C++11 std::thread 队列消费者

Question

我正在考虑实现一个 FIFO 线程队列，其中创建了线程 lambda，但一次只处理 X 个线程。

我假设线程会捕获一些局部变量，但没有别的。

我正在尝试做的一个最小示例是：

#include <stdio.h>
#include <mutex>
#include <thread>
#include <atomic>
#include <vector>
#include <algorithm>
#include <condition_variable>

int main()
{
    std::vector<std::thread> threads;
    std::condition_variable condition;
    std::mutex mutex;
    std::atomic<unsigned int> thread_count( 0 );

    for ( int i = 0; i < 20; i++ )
    {
        std::unique_lock<std::mutex> lock ( mutex );
        condition.wait( lock, [&]{ return thread_count < 8;} );

        std::thread t = std::thread([&,i]()
        {
            printf ("I am Thread #: %d with current thread count: %u\n", i, unsigned( thread_count ) );
            thread_count--;
        });

        threads.push_back( std::move( t ) );
        thread_count++;
    }

    for ( auto & t : threads ) t.join();

    return 0;
}

我想在那个 for 循环中创建线程 objects/lamdas，然后让它们运行最多 8 个线程。

当前输出为：

I am Thread #: 0 with current thread count: 3 
I am Thread #: 3 with current thread count: 4 
I am Thread #: 2 with current thread count: 4 
I am Thread #: 5 with current thread count: 3 
I am Thread #: 1 with current thread count: 3 
I am Thread #: 4 with current thread count: 5 
I am Thread #: 6 with current thread count: 1 
I am Thread #: 7 with current thread count: 1 
I am Thread #: 8 with current thread count: 1 
I am Thread #: 9 with current thread count: 1 
I am Thread #: 10 with current thread count: 1 
I am Thread #: 11 with current thread count: 1 
I am Thread #: 12 with current thread count: 1 
I am Thread #: 13 with current thread count: 1 
I am Thread #: 14 with current thread count: 1 
I am Thread #: 15 with current thread count: 1 
I am Thread #: 16 with current thread count: 1 
I am Thread #: 17 with current thread count: 1 
I am Thread #: 18 with current thread count: 1 
I am Thread #: 19 with current thread count: 1

这显然永远不会达到 8 个线程的最大数量。

你可以自己看看here

Answer 1

您正在创建的线程似乎比主循环创建新线程终止得更快。所以你没有看到 8 个并行线程在运行，而是更少。

请注意，程序输出将是不确定的，因为线程调度有些不可预测。如果调用频率足够高，您的程序实际上可能不时有 8 个并行线程运行...

如果您更改 threadfunc lambda 以使其不会立即终止，您将看到线程堆积起来。您可以通过在 threadfunc lambda 中添加对 sleep 的一些调用来实现。

请注意，您的程序可能会挂起，因为主循环将等待条件变量。但是没有人发出信号，所以它可能会永远挂起。

因此您可能希望将 threadfunc lambda 末尾的 thread_count--; 行更改为：

std::unique_lock<std::mutex> lock ( mutex );
thread_count--;
condition.notify_one();

这不仅会减少线程数，还会向条件变量发出信号，以便主循环可以唤醒并在线程终止时恢复创建额外的线程。

完整的工作示例，每个线程随机延迟休眠（假装一些实际工作）：

#include <stdio.h>
#include <mutex>
#include <thread>
#include <atomic>
#include <vector>
#include <algorithm>
#include <condition_variable>
#include <chrono>
#include <random>

int main()
{
    std::vector<std::thread> threads;
    std::condition_variable condition;
    std::mutex mutex;
    std::atomic<unsigned int> thread_count(0);
    std::mt19937 generator;

    for (int i = 0; i < 20; i++) {
        std::unique_lock<std::mutex> lock (mutex);
        condition.wait(lock, [&]{ return thread_count < 8; });
        // random sleep delay
        auto delay = generator() % 10000;

        std::thread t = std::thread([&, i, delay]() {
            printf("I am Thread #: %d with current thread count: %u\n", i, unsigned(thread_count));
            // ok to access the generator here because 
            std::this_thread::sleep_for(std::chrono::milliseconds(delay));
            printf("I am Thread #: %d and I am done\n", i);

            // signal that this thread is done
            std::unique_lock<std::mutex> lock(mutex);
            thread_count--;
            condition.notify_one();
        });

        threads.push_back(std::move(t));
        thread_count++;
    }

    for (auto& t : threads) {
      t.join();
    }
    return 0;
}

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