Question

I write a 3D model display program using OpenGL、Qt、C++.But I found something strange.That is the FPS(frame per second) in Release mode version is lower than debug mode version.Now I post their FPS:

The left is debug mode version and the right is release mode version:

The function I use to compute FPS is

void displayFPS()
{
    static float framesPerSecond    = 0.0f;       // This will store our fps
    static float lastTime   = 0.0f;       // This will hold the time from the last frame
    float currentTime = GetTickCount() * 0.001f;    
    ++framesPerSecond;
    if( currentTime - lastTime > 1.0f )
    {
        framesPerSecond/=currentTime - lastTime;
        char strFrameRate[256];
        lastTime = currentTime;
        sprintf_s(strFrameRate,256, "FPS : %f", framesPerSecond);
        cout << strFrameRate << endl;
        framesPerSecond = 0;
    }
}

I wonder how could this happen?Shouldn’t release mode be faster than debug mode?Could someone tell me why?

Answer 1

According to this the accuracy of GetTickCount() is much worse than a millisecond. It can even be as bad as 55ms! Use a more reliable method to measure time intervals, like this one:

#include <windows.h>
#include <cstdint>

typedef std::int64_t int64;

// get duration of a single "clock" in microseconds
double
get_clock_duration()
{
  LARGE_INTEGER f;
  QueryPerformanceFrequency(&f);
  return 1000000.0 / double(f.QuadPart);
}

// get number of elapsed clocks from program start
int64
clocks()
{  
  LARGE_INTEGER t;
  QueryPerformanceCounter(&t);
  return t.QuadPart;
}

// convert a duration in clocks to a duration in milliseconds
double
milliseconds(int64 t)
{
  return get_clock_duration() * double(t) * 0.001;
}