Windows下的高精度定时器实现及精确时刻获取

Windows下的高精度定时器实现及精确时刻获取
2010年06月30日
　　通讯、VOIP、视频等领域的很多核心技术对时间精度的要求非常高，比如数据采集、时间同步、媒体流平滑控制、拥塞算法等等，很多技术都是以毫秒为单位来进行计算和控制的。但是Windows设计之初并不是以实时系统为目标的，所以Windows系统的时间精度一直不高，实际最小单位是15ms左右，导致的结果就是所有Windows的时间、线程相关的操作都无法以1ms来实现精确控制。
　　受影响的操作包括Sleep、GetTickCount、_ftime等等。比如你调用Sleep(2)，期待2ms之后线程自动唤醒，但是实际结果可能是15ms甚至2x ms的时候才会唤醒，对于简单应用来说影响不大，但是对于精度要求非常高的系统来说，这样的问题就是非常致命的了。
　　代码思路如下：
　　1、高精度定时器。使用Singleton模式挂起请求Sleep的线程并统一管理，后台使用Windows MultiMedia SDK的定期回调函数不断检测并回复到时的线程，超时时间与当前时间采用QueryPerformanceCounter/QueryPerformanceFrequency的高精度计时，确保整体功能可靠性。
　　2、精确时刻获取。由于可以获取到毫秒级别的_ftime与GetTickCount都受到Windows系统时间精度影响，最小单位只有15ms，所以需要借助QueryPerformanceCounter/QueryPerformanceFrequency进行准确计时。代码首先根据_ftime获取起始时刻的精确刻度，然后根据差量计算当前的精确时刻。
　　代码中的Singleton模式可以找到很多实现，因此本文不进行详述
　　代码（VS2005 c++编译）
　　1、高精度定时器  #pragma once #include  #include  #include  namespace akumaslab{ namespace time{ using std::list; class PreciseTimerProvider { struct WaitedHandle{ HANDLE threadHandle; LONGLONG elapsed;//超时时间 } ; typedef list handle_list_type; typedef akumaslab::system::Singleton timer_type; public: PreciseTimerProvider(void):highResolutionAvailable (false), timerID(0) { InitializeCriticalSection(&critical); static LARGE_INTEGER systemFrequency; if(0 != QueryPerformanceFrequency(&systemFrequency)) { timeBeginPeriod(callbackInterval); highResolutionAvailable = true; countPerMilliSecond = systemFrequency.QuadPart/1000; timerID = timeSetEvent(callbackInterval, 0, &PreciseTimerProvider::TimerFunc, NULL, TIME_PERIODIC); } } //挂起当前线程 //@milliSecond:超时时间，单位：毫秒 bool suspendCurrentThread(int milliSecond) { if(milliSecond = waited.elapsed) { ResumeThread(waited.threadHandle); ir = waitList.erase(ir); continue; } ir++; } LeaveCriticalSection(&critical); } ~PreciseTimerProvider(){ if (0 != timerID) { timeKillEvent(timerID); timerID = 0; timeEndPeriod(callbackInterval); } DeleteCriticalSection(&critical); } private: static void CALLBACK TimerFunc(UINT uID, UINT uMsg, DWORD dwUser, DWORD dw1, DWORD dw2) { static bool initialed = false; if (!initialed) { if (initialWorkThread()) { initialed = true; } else{ return; } } timer_type::getRef().resumeTimeoutThread(); } //调整定时器工作线程优先级 static bool initialWorkThread() { HANDLE realProcessHandle = OpenProcess(PROCESS_ALL_ACCESS, FALSE, _getpid()); if (NULL == realProcessHandle) { return false; } if (0 == SetPriorityClass(realProcessHandle, REALTIME_PRIORITY_CLASS)) { CloseHandle(realProcessHandle); return false; } HANDLE currentThreadHandle = GetCurrentThread(); HANDLE currentProcessHandle = GetCurrentProcess(); HANDLE realThreadHandle(0); DuplicateHandle(currentProcessHandle, currentThreadHandle, currentProcessHandle, &realThreadHandle, 0, FALSE, DUPLICATE_SAME_ACCESS); SetThreadPriority(realThreadHandle, THREAD_PRIORITY_TIME_CRITICAL); //必须关闭复制句柄 CloseHandle(realThreadHandle); CloseHandle(realProcessHandle); return true; } private: const static int callbackInterval = 1; CRITICAL_SECTION critical; MMRESULT timerID; LONGLONG countPerMilliSecond; bool highResolutionAvailable; handle_list_type waitList; }; class PreciseTimer { typedef akumaslab::system::Singleton timer_type; public: static bool wait(int milliSecond) { //static PreciseTimerProvider timer; return timer_type::getRef().suspendCurrentThread(milliSec ond); } }; } }   DEMO  int interval = 1; int repeatCount = 50; cout  #include  #include  #include  namespace akumaslab{ namespace time{ struct HighResolutionTime { int year; int month; int day; int hour; int min; int second; int millisecond; }; class CurrentTimeProvider { public: CurrentTimeProvider():highResolutionAvailable(fals e), countPerMilliSecond(0), beginCount(0) { static LARGE_INTEGER systemFrequency; if(0 != QueryPerformanceFrequency(&systemFrequency)) { highResolutionAvailable = true; countPerMilliSecond = systemFrequency.QuadPart/1000; _timeb tb; _ftime_s(&tb); unsigned short currentMilli = tb.millitm; LARGE_INTEGER now; QueryPerformanceCounter(&now); beginCount = now.QuadPart - (currentMilli*countPerMilliSecond); } }; bool getCurrentTime(HighResolutionTime& _time) { time_t tt; ::time(&tt); tm now; localtime_s(&now, &tt); _time.year = now.tm_year + 1900; _time.month = now.tm_mon + 1; _time.day = now.tm_mday + 1; _time.hour = now.tm_hour; _time.min = now.tm_min; _time.second = now.tm_sec; if (!highResolutionAvailable) { _time.millisecond = 0; } else{ LARGE_INTEGER qfc; QueryPerformanceCounter(&qfc); _time.millisecond = (int)((qfc.QuadPart - beginCount)/countPerMilliSecond)%1000; } return true; } private: bool highResolutionAvailable; LONGLONG countPerMilliSecond; LONGLONG beginCount; }; class CurrentTime { public: static bool get(HighResolutionTime& _time) { return akumaslab::system::Singleton::getRef().getCurrentTime(_time); } }; } }  DEMO：  HighResolutionTime time; CurrentTime::get(time); const int size = 20; char buf[size] = {0}; _snprintf_s(buf, size, size, "%02d:%02d %02d:%02d:%02d.%03d ", time.month, time.day, time.hour, time.min, time.second, time.millisecond);  测试结果如下，下图是高精度计时器按1ms进行Sleep的结果，左侧为使用_ftime计时，右侧为使用精确时刻计时，总体来说，虽然无法达到100%可靠，但是相对原来的15ms已经有较大提升，期望Windows能够尽快提供真正的高精度时间管理技术