Kokkos::parallel_reduce

Kokkos::parallel_reduce()

Header File: Kokkos_Core.hpp

Usage

Kokkos::parallel_reduce( name, policy, functor, result);
Kokkos::parallel_reduce( name, policy, functor, reducer);
Kokkos::parallel_reduce( name, policy, functor);
Kokkos::parallel_reduce( policy, functor, result);
Kokkos::parallel_reduce( policy, functor, reducer);
Kokkos::parallel_reduce( policy, functor);

Dispatches parallel work defined by functor according to the ExecutionPolicy policy and performance a reduction of the contributions provided by the work items. The optional label name is used by profiling and debugging tools. The reduction type is either a sum, is defined by the reducer or is deduced from an optional join operator on the functor. The reduction result is stored in result, or through the reducer handle. It is also provided to the functor.final() function if such a function exists.

Interface

template <class ExecPolicy, class FunctorType>
Kokkos::parallel_reduce(const std::string& name, 
                        const ExecPolicy& policy, 
                        const FunctorType& functor);

template <class ExecPolicy, class FunctorType>
Kokkos::parallel_reduce(const ExecPolicy& policy, 
                        const FunctorType& functor);

template <class ExecPolicy, class FunctorType, class ReducerArgument>
Kokkos::parallel_reduce(const std::string& name, 
                        const ExecPolicy& policy, 
                        const FunctorType& functor, 
                        const ReducerArgument& reducer);

template <class ExecPolicy, class FunctorType, class ReducerArgument>
Kokkos::parallel_reduce(const ExecPolicy& policy, 
                        const FunctorType& functor, 
                        const ReducerArgument& reducer);

template <class ExecPolicy, class FunctorType, class ReducerArgumentNonConst>
Kokkos::parallel_reduce(const std::string& name, 
                        const ExecPolicy& policy, 
                        const FunctorType& functor, 
                        ReducerArgumentNonConst& reducer);

template <class ExecPolicy, class FunctorType, class ReducerArgumentNonConst>
Kokkos::parallel_reduce(const ExecPolicy& policy, 
                        const FunctorType& functor, 
                        ReducerArgumentNonConst& reducer);

Parameters:

• name: A user provided string which is used in profiling and debugging tools via the Kokkos Profiling Hooks.
• ExecPolicy: An ExecutionPolicy which defines iteration space and other execution properties. Valid policies are:
  • IntegerType: defines a 1D iteration range, starting from 0 and going to a count.
  • RangePolicy: defines a 1D iteration range.
  • MDRangePolicy: defines a multi-dimensional iteration space.
  • TeamPolicy: defines a 1D iteration range, each of which is assigned to a thread team.
  • TeamThreadRange: defines a 1D iteration range to be executed by a thread-team. Only valid inside a parallel region executed through a TeamPolicy or a TaskTeam.
  • ThreadVectorRange: defines a 1D iteration range to be executed through vector parallelization dividing the threads within a team. Only valid inside a parallel region executed through a TeamPolicy or a TaskTeam.
• FunctorType: A valid functor with (at minimum) an operator() with a matching signature for the ExecPolicy combined with the reduced type.
• ReducerArgument: Either a class fullfilling the "Reducer" concept or a Kokkos::View
• ReducerArgumentNonConst: a class fullfilling the "Reducer" concept, a POD type with operator += and operator =, or a Kokkos::View. The ReducerArgumentNonConst can also be an array or a pointer; see below for functor requirements.

Requirements:

• If ExecPolicy is not MDRangePolicy the functor has a member function of the form operator() (const HandleType& handle, ReducerValueType& value) const or operator() (const WorkTag, const HandleType& handle, ReducerValueType& value) const
  • The WorkTag free form of the operator is used if ExecPolicy is an IntegerType or ExecPolicy::work_tag is void.
  • HandleType is an IntegerType if ExecPolicy is an IntegerType else it is ExecPolicy::member_type.
• If ExecPolicy is MDRangePolicy the functor has a member function of the form operator() (const IntegerType& i0, ... , const IntegerType& iN, ReducerValueType& value) const or operator() (const WorkTag, const IntegerType& i0, ... , const IntegerType& iN, ReducerValueType& value) const
  • The WorkTag free form of the operator is used if ExecPolicy::work_tag is not void.
  • N must match ExecPolicy::rank
• If the functor is a lambda, ReducerArgument must satisfy the Reducer concept or ReducerArgumentNonConst must be a POD type with operator += and operator = or a Kokkos::View. In the latter case, the default Sum reduction is applied.
• If ExecPolicy is TeamThreadRange a "reducing" functor is not allowed and the ReducerArgument must satisfy the Reducer concept or ReducerArgumentNonConst must be a POD type with operator += and operator = or a Kokkos::View. In the latter case, the default Sum reduction is applied.
• The reduction argument type ReducerValueType of the functor operator must be compatible with the ReducerArgument (or ReducerArgumentNonConst) and must match the arguments of the init, join and final functions of the functor if those exist.
• If ReducerArgument (or ReducerArgumentNonConst)
  • is a scalar type: ReducerValueType must be of the same type.
  • is a Kokkos::View: ReducerArgument::rank must be 0 and ReducerArgument::non_const_value_type must match ReducerValueType.
  • satisfies the Reducer concept: ReducerArgument::value_type must match ReducerValueType
  • is an array or a pointer
    • ReducerValueType must match the array or the pointer signature
    • the functor must define FunctorType::value_type the same as ReducerValueType
    • the functor must declare a public member variable int value_count which is the length of the array
    • the functor must implement the function void init( ReducerValueType dst [] ) constor void init( ReducerValueType * dst) const depending on whether ReducerArgumentNonConst is an array or pointer respectively.
    • the functor must implement the function void join( ReducerValueType dst[], ReducerValueType src[] ) const or void join( ReducerValueType * dst, ReducerValueType * src ) const depending on whether ReducerArgumentNonConst is an array or pointer respectively.
    • If the functor implements the final function, the argument must also match those of init and join.

Semantics

• Neither concurrency nor order of execution are guaranteed.
• The call is potentially asynchronous if the ReducerArgument is not a scalar type.
• The ReducerArgument content will be overwritten, i.e. the value does not need to be initialized to the reduction-neutral element.
• The input value to the operator may contain a partial reduction result, Kokkos may only combine the thread local contributions in the end. The operator must modify the input reduction value according to the requested reduction type.

Examples

More Detailed Examples are provided in the Custom Reductions and ExecutionPolicy documentation.

#include<Kokkos_Core.hpp>
#include<cstdio> 

int main(int argc, char* argv[]) {
   Kokkos::initialize(argc,argv);

   int N = atoi(argv[1]);
   double result;
   Kokkos::parallel_reduce("Loop1", N, KOKKOS_LAMBDA (const int& i, double& lsum ) {
     lsum += 1.0*i;
   },result);

   printf("Result: %i %lf\n",N,result);
   Kokkos::finalize();
}

#include<Kokkos_Core.hpp>
#include<cstdio> 

struct TagMax {};
struct TagMin {};

struct Foo {
  KOKKOS_INLINE_FUNCTION
  void operator() (const TagMax, const Kokkos::TeamPolicy<>::member_type& team, double& lmax) const {
    if( team.league_rank % 17 + team.team_rank % 13 > lmax )
      lmax = team.league_rank % 17 + team.team_rank % 13;
  }
  KOKKOS_INLINE_FUNCTION
  void operator() (const TagMin, const Kokkos::TeamPolicy<>::member_type& team, double& lmin ) const {
    if( team.league_rank % 17 + team.team_rank % 13 < lmin )
      lmin = team.league_rank % 17 + team.team_rank % 13;
  }
};

int main(int argc, char* argv[]) {
   Kokkos::initialize(argc,argv);

   int N = atoi(argv[1]);

   Foo foo;
   double max,min;
   Kokkos::parallel_reduce(Kokkos::TeamPolicy<TagMax>(N,Kokkos::AUTO), foo, Kokkos::Max<double>(max));
   Kokkos::parallel_reduce("Loop2", Kokkos::TeamPolicy<TagMin>(N,Kokkos::AUTO), foo, Kokkos::Min<double>(min));
   Kokkos::fence();

   printf("Result: %lf %lf\n",min,max);

   Kokkos::finalize();
}