54 lines
1.6 KiB
C++
54 lines
1.6 KiB
C++
template <typename Iterator, typename T>
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struct accumulate_block {
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T operator()(Iterator first, Iterator last)
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{
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return std::accumulate(first, last, T());
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}
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};
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template <typename Iterator, typename T>
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T
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parallel_accumulate(Iterator first, Iterator last, T init)
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{
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unsigned long const length = std::distance(first, last);
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if (!length)
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return init;
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unsigned long const min_per_thread = 25;
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unsigned long const max_threads =
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(length + min_per_thread - 1) / min_per_thread;
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unsigned long const hardware_threads = std::thread::hardware_concurrency();
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unsigned long const num_threads =
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std::min(hardware_threads != 0 ? hardware_threads : 2, max_threads);
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unsigned long const block_size = length / num_threads;
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std::vector<std::future<T>> futures(num_threads - 1);
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std::vector<std::thread> threads(num_threads - 1);
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Iterator block_start = first;
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for (unsigned long i = 0; i < (num_threads - 1); ++i) {
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Iterator block_end = block_start;
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std::advance(block_end, block_size);
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std::packaged_task<T(Iterator, Iterator)> task(
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accumulate_block<Iterator, T>());
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futures[i] = task.get_future();
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threads[i] = std::thread(std::move(task), block_start, block_end);
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block_start = block_end;
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}
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T last_result = accumulate_block()(block_start, last);
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std::for_each(
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threads.begin(), threads.end(), std::mem_fn(&std::thread::join));
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T result = init;
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for (unsigned long i = 0; i < (num_threads - 1); ++i) {
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result += futures[i].get();
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}
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result += last_result;
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return result;
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}
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