【发布时间】：2016-01-25 05:10:39
【问题描述】：
首先，我确实查看了该网站上的其他主题，发现它们与我的问题无关，因为这些主题主要涉及使用 I/O 操作或线程创建开销的人。我的问题是我的线程池或工作任务结构实现（在这种情况下）比单线程慢很多。我对此感到非常困惑，不确定是线程池、任务本身、我如何测试它、线程的性质还是我无法控制的东西。
// Sorry for the long code
#include <vector>
#include <queue>
#include <thread>
#include <mutex>
#include <future>
#include "task.hpp"
class ThreadPool
{
public:
ThreadPool()
{
for (unsigned i = 0; i < std::thread::hardware_concurrency() - 1; i++)
m_workers.emplace_back(this, i);
m_running = true;
for (auto&& worker : m_workers)
worker.start();
}
~ThreadPool()
{
m_running = false;
m_task_signal.notify_all();
for (auto&& worker : m_workers)
worker.terminate();
}
void add_task(Task* task)
{
{
std::unique_lock<std::mutex> lock(m_in_mutex);
m_in.push(task);
}
m_task_signal.notify_one();
}
private:
class Worker
{
public:
Worker(ThreadPool* parent, unsigned id) : m_parent(parent), m_id(id)
{}
~Worker()
{
terminate();
}
void start()
{
m_thread = new std::thread(&Worker::work, this);
}
void terminate()
{
if (m_thread)
{
if (m_thread->joinable())
{
m_thread->join();
delete m_thread;
m_thread = nullptr;
m_parent = nullptr;
}
}
}
private:
void work()
{
while (m_parent->m_running)
{
std::unique_lock<std::mutex> lock(m_parent->m_in_mutex);
m_parent->m_task_signal.wait(lock, [&]()
{
return !m_parent->m_in.empty() || !m_parent->m_running;
});
if (!m_parent->m_running) break;
Task* task = m_parent->m_in.front();
m_parent->m_in.pop();
// Fixed the mutex being locked while the task is executed
lock.unlock();
task->execute();
}
}
private:
ThreadPool* m_parent = nullptr;
unsigned m_id = 0;
std::thread* m_thread = nullptr;
};
private:
std::vector<Worker> m_workers;
std::mutex m_in_mutex;
std::condition_variable m_task_signal;
std::queue<Task*> m_in;
bool m_running = false;
};
class TestTask : public Task
{
public:
TestTask() {}
TestTask(unsigned number) : m_number(number) {}
inline void Set(unsigned number) { m_number = number; }
void execute() override
{
if (m_number <= 3)
{
m_is_prime = m_number > 1;
return;
}
else if (m_number % 2 == 0 || m_number % 3 == 0)
{
m_is_prime = false;
return;
}
else
{
for (unsigned i = 5; i * i <= m_number; i += 6)
{
if (m_number % i == 0 || m_number % (i + 2) == 0)
{
m_is_prime = false;
return;
}
}
m_is_prime = true;
return;
}
}
public:
unsigned m_number = 0;
bool m_is_prime = false;
};
int main()
{
ThreadPool pool;
unsigned num_tasks = 1000000;
std::vector<TestTask> tasks(num_tasks);
for (auto&& task : tasks)
task.Set(randint(0, 1000000000));
auto s = std::chrono::high_resolution_clock::now();
#if MT
for (auto&& task : tasks)
pool.add_task(&task);
#else
for (auto&& task : tasks)
task.execute();
#endif
auto e = std::chrono::high_resolution_clock::now();
double seconds = std::chrono::duration_cast<std::chrono::nanoseconds>(e - s).count() / 1000000000.0;
}
VS2013 Profiler 的基准测试：
10,000,000 tasks:
MT:
13 seconds of wall clock time
93.36% is spent in msvcp120.dll
3.45% is spent in Task::execute() // Not good here
ST:
0.5 seconds of wall clock time
97.31% is spent with Task::execute()
【问题讨论】：
-
首先展示您的“耗时”代码、如何进行测量以及如何编译它。可能很重要。
-
@deviantfan 我发现这个错误太晚了。修改后的答案。
-
多少个核心？如果只有一个，多线程代码很容易比单线程慢。
-
您的程序不会改变环境（stdio、文件系统、网络、渲染等）。理论上，过于聪明的编译器可以优化您的程序以使其无操作或接近它（因为它不会改变可观察的结果）。可以发一个ST变种
main的反汇编吗？ -
在MT版本中，您不应该等待任务完成吗？
标签： c++ multithreading