Modules hook system. Performance. Hooks cashing.

Hello All,

While I'm new in PHP and Drupal I have some questions about modules hook system in Drupal. I've read book 'Pro Drupal Development' and watched 'lynda.com' training course but still hadn't got any answers to these questions.

1. As I understand that on each action the notifications are send to all enabled modules. So, more modules are activated the bigger this list becomes. What influence is on performance when registering more and more modules?

2. I see in the code(include\module.inc) that hooks' list is cashed into $implementations variable in function

function module_implements($hook, $sort = FALSE, $refresh = FALSE)

, but I've found only two places where this functions is called with param $refresh as TRUE. And it's in functions module_disable, module_enable. As I understand these functions called only when admin is enabling/disabeling modules from front-end.

So when hooks list is initialized, how often it's refreshed and were is stored during the ssesion? Is it stored in $_SESSION variable or in DB?

Thank you for the answers and links into similar discussions.
marukas