How to keep the CPU usage down while running an SDL program?

Question

I've done a very basic window with SDL and want to keep it running until I press the X on window.

#include "SDL.h"
const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;

int main(int argc, char **argv)
{
    SDL_Init( SDL_INIT_VIDEO );
    SDL_Surface* screen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, 0, 
                                            SDL_HWSURFACE | SDL_DOUBLEBUF );
    SDL_WM_SetCaption( "SDL Test", 0 ); 
    SDL_Event event;
    bool quit = false;
    while (quit != false)
    {
        if (SDL_PollEvent(&event)) {
            if (event.type == SDL_QUIT) {
                quit = true;
            }
        }
        SDL_Delay(80);
    }
    SDL_Quit();
    return 0;
}

I tried adding SDL_Delay() at the end of the while-clause and it worked quite well.

However, 80 ms seemed to be the highest value I could use to keep the program running smoothly and even then the CPU usage is about 15-20%.

Is this the best way to do this and do I have to just live with the fact that it eats this much CPU already on this point?

Answer 1

I know this is an older post, but I myself just came across this issue with SDL when starting up a little demo project. Like user 'thebuzzsaw' noted, the best solution is to use SDL_WaitEvent to reduce the CPU usage of your event loop.

Here's how it would look in your example for anyone looking for a quick solution to it in the future. Hope it helps!

#include "SDL.h"
const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;

int main(int argc, char **argv)
{
    SDL_Init( SDL_INIT_VIDEO );
    SDL_Surface* screen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, 0, 
                                            SDL_HWSURFACE | SDL_DOUBLEBUF );
    SDL_WM_SetCaption( "SDL Test", 0 ); 
    SDL_Event event;
    bool quit = false;
    while (quit == false)
    {
        if (SDL_WaitEvent(&event) != 0) {
            switch (event.type) {
            case SDL_QUIT:
            quit = true;
            break;
            }
        }
    }
    SDL_Quit();
    return 0;
}

Answer 2

I would definitely experiment with fully blocking functions (such as SDL_WaitEvent). I have an OpenGL application in Qt, and I noticed the CPU usage hovers between 0% and 1%. It spikes to maybe 4% during "usage" (moving the camera and/or causing animations).

I am working on my own windowing toolkit. I have noticed I can achieve similar CPU usage when I use blocking event loops. This will complicate any timers you may depend on, but it is not terribly difficult to implement timers with this new approach.

Answer 3

I just figured out how to reduce CPU usage in my game from 50% down to < 10%. Your program is much more simple and simply using SDL_Delay() should be enough.

What I did was: Use SDL_DisplayFormat() when loading images, so the blitting would be faster. This brought its CPU usage down to about 30%.

So I found out that blitting the games background (big one-piece .png file) was eating the most out of my CPU. I searched the Internet for a solution, but all I found was the same answer - just use SDL_Delay(). Finally, I found out that the problem was embarrassingly simple - the SDL_DisplayFormat() was converting my 24-bit images to 32-bit. So I set my display BPP to 24, which brought CPU usage to ~20%. Bringing it down to 16 bit solved the problem for me and the CPU usage is under 10% now.

Of course this means loss of color detail, but as my game is a simplistic 2D game with not too detailed graphics, this was OK.