为什么在带有Chrono的Windows上的C ++中，为什么我的循环不能旋转不到一毫秒？

Question

EDIT: I am using VS2013 and on Windows 7.

With the below code I'd expect to be able to have a time difference of at least one microsecond, however, when executed it builds it up to at least 1000 microseconds (one millisecond). What is the reasoning I'm not able to get a time lower then one millisecond? Is there any way around this?

// SleepTesting.cpp : Defines the entry point for the console application.
//

#include <chrono>
#include "windows.h"
#include <iostream>

int _tmain(int argc, _TCHAR* argv[])
{
    FILETIME startFileTime, endFileTime;
    uint64_t ullStartTime, ullEndTime;
    bool sleep = true;
    auto start = std::chrono::system_clock::now();
    auto now = std::chrono::system_clock::now();
    auto elapsedTime = std::chrono::duration_cast<std::chrono::microseconds>(now - start);

    GetSystemTimeAsFileTime(&startFileTime);
    ullStartTime = static_cast<uint64_t>(startFileTime.dwHighDateTime) << 32 | startFileTime.dwLowDateTime;

    while (sleep)
    {
        now = std::chrono::system_clock::now();
        elapsedTime = std::chrono::duration_cast < std::chrono::microseconds > (now - start);
        if (elapsedTime.count() > 0)
        {
            sleep = false;
        }
    }

    GetSystemTimeAsFileTime(&endFileTime);
    ullEndTime = static_cast<uint64_t>(endFileTime.dwHighDateTime) << 32 | endFileTime.dwLowDateTime;
    uint64_t timeDifferenceHundredsOfNano = ullEndTime - ullStartTime;

    std::cout << "Elapsed time with Chrono library: " << elapsedTime.count() << " micro-seconds" << std::endl;
    std::cout << "Elapsed time with Windows.h FILETIME: " << timeDifferenceHundredsOfNano << " hundreds of nanoseconds" << std::endl;

    return 0;
}

Answer 1

since you're using system_clock, I think that you can't get micro-second resolution on windows 7 (at least from what I've seen). try high-resolution clock, but even that won't always work, since windows doesn't even guarantee that time elapsed between two consecutive operations is less then one millisecond, even without sleeping

Answer 2

IIRC in VS2013 the system_clock (and highres_clock) are in clock ticks ie ms. If you need higher resolution you can go all Windows and take a look at QueryPerformanceCounter .

LARGE_INTEGER startCount;
LARGE_INTEGER endCount;
LARGE_INTEGER frequency;
QueryPerformanceFrequency(&frequency);
QueryPerformanceCounter(&startCount);
{...}
QueryPerformanceCounter(&endCount);

double startTimeInMicroSec = startCount.QuadPart * (1000000.0 / frequency.QuadPart);
double endTimeInMicroSec = endCount.QuadPart * (1000000.0 / frequency.QuadPart);

// endTimeInMicroSec - startTimeInMicroSec

disclaimer: ocular compilation