std::experimental::parallel::reduce

The behavior is non-deterministic if binary_op is not associative or not commutative.

The behavior is undefined if binary_op modifies any element or invalidates any iterator in [first, last).

[edit] Parameters

[edit] Return value

Generalized sum of init and *first, *(first + 1), ... *(last - 1) over binary_op,

where generalized sum GSUM(op, a1, ..., aN) is defined as follows:

• if N=1, a1
• if N > 1, op(GSUM(op, b1, ..., bK), GSUM(op, bM, ..., bN)) where

• b1, ..., bN may be any permutation of a1, ..., aN and
• 1 < K+1 = M ≤ N

in other words, the elements of the range may be grouped and rearranged in arbitrary order.

[edit] Complexity

O(last - first) applications of binary_op.

[edit] Exceptions

• If execution of a function invoked as part of the algorithm throws an exception,

• if policy is parallel_vector_execution_policy, std::terminate is called.
• if policy is sequential_execution_policy or parallel_execution_policy, the algorithm exits with an exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
• if policy is some other type, the behavior is implementation-defined.

• If the algorithm fails to allocate memory (either for itself or to construct an exception_list when handling a user exception), std::bad_alloc is thrown.

[edit] Notes

If the range is empty, init is returned, unmodified.

• If policy is an instance of sequential_execution_policy, all operations are performed in the calling thread.
• If policy is an instance of parallel_execution_policy, operations may be performed in unspecified number of threads, indeterminately sequenced with each other.
• If policy is an instance of parallel_vector_execution_policy, execution may be both parallelized and vectorized: function body boundaries are not respected and user code may be overlapped and combined in arbitrary manner (in particular, this implies that a user-provided Callable must not acquire a mutex to access a shared resource).

[edit] Example

#include <chrono>
#include <experimental/execution_policy>
#include <experimental/numeric>
#include <iostream>
#include <numeric>
#include <vector>
 
int main()
{
 std::vector <double> v(10'000'007, 0.5);
 
 {
 auto t1 = std::chrono::high_resolution_clock::now ();
 double result = std::accumulate (v.begin(), v.end(), 0.0);
 auto t2 = std::chrono::high_resolution_clock::now ();
 std::chrono::duration <double, std::milli > ms = t2 - t1;
 std::cout << std::fixed << "std::accumulate result " << result
 << " took " << ms.count() << " ms\n";
 }
 
 {
 auto t1 = std::chrono::high_resolution_clock::now ();
 double result = std::experimental::parallel::reduce(
 std::experimental::parallel::par,
 v.begin(), v.end());
 auto t2 = std::chrono::high_resolution_clock::now ();
 std::chrono::duration <double, std::milli > ms = t2 - t1;
 std::cout << "parallel::reduce result "
 << result << " took " << ms.count() << " ms\n";
 }
}

std::accumulate result 5000003.50000 took 12.7365 ms
parallel::reduce result 5000003.50000 took 5.06423 ms

[edit] See also

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