        } 
    } 
 
    /* and, to call it... 
     */ 
    ...
    /* each process has an array of 100 Complexes 
         */ 
        Complex a[100], answer[100]; 
        MPI_Op myOp; 
        MPI_Datatype ctype; 
    
    /* explain to MPI how type Complex is defined 
         */ 
       MPI_Type_contiguous( 2, MPI_DOUBLE, &ctype ); 
        MPI_Type_commit( &ctype ); 
        /* create the complex-product user-op 
         */ 
        MPI_Op_create( myProd, True, &myOp ); 
     
        MPI_Reduce( a, answer, 100, ctype, myOp, root, comm ); 
     
        /* At this point, the answer, which consists of 100 Complexes, 
         * resides on process root 
         */ 

The Fortran version of MPI_Reduce will invoke a user-defined reduce function using the Fortran calling conventions and will pass a Fortran-type datatype argument; the C version will use C calling convention and the C representation of a datatype handle. Users who plan to mix languages should define their reduction functions accordingly.

NOTES ON COLLECTIVE OPERATIONS

The reduction functions ( MPI_Op ) do not return an error value. As a result, if the functions detect an error, all they can do is either call MPI_Abort or silently skip the problem. Thus, if you change the error handler from MPI_ERRORS_ARE_FATAL to something else, for example, MPI_ERRORS_RETURN , then no error may be indicated.

The reason for this is the performance problems in ensuring that all collective routines return the same error value.