SDL_GL_SwapBuffers() is intermittently slow - c++

Question

I have a sdl/opengl game I am working on for fun. I get a decent fps on average, but movement is really choppy because SDL_GL_SwapBuffers() will randomly take a crazy long amount of time to process. With textures loaded and written to the buffer sometimes it will take over 100ms! I cut out a lot of my code to try and figure out if it was something I did wrong but I haven't had much luck. When I run this bare bones program it will still block for up to 70ms at times.
Main:
// Don't forget to link to opengl32, glu32, SDL_image.lib
// includes
#include <stdio.h>
// SDL
#include <cstdlib>
#include <SDL/SDL.h>
// Video
#include "videoengine.h"
int main(int argc, char *argv[])
{
// begin SDL
if ( SDL_Init(SDL_INIT_VIDEO) != 0 )
{
printf("Unable to initialize SDL: %s\n", SDL_GetError());
}
// begin video class
VideoEngine videoEngine;
// BEGIN MAIN LOOP
bool done = false;
while (!done)
{
int loopStart = SDL_GetTicks();
printf("STARTING SWAP BUFFER : %d\n", SDL_GetTicks() - loopStart);
SDL_GL_SwapBuffers();
int total = SDL_GetTicks() - loopStart;
if (total > 6)
printf("END LOOP : %d ------------------------------------------------------------>\n", total);
else
printf("END LOOP : %d\n", total);
}
// END MAIN LOOP
return 0;
}
My "VideoEngine" constructor:
VideoEngine::VideoEngine()
{
UNIT = 16;
SCREEN_X = 320;
SCREEN_Y = 240;
SCALE = 1;
// Begin Initalization
SDL_Surface *screen;
SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 ); // [!] SDL_GL_SetAttributes must be done BEFORE SDL_SetVideoMode
screen = SDL_SetVideoMode( SCALE*SCREEN_X, SCALE*SCREEN_Y, 16, SDL_OPENGL ); // Set screen to the window with opengl
if ( !screen ) // make sure the window was created
{
printf("Unable to set video mode: %s\n", SDL_GetError());
}
// set opengl state
opengl_init();
// End Initalization
}
void VideoEngine::opengl_init()
{
// Set the OpenGL state after creating the context with SDL_SetVideoMode
//glClearColor( 0, 0, 0, 0 ); // sets screen buffer to black
//glClearDepth(1.0f); // Tells OpenGL what value to reset the depth buffer when it is cleared
glViewport( 0, 0, SCALE*SCREEN_X, SCALE*SCREEN_Y ); // sets the viewport to the default resolution (SCREEN_X x SCREEN_Y) multiplied by SCALE. (x,y,w,h)
glMatrixMode( GL_PROJECTION ); // Applies subsequent matrix operations to the projection matrix stack.
glLoadIdentity(); // Replaces the current matrix with the identity matrix
glOrtho( 0, SCALE*SCREEN_X, SCALE*SCREEN_Y, 0, -1, 1 ); //describes a transformation that produces a parallel projection
glMatrixMode( GL_MODELVIEW ); // Applies subsequent matrix operations to the projection matrix stack.
glEnable(GL_TEXTURE_2D); // Need this to display a texture
glLoadIdentity(); // Replaces the current matrix with the identity matrix
glEnable(GL_BLEND); // Enable blending for transparency
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // Specifies pixel arithmetic
//glDisable( GL_LIGHTING ); // Disable lighting
//glDisable( GL_DITHER ); // Disable dithering
//glDisable( GL_DEPTH_TEST ); // Disable depth testing
//Check for error
GLenum error = glGetError();
if( error != GL_NO_ERROR )
{
printf( "Error initializing OpenGL! %s\n", gluErrorString( error ) );
}
return;
}
I'm starting to think possibly I have a hardware issue? I have never had this problem with a game though.

Answer 1

SDL does use the SwapIntervalEXT extension so you can make sure that the buffer swaps are as fast as possible (VSYNC disabled). Also, buffer swap is not a simple operation, OpenGL needs to copy contents of back buffers to front buffers for case that you want to glReadPixels(). This behavior can be controlled using WGL_ARB_pixel_format, using WGL_SWAP_EXCHANGE_ARB (you can read about all this stuff in the specs; now I'm not sure if there is an alternative to that for Linux).
And then on top of all that, there is the windowing system. That can actually cause a lot of trouble. Also, if some errors are generated ...
This behavior is probably ok if you're running on a small mobile GPU.
SDL_GL_SwapBuffers() only contains a call to glxSwapBuffers() / wglSwapBuffers() so there is no time spent in there.