Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Function all_reduce

boost::mpi::all_reduce — Combine the values stored by each process into a single value available to all processes.

Synopsis

// In header: <boost/mpi/collectives.hpp>


template<typename T, typename Op> 
  void all_reduce(const communicator & comm, const T * value, int n, 
                  T * out_value, Op op);
template<typename T, typename Op> 
  void all_reduce(const communicator & comm, const T & value, T & out_value, 
                  Op op);
template<typename T, typename Op> 
  T all_reduce(const communicator & comm, const T & value, Op op);
template<typename T, typename Op> 
  void all_reduce(const communicator & comm, inplace_t< T * > value, int n, 
                  Op op);
template<typename T, typename Op> 
  void all_reduce(const communicator & comm, inplace_t< T > value, Op op);

Description

all_reduce is a collective algorithm that combines the values stored by each process into a single value available to all processes. The values are combined in a user-defined way, specified via a function object. The type T of the values may be any type that is serializable or has an associated MPI data type. One can think of this operation as a all_gather, followed by an std::accumulate() over the gather values and using the operation op.

When the type T has an associated MPI data type, this routine invokes MPI_Allreduce to perform the reduction. If possible, built-in MPI operations will be used; otherwise, all_reduce() will create a custom MPI_Op for the call to MPI_Allreduce.

If wrapped in a inplace_t object, combine the usage of both input and $c out_value and the local value will be overwritten (a convenience function inplace is provided for the wrapping).

Parameters:

comm

The communicator over which the reduction will occur.

n

Indicated the size of the buffers of array type.

op

The binary operation that combines two values of type T and returns a third value of type T. For types T that has ssociated MPI data types, op will either be translated into an MPI_Op (via MPI_Op_create) or, if possible, mapped directly to a built-in MPI operation. See is_mpi_op in the operations.hpp header for more details on this mapping. For any non-built-in operation, commutativity will be determined by the is_commmutative trait (also in operations.hpp): users are encouraged to mark commutative operations as such, because it gives the implementation additional lattitude to optimize the reduction operation.

out_value

Will receive the result of the reduction operation. If this parameter is omitted, the outgoing value will instead be returned.

value

The local value to be combined with the local values of every other process. For reducing arrays, in_values is a pointer to the local values to be reduced and n is the number of values to reduce. See reduce for more information.

Returns:

If no out_value parameter is supplied, returns the result of the reduction operation.


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