Intel Threading Building Blocks

ParallelSum without Having to Specify a Grain Size

Example 11-20 does away with the need to supply a grain size by converting the prior example to use an auto_partitioner. Note how the block_range loses the grainsize parameter, and the parallel_reduce has a parameter added specifying our desire to use the auto_partitioner.

Example 11-20. ParallelSum with auto_partitioner

#include "tbb/parallel_reduce.h"
#include "tbb/blocked_range.h"

using namespace tbb;

struct Sum {
    float value;
    Sum() : value(0) {}
    Sum( Sum& s, split ) {value = 0;}
    void operator()( const blocked_range<float*>& range ) {
        float temp = value;
        for( float* a=range.begin(); a!=range.end(); ++a ) {
            temp += *a;
        }
        value = temp;
    }
    void join( Sum& rhs ) {value += rhs.value;}
};

float ParallelSum( float array[], size_t n ) {
    Sum total;
    parallel_reduce( blocked_range<float*>( array, array+n ),
                     total, auto_partitioner() );
    return total.value;
}