SDL_GL_SwapBuffers() is intermittently slow

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.