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94 lines
2.9 KiB
C++
94 lines
2.9 KiB
C++
#include <iostream>
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#include <thread>
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#include <chrono>
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#include <vector>
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#include <random>
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#include <algorithm>
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#include <cstdint>
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#include <functional>
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using std::cout, std::endl, std::size_t, std::vector;
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using namespace std::chrono_literals;
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uint64_t get_max(vector<uint64_t>::iterator start, vector<uint64_t>::iterator end)
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{
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uint64_t result = *start;
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while (start++ != end)
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if (*start > result)
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result = *start;
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return result;
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}
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struct max_block {
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void operator() (vector<uint64_t>::iterator start, vector<uint64_t>::iterator end, uint64_t& res) {
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res = get_max(start, end);
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}
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};
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uint64_t parallel_max(vector<uint64_t>::iterator start, vector<uint64_t>::iterator end) {
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int const length = std::distance(start, end);
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int const min_per_thread = 25;
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int const max_threads = (length + min_per_thread - 1) / min_per_thread;
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int const hardware_threads = std::thread::hardware_concurrency();
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int const num_threads = std::min(hardware_threads!=0?hardware_threads:2, max_threads);
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int const block_size = length / num_threads;
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#ifdef _DEBUG
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cout << "num_threads = " << num_threads << endl;
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#endif
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vector<uint64_t> results(num_threads);
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vector<std::thread> threads(num_threads - 1);
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auto block_start = start;
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for(auto i = 0; i < num_threads - 1; ++i) {
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auto block_end = block_start;
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std::advance(block_end, block_size);
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threads[i] = std::thread(max_block(), block_start, block_end, std::ref(results[i]));
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block_start = block_end;
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}
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max_block() (block_start, end, results[num_threads - 1]);
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std::for_each(threads.begin(), threads.end(), std::mem_fn(&std::thread::join));
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return get_max(results.begin(), results.end());
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}
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int main()
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{
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cout << "Generating numbers!" << endl;
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#ifdef _DEBUG
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std::vector<uint64_t> numbers(1000);
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numbers[0] = 5;
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for (size_t i = 1; i < numbers.size(); ++i)
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{
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numbers[i] = numbers[i - 1] * i + 1;
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}
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#else
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std::vector<uint64_t> numbers(5e8);
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numbers[0] = 123456789;
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for (size_t i = 1; i < numbers.size(); ++i)
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{
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numbers[i] = numbers[i - 1] * i + 1;
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}
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cout << "Numbers generated!" << endl;
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#endif
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auto start1 = std::chrono::high_resolution_clock::now();
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uint64_t m1 = get_max(numbers.begin(), numbers.end());
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auto end1 = std::chrono::high_resolution_clock::now();
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cout << "Maximum = " << m1 << endl;
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cout << "Time to calculate max = " << std::chrono::duration_cast<std::chrono::milliseconds>(end1 - start1).count()
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<< " milliseconds." << endl;
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auto start2 = std::chrono::high_resolution_clock::now();
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uint64_t m2 = parallel_max(numbers.begin(), numbers.end());
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auto end2 = std::chrono::high_resolution_clock::now();
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cout << "Maximum = " << m2 << endl;
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cout << "Time to calculate max = " << std::chrono::duration_cast<std::chrono::milliseconds>(end2 - start2).count()
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<< " milliseconds." << endl;
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}
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