Measurement with boost::posix_time::microsec_clock has error more than ten microseconds?

Question

I have the following code:

long long unsigned int GetCurrentTimestamp()
{
   LARGE_INTEGER res;
   QueryPerformanceCounter(&res);
   return res.QuadPart;         


        

Answer 1

Posix time: microsec_clock:

Get the UTC time using a sub second resolution clock. On Unix systems this is implemented using GetTimeOfDay. On most Win32 platforms it is implemented using ftime. Win32 systems often do not achieve microsecond resolution via this API. If higher resolution is critical to your application test your platform to see the achieved resolution.

ftime simply does not provide microsecond resolution. The argument may contain the word microsecond but the implementation does not provide any accuracy in that range. It's granularity is in the ms regime.

You'd get something different than ZERO when you operation needs more time, say more than at least 20ms.

Edit: Note: In the long run the microsec_clock implementation for Windows should use the GetSystemTimePreciseAsFileTime function when possible (min. req. Windows 8 desktop, Windows Server 2012 desktop) to achieve microsecond resolution.

Answer 2

Unfortunately current Boost implementation of boost::posix_time::microsec_clock doesn't uses QueryPerformanceCounter Win32 API, it uses GetSystemTimeAsFileTime instead which in its turn uses GetSystemTime. But system time resolution is milliseconds (or even worse).