Function all

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.