std::shared_mutex 不与执行 lock_shared() 的线程一起缩放

Question

我编写了一个简单的程序来测试std::shared_mutex在多个循环lock_shared()的线程中的性能。 但从结果来看，它似乎并没有随着添加更多线程而扩展，这对我来说真的没有意义。

您可能会争辩说这是因为stopFlag限制了性能，所以第二个 for 循环测试增加本地计数器，这在开始时几乎是完美的缩放

注释中的结果是使用带有Release标志的MSVC编译的。

int main()
{
    const auto threadLimit = std::thread::hardware_concurrency() - 1; //for running main() 
    struct SharedMutexWrapper
    {
        std::shared_mutex mut;
        void read()
        {
            mut.lock_shared();
            mut.unlock_shared();
        }
    };
    /*Testing shared_mutex */
    for (auto i = 1; i <= threadLimit; ++i)
    {
        std::cerr << "Testing " << i << " threads: ";
        SharedMutexWrapper test;
        std::atomic<unsigned long long> count = 0;
        std::atomic_bool stopFlag = false;
        std::vector<std::thread> threads;
        threads.reserve(i);
        for (auto j = 0; j < i; ++j)
            threads.emplace_back([&] {unsigned long long local = 0; while (!stopFlag) { test.read(); ++local; } count += local;  });
        std::this_thread::sleep_for(std::chrono::seconds{ 1 });
        stopFlag = true;
        for (auto& thread : threads)
            thread.join();
        std::cerr << count << '\n';
    }
/*
Testing 1 threads: 60394076
Testing 2 threads: 39703889
Testing 3 threads: 23461029
Testing 4 threads: 16961003
Testing 5 threads: 12750838
Testing 6 threads: 12227898
Testing 7 threads: 12245818
*/
    for (auto i = 1; i <= threadLimit; ++i)
    {
        std::cerr << "Testing " << i << " threads: ";
        std::atomic<unsigned long long> count = 0;
        std::atomic_bool stopFlag = false;
        std::vector<std::thread> threads;
        threads.reserve(i);
        for (auto j = 0; j < i; ++j)
            threads.emplace_back([&] {unsigned long long local = 0;  while (!stopFlag) ++local; count += local; });
        std::this_thread::sleep_for(std::chrono::seconds{ 1 });
        stopFlag = true;
        for (auto& thread : threads)
            thread.join();
        std::cerr << count << '\n';
    }
/*
Testing 1 threads: 3178867276
Testing 2 threads: 6305783667
Testing 3 threads: 9388659151
Testing 4 threads: 12472666861
Testing 5 threads: 15230810694
Testing 6 threads: 18130479890
Testing 7 threads: 20151074046
*/
}

Answer 1

在共享互斥锁上放置读锁会修改该互斥锁的 state。 您的所有线程都只尝试更改共享互斥锁 object 的 state 。 所以当然，这段代码的扩展性很差。

共享互斥锁的意义在于允许访问未按比例修改的共享数据。 您无权访问未修改的共享数据。 因此，您没有在这里测量共享互斥锁的任何有益属性。