Autoload function

Hello, I have some doubts about autoload in CodeIgniter.
My first question is that currently, I use __autoload, it has been discontinued? Is not it good practice to use it?
I read about the spl_autoload_register, is it better to use it instead of __autoload?
Also, my other doubt is related to server demand when using autoload, does it load EVERYTHING as soon as the server is loaded or is it on demand?
Because if it loads everything, it would not be good to put many includes there, right?

Thanks a lot !