## 4.9.4. User-Defined Operations

Up: Global Reduction Operations Next: Example of User-defined Reduce Previous: MINLOC and MAXLOC

MPI_OP_CREATE( function, commute, op)
[ IN function] user defined function (function)
[ IN commute] `true` if commutative; `false` otherwise.
[ OUT op] operation (handle)

`int MPI_Op_create(MPI_User_function *function, int commute, MPI_Op *op)
`

`MPI_OP_CREATE( FUNCTION, COMMUTE, OP, IERROR)
EXTERNAL FUNCTION
LOGICAL COMMUTE
INTEGER OP, IERROR
`

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, talking 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] v[i], for i=0 , ... , len-1, where 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 , for , where is the combining operation computed by the function.

[] Rationale.

The len argument allows MPI_REDUCE to avoid calling the function for each element in the input buffer. Rather, the system can choose to apply the function to chunks of input. In C, it is passed in as a reference for reasons of compatibility with Fortran.

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. ( End of rationale.)
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.

[] Advice to users.

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.

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. ( End of advice to users.)
[] Advice to implementors.

We outline below a naive and inefficient implementation of MPI_REDUCE.

    if (rank > 0) {
         RECV(tempbuf, count, datatype, rank-1,...)
         User_reduce( tempbuf, sendbuf, count, datatype)
     }
     if (rank < groupsize-1) {
         SEND( sendbuf, count, datatype, rank+1, ...)
     }
     /* answer now resides in process groupsize-1 ... now send to root
      */
     if (rank == groupsize-1) {
         SEND( sendbuf, count, datatype, root, ...)
     }
     if (rank == root) {
         RECV(recvbuf, count, datatype, groupsize-1,...)
     }

The reduction computation proceeds, sequentially, from process `0` to process `group-size-1`. Thisorder is chosen so as to respect the order of a possibly non-commutative operator defined by the function `User_reduce()`. A more efficient implementation is achieved by taking advantage of associativity and using a logarithmic tree reduction. Commutativity can be used to advantage, for those cases in which the commute argument to MPI_OP_CREATE is true. Also, the amount of temporary buffer required can be reduced, and communication can be pipelined with computation, by transferring and reducing the elements in chunks of size `len` <`count`.

The predefined reduce operations can be implemented as a library of user-defined operations. However, better performance might be achieved if MPI_REDUCE handles these functions as a special case. ( End of advice to implementors.)
MPI_OP_FREE( op)
[ IN op] operation (handle)

`int MPI_op_free( MPI_Op *op)
`

`MPI_OP_FREE( OP, IERROR)
INTEGER OP, IERROR
`

Marks a user-defined reduction operation for deallocation and sets op to MPI_OP_NULL.

Up: Global Reduction Operations Next: Example of User-defined Reduce Previous: MINLOC and MAXLOC

### 4.9.4.1. Example of User-defined Reduce

Up: User-Defined Operations Next: All-Reduce Previous: User-Defined Operations

It is time for an example of user-defined reduction.

Example 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
         */

Up: User-Defined Operations Next: All-Reduce Previous: User-Defined Operations

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