join_thread.hpp
#pragma once
#include <vector>
#include <thread>
class join_threads
{
public:
explicit join_threads(std::vector<std::thread>& threads)
: threads_(threads) {}
~join_threads()
{
for (size_t i = 0; i < threads_.size(); ++i)
{
if(threads_[i].joinable())
{
threads_[i].join();
}
}
}
private:
std::vector<std::thread>& threads_;
};
parallel_for_each.hpp
#pragma once
#include <future>
#include <algorithm>
#include "join_threads.hpp"
template<typename Iterator,typename Func>
void parallel_for_each(Iterator first,Iterator last,Func func)
{
const auto length = std::distance(first,last);
if (0 == length) return;
const auto min_per_thread = 25u;
const unsigned max_threads = (length + min_per_thread - 1) / min_per_thread;
const auto hardware_threads = std::thread::hardware_concurrency();
const auto num_threads= std::min(hardware_threads != 0 ?
hardware_threads : 2u,max_threads);
const auto block_size = length / num_threads;
std::vector<std::future<void>> futures(num_threads - 1);
std::vector<std::thread> threads(num_threads-1);
join_threads joiner(threads);
auto block_start = first;
for (unsigned i = 0; i < num_threads - 1; ++i)
{
auto block_end = block_start;
std::advance(block_end,block_size);
std::packaged_task<void (void)> task([block_start,block_end,func]()
{
std::for_each(block_start,func);
});
futures[i] = task.get_future();
threads[i] = std::thread(std::move(task));
block_start = block_end;
}
std::for_each(block_start,last,func);
for (size_t i = 0; i < num_threads - 1; ++i)
{
futures[i].get();
}
}
我使用以下程序使用顺序版本std::for_each对其进行基准测试：
main.cpp中
#include <iostream>
#include <random>
#include <chrono>
#include "parallel_for_each.hpp"
using namespace std;
constexpr size_t ARRAY_SIZE = 500'000'000;
typedef std::vector<uint64_t> Array;
template <class FE,class F>
void test_for_each(const Array& a,FE fe,F f,atomic<uint64_t>& result)
{
auto time_begin = chrono::high_resolution_clock::now();
result = 0;
fe(a.begin(),a.end(),f);
auto time_end = chrono::high_resolution_clock::now();
cout << "Result = " << result << endl;
cout << "Time: " << chrono::duration_cast<chrono::milliseconds>(
time_end - time_begin).count() << endl;
}
int main()
{
random_device device;
default_random_engine engine(device());
uniform_int_distribution<uint8_t> distribution(0,255);
Array a;
a.reserve(ARRAY_SIZE);
cout << "Generating array ... " << endl;
for (size_t i = 0; i < ARRAY_SIZE; ++i)
a.push_back(distribution(engine));
atomic<uint64_t> result;
auto acc = [&result](uint64_t value) { result += value; };
cout << "parallel_for_each ..." << endl;
test_for_each(a,parallel_for_each<Array::const_iterator,decltype(acc)>,acc,result);
cout << "for_each ..." << endl;
test_for_each(a,for_each<Array::const_iterator,result);
return 0;
}
我机器上算法的并行版本比顺序版本慢两倍：
parallel_for_each ... Result = 63750301073 Time: 5448 for_each ... Result = 63750301073 Time: 2496
我在运行在Intel(R)Core(TM)i3-6100 CPU @ 3.70GHz上的Ubuntu Linux上使用GCC 6.2编译器.
如何解释这种行为？这是因为共享原子< uint64_t>线程和缓存乒乓之间的变量？
我用perf分别对它们进行了分析.对于并行版本,统计数据如下：
1137982167 cache-references
247652893 cache-misses # 21,762 % of all cache refs
60868183996 cycles
27409239189 instructions # 0,45 insns per cycle
3287117194 branches
80895 faults
4 migrations
对于顺序的：
402791485 cache-references
246561299 cache-misses # 61,213 % of all cache refs
40284812779 cycles
26515783790 instructions # 0,66 insns per cycle
3188784664 branches
48179 faults
3 migrations
很明显,并行版本会产生更多的缓存引用,循环和故障,但为什么呢？
解决方法
每次线程写入结果时,其他核心中的所有高速缓存都将失效：这会导致高速缓存行争用.
更多信息：
> “Sharing Is the Root of All Contention”.
[…] to write to a memory location a core must additionally have exclusive ownership of the cache line containing that location. While one core has exclusive use,all other cores trying to write the same memory location must wait and take turns — that is,they must run serially. Conceptually,it’s as if each cache line were protected by a hardware mutex,where only one core can hold the hardware lock on that cache line at a time.
> This article on “false sharing”,其中涉及类似问题,更深入地解释了缓存中发生的情况.
我对您的程序进行了一些修改并获得了以下结果(在具有i7-4770K [8线程超线程]的机器上)：
Generating array ... parallel_for_each ... Result = 63748111806 Time: 195 for_each ... Result = 63748111806 Time: 2727
并行版本比串行版本快约92％.
> std :: future和std :: packaged_task是重量级抽象.在这种情况下,std::experimental::latch就足够了.
>每个任务都发送到线程池这可以最大限度地减少线程创建开销.
>每个任务都有自己的累加器.这消除了共享.
代码可用于here on my GitHub.它使用了一些个人依赖项,但您应该了解这些更改.
以下是最重要的变化：
// A latch is being used instead of a vector of futures.
ecst::latch l(num_threads - 1);
l.execute_and_wait_until_zero([&]
{
auto block_start = first;
for (unsigned i = 0; i < num_threads - 1; ++i)
{
auto block_end = block_start;
std::advance(block_end,block_size);
// `p` is a thread pool.
// Every task posted in the thread pool has its own `tempacc` accumulator.
p.post([&,block_start,tempacc = 0ull]() mutable
{
// The task accumulator is filled up...
std::for_each(block_start,[&tempacc](auto x){ tempacc += x; });
// ...and then the atomic variable is incremented ONCE.
func(tempacc);
l.decrement_and_notify_all();
});
block_start = block_end;
}
// Same idea here: accumulate to local non-atomic counter,then
// add the partial result to the atomic counter ONCE.
auto tempacc2 = 0ull;
std::for_each(block_start,[&tempacc2](auto x){ tempacc2 += x; });
func(tempacc2);
});
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