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gleurop
11-16-2012, 04:26 PM
I'm developing a game using OpenGL and have run into a strange issue. Exactly 30 seconds after creating a context, the frame time increases by 2 - 3x depending on the scene, and then remains constant. I am using query objects with GL_TIME_ELAPSED to get the frame time. Below is a small demo that demonstrates the issue.


#include <stdio.h>
#include <GL/glew.h>
#include <GL/freeglut.h>

int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE);
glutCreateWindow("Frame time test");
glewExperimental = GL_TRUE;
glewInit();
GLuint query;
glGenQueries(1, &query);

while(1) {
glClear(GL_COLOR_BUFFER_BIT);
glBeginQuery(GL_TIME_ELAPSED, query);
glBegin(GL_TRIANGLES);
glVertex3f(-1, -1, 0);
glVertex3f(1, -1, 0);
glVertex3f(0, 1, 0);
glEnd();
glEndQuery(GL_TIME_ELAPSED);

GLuint drawTime;
glGetQueryObjectuiv(query, GL_QUERY_RESULT, &drawTime);
char timeStr[32];
sprintf(timeStr, "%f", drawTime / 1000000.0f);
glutSetWindowTitle(timeStr);

glutSwapBuffers();
glutMainLoopEvent();
}
return 0;
}
I know I shouldn't be using glBegin/glEnd, my actual game uses vertex buffers and the issue is exactly the same. I've also tried using GLFW, but the exact same thing happened.

Is there something I'm doing wrong, or is this a driver bug? I've been doing OpenGL development for quite a while and have never seen this before.

I'm on Linux with a NVIDIA GTX 560 Ti that has the latest drivers (310.19).

Alfonse Reinheart
11-16-2012, 04:50 PM
the frame time increases by 2 - 3x depending on the scene, and then remains constant

So? You're not actually doing anything yet. Your frame time is negligibly small.

What does it matter if a frame time measured in microseconds jumps by a factor of 3?

gleurop
11-16-2012, 05:42 PM
It may not equate to much in this example, but it can mean a significant FPS drop in a real world application.

Anyways, I managed to fix it by disabling adaptive clocking in nvidia-settings.

Alfonse Reinheart
11-16-2012, 06:40 PM
It may not equate to much in this example, but it can mean a significant FPS drop in a real world application.

Or... it may not. If it's only a few microseconds, real-world applications wouldn't notice. In short, your profiling is too artificial to be detecting anything that is worrisome.

The fact that disabling adaptive clocking turned it "fixed" it is evidence of that. You were detecting the fact that your graphics card looked at your workload and said, "Oh, nothing much here. No need to stress myself."

Real-world applications will not have this problem.

Mikael Ara
11-19-2012, 08:15 AM
you have wrote no code here, but it may be because of writing rendering functions in main display function(they will be re-rendered every 30 secs).
Try having some void init(){//hardworking functions, or rendering functions}., and just attaching it to your display.
It may fix the problem you have.