xcb mouse motion causes input lag

Question

I have written some basic OpenGL applications with XCB as the backend(xlib for GLX, of course) and in every test I have written when I move my mouse over the window it causes all input to get sort of "buffered" and only responds to the events after a period of time(varies depending on how many inputs).

I'm calling xcb_poll_events and getting the event information that way then loading it into a custom event class, but this was never slow on my old xlib implementation.

What could be causing this lag?

The event polling:

Event_c system_class::poll_for_event(){
    Event_c temp;

    xcb_generic_event_t *event;
    event = xcb_poll_for_event(this->connection_xcb);

    if(!event)
        return temp;

    switch(event->response_type){
        handle events...
    }

    free(event);
    return temp;
}

and the event loop in the test app:

int main(int argc, char *argv[]){

    init stuff...

    system_class app;
    window_class window;

    Event_c event;
    while(running){
        event = app.poll_for_event();
        if(event.detail){
            handle user input...
        }

        window.swap_buffers(); // just calls glXSwapBuffers
    }

    return 0;
}

Answer 1

Your problem is, that you are calling glXSwapBuffers between two calls of xcb_poll_for_event. Therefore you only can handle one message per screen refresh.

Apart from your multithreading solution you could simply process events until xcb_poll_for_event would return zero. When you are finished with handling all pending events, you could return to rendering.