Windows下多媒体定时器对waitabletimer/sleep的精度影响

有篇写的不错游戏主循环框架的文章，https://blog.csdn.net/u014038143/article/details/78192076，提到了如何在避免死等的情况下实现高精度定时，另外一个程序员基于写了个测试程序https://blog.csdn.net/Gammy_Ye/article/details/90272904。这个程序每个循环都有输出，没法测试总体的偏差，注释掉其中的cout输出，统计一下6000次循环，每次等待0.01秒的总耗时。编译运行

• 总时间为60秒，期望在60秒左右完成，如果系统压力不大，实际上在92秒左右（观察Windows CPU，此期间CPU频率会降低到2Ghz以下）
• 打开视频播放器，基本在62秒左右完成

所以看起来还是有其它因素影响这个精度。搜索Windows API，看到多次提到mmtimer提供高精度计时，所以修改了一下代码，核心是通过timeBeginPeriod设置系统计时精度，如果设置为1或者2毫秒，则测试程序总是可以在62秒左右完成，输出如下

Total excu time for Loop 6000 times by 10ms=62684ms
Timer interval is 0.01
Use timer times is 6000
Total deviation is 26394715|2.63947
Average deviation is 0.000439912

其实把setwaitabletimer换成sleep()，完成时间也差不多，所以看起来要得到高精度计时，必须通过timeBeginPeriod来设置系统timer精度，按微软文档，在Win10 2004前，这个设置甚至是全局的，即任何一个程序设置过，所以程序都会使用最小精度值，2004后则只影响同样调用过此函数的进程。

代码如下：

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

using namespace std;

#pragma comment(lib,"winmm.lib")

int main()
{

#define TARGET_RESOLUTION 1         // 1-millisecond target resolution

    TIMECAPS tc;
    UINT     wTimerRes;

    if (timeGetDevCaps(&tc, sizeof(TIMECAPS)) != TIMERR_NOERROR)
    {
        // Error; application can't continue.
    }

    wTimerRes = min(max(tc.wPeriodMin, TARGET_RESOLUTION), tc.wPeriodMax);
    wTimerRes = 2;
    timeBeginPeriod(wTimerRes);


    HANDLE hTimer = CreateWaitableTimer(nullptr, true, nullptr);
    if (hTimer == nullptr)
    {
        return 1;
    }

    LARGE_INTEGER DueTime;
    DueTime.QuadPart = -10000000LL / 100; //定时的间隔为0.01秒
    int LoopTimes = 6000; //做6000次定时
    LARGE_INTEGER Counter_1, Counter_2, Counter_3, Counter_Dif, Frequency;
    Counter_Dif.QuadPart = 0;

    if (QueryPerformanceFrequency(&Frequency) == false)
    {
        return 1;
    }

    QueryPerformanceCounter(&Counter_1);

    for (int I = 0; I < LoopTimes; ++I)
    {
        SetWaitableTimer(hTimer, &DueTime, 0, nullptr, nullptr, 0);
        if (QueryPerformanceCounter(&Counter_2) == false)
        {
            return 1;
        }
        
        WaitForSingleObject(hTimer, INFINITE);
        //Sleep(10);

        if (QueryPerformanceCounter(&Counter_3) == false)
        {
            return 1;
        }
        LARGE_INTEGER Delta;
        Delta.QuadPart = abs(Counter_3.QuadPart - Counter_2.QuadPart + DueTime.QuadPart);//本次定时的误差值(单位是硬件计数)
        Counter_Dif.QuadPart += Delta.QuadPart;

        //cout << Counter_3.QuadPart - Counter_2.QuadPart << "|" << Delta.QuadPart << "|"
        //    << (long double)Delta.QuadPart / Frequency.QuadPart << endl; //本次定时的误差时间(单位为秒)
    }

    timeEndPeriod(wTimerRes);


    cout << "Total excu time for Loop 6000 times by 10ms=" << (Counter_3.QuadPart - Counter_1.QuadPart)*1000 / Frequency.QuadPart << "ms" << endl;

    cout << "Timer interval is " << (long double)-DueTime.QuadPart / 10000000 << endl;
    cout << "Use timer times is " << LoopTimes << endl;
    cout << "Total deviation is " << Counter_Dif.QuadPart << "|" << (long double)Counter_Dif.QuadPart / Frequency.QuadPart << endl;
    cout << "Average deviation is " << (long double)Counter_Dif.QuadPart / Frequency.QuadPart / LoopTimes << endl;//平均误差时间
    system("pause");
}