user-defined operations reduce, user-defined scan, user-defined

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 operations 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.
associativity, and user-defined operation
commutativity, and user-defined operation

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]ov[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]oinoutvec[i],* for i = 0,.....,count-1,
where *o* is the combining operation computed by the function.

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.

Fri Sep 1 06:16:55 EDT 1995