presentRenderbuffer : GL_RENDERBUFFER_OES takes long time

Question

I added a profiler to my game and isolated this function. It, sometimes, cause FPS drop. This is my result:

Present buffer time: 22
Present buffer time: 1
Present buffer time: 9
Present buffer time: 1
Present buffer time: 20
Present buffer time: 6
Present buffer time: 4
Present buffer time: 4
Present buffer time: 22
Present buffer time: 4
Present buffer time: 8
Present buffer time: 4
Present buffer time: 4
Present buffer time: 15
Present buffer time: 1
Present buffer time: 1
Present buffer time: 1
Present buffer time: 11
Present buffer time: 1
Present buffer time: 6
Present buffer time: 1
Present buffer time: 1
Present buffer time: 25
Present buffer time: 6
Present buffer time: 7
Present buffer time: 1
Present buffer time: 10
Present buffer time: 7
Present buffer time: 3
Present buffer time: 1
Present buffer time: 12
Present buffer time: 9
Present buffer time: 6
Present buffer time: 9

Do you know why at every few frames this function takes more time?

This is my code:

    f64 time = -System::GetTime();      
    [m_context presentRenderbuffer:GL_RENDERBUFFER_OES];
    Log("Present buffer time: %d\n", u32((System::GetTime() + time) * 1000.));

Answer 1

If you are not calling glFlush() explicitly, the driver is forced to do it when you present the buffer, because at the point of presentRenderbuffer call the rendering operations on the buffer may not be completed yet.

Try calling glFlush before presenting the buffer and measuring time of it separately.