Reducer< T, R > Class Template Reference

Reducers are a special kind of symmetric object that, when read, compute a reduction over all instances. More...

#include <Reducer.hpp>

Inheritance diagram for Reducer< T, R >:

Additional Inherited Members
Public Member Functions inherited from ReducerImpl< T, collective_add >
	ReducerImpl ()
	operator T () const
	Read out value; does expensive global reduce. More...
void	operator= (const T &v)
	Globally set the value; expensive global synchronization. More...
void	reset ()
	Globally reset to default value for the type. More...
Protected Attributes inherited from ReducerImpl< T, collective_add >
T	local_value

Detailed Description

template<typename T, ReducerType R>
class Reducer< T, R >

Reducers are a special kind of symmetric object that, when read, compute a reduction over all instances.

They can be inexpensively updated (incremented/decremented, etc), so operations that modify the value without observing it are cheap and require no communication. However, reading the value forces a global reduction operation each time, and directly setting the value forces global synchronization.

Reducers must be declared in the C++ global scope ("global variables") so they have identical pointers on each core. Declaring one as a local variable (on the stack), or in a symmetric allocation will lead to segfaults or worse.

By default, Reducers do a global "sum", using the + operator; other operations can be specified by ReducerType (second template parameter).

Example usage (count non-negative values in an array):

• ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ // declare symmetric global object Reducer<int> non_negative_count;

  int main(int argc, char* argv[]) { Grappa::init(&argc, &argv); Grappa::run([]{ GlobalAddress<double> A; // allocate & initialize array A...

  // set global count to 0 (expensive global sync.) non_negative_count = 0;

  forall(A, N, [](double& A_i){ if (A_i > 0) { // cheap local increment non_negative_count += 1; } });

  // read out total value and print it (expensive global reduction) LOG(INFO) << non_negative_count << " non negative values"; }); Grappa::finalize(); }

• ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Definition at line 308 of file Reducer.hpp.

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The documentation for this class was generated from the following file:

• Reducer.hpp