Best way to get elapsed time in miliseconds in windows

Question

I'm trying to do it using two FILETIMEs, casting them to ULONGLONGs, substracting the ULONGLONGs, and dividing the result by 10000. But it's pretty slow, and I want to know if there is a better way to do it.I use c++ with visual studio 2008 express edition. This is what I'm using:

FILETIME filetime,filetime2;
GetSystemTimeAsFileTime(&filetime);
Sleep(100);
GetSystemTimeAsFileTime(&filetime2);
ULONGLONG time1,time2;
time1 = (((ULONGLONG) filetime.dwHighDateTime) << 32) + filetime.dwLowDateTime;
time2 = (((ULONGLONG) filetime2.dwHighDateTime) << 32) + filetime2.dwLowDateTime;
printf("ELAPSED TIME IN MS:%d",(int)((time2-time1)/10000));

Answer 1

Use QueryPerformanceCounter.

long long milliseconds_now() {
    static LARGE_INTEGER s_frequency;
    static BOOL s_use_qpc = QueryPerformanceFrequency(&s_frequency);
    if (s_use_qpc) {
        LARGE_INTEGER now;
        QueryPerformanceCounter(&now);
        return (1000LL * now.QuadPart) / s_frequency.QuadPart;
    } else {
        return GetTickCount();
    }
}

// Somewhere else...
    // ...
    long long start = milliseconds_now();
    // ....
    long long elapsed = milliseconds_now() - start;

Answer 2

To answer your question : there is no "best" solution to measure time. Every method has its own advantages/disadvantages. It all depends of your needs.

Here is several questions you should ask yourself before choosing one solution :

• timer resolution. Do you really need a precise timer (< 1ms) or can you live with something less precise (eg : 10ms precision) ?

• cpu cost. some methods will require more cpu than some others. generally more precise timers require more cpu. also, calling some methods can have consequences for the other applications (eg : calling timeBeginPeriod(1) in win32 stress the os and reduce laptop battery life)

• works with SMP. some methods use specific CPU data (eg: number of cycles elapsed) and can have some problems in multiprocessor environments.

• works with CPU that have power saving feature. Some cpu have ability to change frequency over the time, which can cause some trouble to some timing methods.

For the example you give, I would use a combination of QueryPerformanceCounter() / QueryPerformanceFrequency().

Also please note that in the example you give sleep(100) can actually wait more than 100ms. So even with very precise timer it can returns some results other than 100ms.

Answer 3

Take a look at the GetTickCount() function it's return value is the number of milliseconds that have elapsed since the system was started.

unsigned int Last = 0;
unsigned int Now = 0;
Last = GetTickCount();
//...Run your functions..
Now = GetTickCount();
printf("Elapsed Time in Milliseconds: %i",(Now-Last));