#include <mpi.h>
int MPI_Op_create(MPI_User_function *function, int commute,
MPI_Op *op)
INCLUDE 'mpif.h'
MPI_OP_CREATE(FUNCTION, COMMUTE, OP, IERROR)
EXTERNAL FUNCTION
LOGICAL COMMUTE
INTEGER OP, IERROR
#include <mpi.h> void Op::Init(User function* function, bool commute)
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.
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.
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.
Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.
MPI_Reduce MPI_Reduce_scatter MPI_Allreduce MPI_Scan MPI_Op_free