std::chrono::high_resolution_clock 的分辨率与测量值不对应

Resolution of std::chrono::high_resolution_clock doesn#39;t correspond to measurements(std::chrono::high_resolution_clock 的分辨率与测量值不对应)

问题描述

让我通过这个测试程序问我的问题:

Let me ask my question by this test program:

#include <iostream>
#include <chrono>

using std::chrono::nanoseconds;
using std::chrono::duration_cast;

int main(int argc, char* argv[])
{
    std::cout 
      << "Resolution (nano) = " 
      << (double) std::chrono::high_resolution_clock::period::num / 
                  std::chrono::high_resolution_clock::period::den * 
                  1000 * 1000 * 1000 
      << std::endl;

    auto t1 = std::chrono::high_resolution_clock::now();
    std::cout << "How many nanoseconds does std::cout take?" << std::endl;
    auto t2 = std::chrono::high_resolution_clock::now();

    auto diff = t2-t1;
    nanoseconds ns = duration_cast<nanoseconds>(diff);

    std::cout << "std::cout takes " << ns.count() << " nanoseconds" 
              << std::endl;
    return 0;
}

我机器上的输出:

分辨率(纳米)= 100

Resolution (nano) = 100

std::cout 需要多少纳秒?

How many nanoseconds does std::cout take?

std::cout 需要 1000200 纳秒

std::cout takes 1000200 nanoseconds

我收到 1000200 或 1000300 或 1000400 或 1000500 或 1000600或 2000600 结果(= 1 或 2 微秒).很明显，要么std::chrono的分辨率不是 100纳秒或者我测量std时间的方式::cout 是错误的.(为什么我从来没有收到 1 到 2 微秒之间的信息，例如 1500000?)

I receive either 1000200 or 1000300 or 1000400 or 1000500 or 1000600 or 2000600 as a result (= 1 or 2 microsecond). Obviously, either the resolution of std::chrono is not 100 nano-seconds or the way I measure the time of std::cout is wrong. (Why do I never receive something between 1 and 2 microseconds, for example 1500000?)

我需要一个高分辨率的 C++ 计时器.操作系统本身提供了一个高分辨率计时器，因为我能够在同一台机器上使用 C# Stopwatch 类以微秒精度测量事物.所以我只需要正确使用操作系统拥有的高分辨率计时器！

I need a high-resolution timer in C++. The OS itself provides a high-resolution timer, because I'm able to measure things with microsecond-precision using the C# Stopwatch class on the same machine. So I would just need to correctly use the high-resolution timer that the OS has!

如何修复我的程序以产生预期的结果?

How do I fix my program to produce the expected results?

推荐答案

我猜您正在使用 Visual Studio 2012.如果不是，请忽略此答案.Visual Studio 2012 typedef 的 high_resolution_clock 到 system_clock.可悲的是，这意味着它的精度很差(大约 1 毫秒).我写了一个更好的高分辨率时钟，它使用 QueryPerformanceCounter 在 Visual Studio 2012 中使用...

I'm going to guess you are using Visual Studio 2012. If not, disregard this answer. Visual Studio 2012 typedef's high_resolution_clock to system_clock. Sadly, this means it has crappy precision (around 1 ms). I wrote a better high-resolution clock which uses QueryPerformanceCounter for use in Visual Studio 2012...

HighResClock.h:

    struct HighResClock
    {
        typedef long long                              rep;
        typedef std::nano                              period;
        typedef std::chrono::duration<rep, period>     duration;
        typedef std::chrono::time_point<HighResClock>  time_point;
        static const bool is_steady = true;

        static time_point now();
    };

HighResClock.cpp:

namespace
{
    const long long g_Frequency = []() -> long long
    {
        LARGE_INTEGER frequency;
        QueryPerformanceFrequency(&frequency);
        return frequency.QuadPart;
    }();
}

HighResClock::time_point HighResClock::now()
{
    LARGE_INTEGER count;
    QueryPerformanceCounter(&count);
    return time_point(duration(count.QuadPart * static_cast<rep>(period::den) / g_Frequency));
}

(我省略了断言和 #ifs 以查看它是否在 Visual Studio 2012 上从上述代码编译.)

(I left out an assert and #ifs to see if it's being compiled on Visual Studio 2012 from the above code.)

您可以在任何地方使用此时钟，使用方式与标准时钟相同.

You can use this clock anywhere and in the same way as standard clocks.

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