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General / SFML 2.1 - High CPU usage (SOLVED) (Need explanation still)

« on: October 05, 2013, 01:45:52 pm »

I'm experiencing high CPU usage with SFML 2.1.

sf::RenderWindow window(sf::VideoMode(1280, 720), "Hello SFML", sf::Style::Default, settings);
window.setFramerateLimit(60);
while(window.isOpen())
{
    window.clear(sf::Color::Black);
    window.display();
}

 

This does not seem to limit the FPS correctly. I've also tried using sf::Clock and sleeping for the remainder (1.f / 60.f - clock.getElapsedTime().asSeconds()), but it doesn't seem to help. I clocked the sleep time, and it seems to be correct as well, but something hogs the CPU. I use only a single thread (was using multiple threads, but stripped the code down and it still does that). Any ideas?

EDIT:
I did some testing. My sleeping code is like this:

// Inside the main loop
clock.restart();

// Do the drawing etc.

sf::Time elapsed = clock.getElapsedTime();
float sleepTime = 1.f / 60.f - elapsed.asSeconds();
if(sleepTime > 0.f)
{
    sf::sleep(sf::seconds(sleepTime));
}
 

Now, with 1.f / 60.f (60 FPS limiter), it hogs the CPU, but drops to zero when using 1 FPS limiter. If I set it to 1.f / 30.f (30 FPS), it stays around 4-5 % usage. Profiler says that window.clear(sf::Color::Black) takes the most of the processing time.


EDIT 2:

Okay, I solved it by turning off VSync. Why is that? Why does enabling VSync cause CPU usage to skyrocket?