Kokkos::ReducerConcept

Kokkos::ReducerConcept

The concept of a Reducer is the abstraction that defines the "how" a "Reduction" is performed during the parallel reduce execution pattern. The abstraction of "what" is given as a template parameter and corresponds to the "what" that is being reduced in the parallel_reduce operation. This page describes the definitions and functions expected from a Reducer with a hypothetical 'Reducer' class definition. A brief description of built-in reducers is also included.

Header File: Kokkos_Core.hpp

Usage:

T result;
parallel_reduce(N,Functor,ReducerConcept<T>(result));

.

Synopsis

class Reducer {
  public:
    //Required for Concept
    typedef Reducer reducer;
    typedef ... value_type;
    typedef Kokkos::View<value_type, ... > result_view_type;
    
    KOKKOS_INLINE_FUNCTION
    void join(value_type& dest, const value_type& src)  const

    KOKKOS_INLINE_FUNCTION
    void join(volatile value_type& dest, const volatile value_type& src) const;

    KOKKOS_INLINE_FUNCTION
    void init( value_type& val)  const;

    KOKKOS_INLINE_FUNCTION
    value_type& reference() const;

    KOKKOS_INLINE_FUNCTION
    result_view_type view() const;

    
    //Part of Build-In reducers for Kokkos
    KOKKOS_INLINE_FUNCTION
    Reducer(value_type& value_);

    KOKKOS_INLINE_FUNCTION
    Reducer(const result_view_type& value_);
};

Public Class Members

Typedefs

• reducer: The self type.
• value_type: The reduction scalar type.
• result_view_type: A Kokkos::View referencing where the reduction result should be placed. Can be an unmanaged view of a scalar or complex datatype (class or struct). Unmanaged views must specify the same memory space where the referenced scalar (or complex datatype) resides.

Constructors

Constructors are not part of the concept. A custom reducer can have complex custom constructors. All Build-In reducers in Kokkos have the following two constructors:

• Reducer(value_type& result)

  Constructs a reducer which references a local variable as its result location.

• Reducer(const result_view_type result)`

  Constructs a reducer which references a specific view as its result location.

Functions

• void join(value_type& dest, const value_type& src)  const;`

  Combine src into dest. For example, Add performs dest+=src;.

• void join(volatile value_type& dest, const volatile value_type& src) const;

  Combine src into dest. For example, Add performs dest+=src;.

• void init( value_type& val)  const;

  Initialize val with appropriate initial value. For example, 'Add' assigns val = 0;, but Prod assigns val = 1;

• value_type& reference() const;

  Returns a reference to the result place.

• result_view_type view() const;

  Returns a view of the result place.

Built-In Reducers

Kokkos provides a number of built-in reducers that automatically work with the intrinsic C++ types as well as Kokkos::complex. In order to use a Built-in reducer with a custom type, a template specialization of Kokkos::reduction_identity must be defined. A simple example is shown below and more information can be found under Custom Reductions.

• Kokkos::Sum

• Kokkos::Prod

• Kokkos::Min

• Kokkos::Max

• Kokkos::LAnd

• Kokkos::LOr

• Kokkos::BAnd

• Kokkos::BOr

• Kokkos::MinMax

• Kokkos::MinLoc

• Kokkos::MaxLoc

• Kokkos::MinMaxLoc

Examples

  #include<Kokkos_Core.hpp>
  
  int main(int argc, char* argv[]) {

    long N = argc>1 ? atoi(argv[1]):100; 
    long result;
    Kokkos::parallel_reduce("ReduceSum: ", N, KOKKOS_LAMBDA (const int i, long& lval) {
      lval += i;
    },Sum<long>(result));

    printf("Result: %l Expected: %l\n",result,N*(N-1)/2);
  }