Sleep(1)和SDL_Delay(1)需要15 ms

Question

我正在编写一个C++/SDL/OpenGL应用程序,我遇到了最奇怪的错误.通过简单的可变时间步长,游戏似乎运行良好.但随后FPS开始表现得很奇怪.我发现Sleep(1)和SDL_Delay(1)都需要15 ms才能完成.

在0-15之间的任何输入都需要15ms来完成,将FPS锁定在大约64.如果我将其设置为16,则需要30 MS OO

我不知道为什么会这样.这是我遇到过的最奇怪的错误.

我的循环看起来像这样:

while (1){
    GLuint t = SDL_GetTicks();
    Sleep(1); //or SDL_Delay(1)
    cout << SDL_GetTicks() - t << endl; //outputs 15
}

它应该很少花费1ms,但大部分时间需要15ms.

我的操作系统是Windows 8.1.CPU是intel i7.我正在使用SDL2.任何想法都会受到赞赏,因为我一无所知.

Answer 1

股票代码默认为64赫兹,或15.625毫秒/滴答.您需要使用timeBeginPeriod(1)将其更改为1000hz == 1ms.MSDN文章:

http://msdn.microsoft.com/en-us/library/windows/desktop/dd757624(v=vs.85).aspx

如果这里的目标是获得固定频率序列,则应该使用更高分辨率的计时器,但遗憾的是这些只能被轮询,因此需要轮询和休眠的组合以减少CPU开销.示例代码,假设Sleep(1)可能需要几乎2毫秒(在Windows XP中会发生这种情况,但在Windows的更高版本中不会发生).

/* code for a thread to run at fixed frequency */
#define FREQ 400                        /* frequency */

typedef unsigned long long UI64;        /* unsigned 64 bit int */

LARGE_INTEGER liPerfFreq;               /* used for frequency */
LARGE_INTEGER liPerfTemp;               /* used for query */
UI64 uFreq = FREQ;                      /* process frequency */
UI64 uOrig;                             /* original tick */
UI64 uWait;                             /* tick rate / freq */
UI64 uRem = 0;                          /* tick rate % freq */
UI64 uPrev;                             /* previous tick based on original tick */
UI64 uDelta;                            /* current tick - previous */
UI64 u2ms;                              /* 2ms of ticks */
#if 0                                   /* for optional error check */
static DWORD dwLateStep = 0;
#endif
    /* get frequency */
    QueryPerformanceFrequency(&liPerfFreq);
    u2ms = ((UI64)(liPerfFreq.QuadPart)+499) / ((UI64)500);

    /* wait for some event to start this thread code */
    timeBeginPeriod(1);                 /* set period to 1ms */
    Sleep(128);                         /* wait for it to stabilize */

    QueryPerformanceCounter((PLARGE_INTEGER)&liPerfTemp);
    uOrig = uPrev = liPerfTemp.QuadPart;

    while(1){
        /* update uWait and uRem based on uRem */
        uWait = ((UI64)(liPerfFreq.QuadPart) + uRem) / uFreq;
        uRem  = ((UI64)(liPerfFreq.QuadPart) + uRem) % uFreq;
        /* wait for uWait ticks */
        while(1){
            QueryPerformanceCounter((PLARGE_INTEGER)&liPerfTemp);
            uDelta = (UI64)(liPerfTemp.QuadPart - uPrev);
            if(uDelta >= uWait)
                break;
            if((uWait - uDelta) > u2ms)
                Sleep(1);
        }
        #if 0                    /* optional error check */
        if(uDelta >= (uWait*2))
            dwLateStep += 1;
        #endif
        uPrev += uWait;
        /* fixed frequency code goes here */
        /*  along with some type of break when done */
    }

    timeEndPeriod(1);                   /* restore period */