session - PHP Garbage Collection clarification

Question

From the PHP manual, session.gc_probability and session.gc_divisor state that gc will occur based on this probability. I get that.

What I'm not clear on is whether this probability is on a session by session basis or overall.

So if my probability is 1% (1/100) that GC will occur, does that mean that if one session keeps getting extended, each time there is a 1% change that specific session will be cleaned up? Or does this mean that 1% of all existing sessions (as well as new ones) will trigger GC for all other existing sessions?

I'm pretty sure it's the latter, I just want to make sure.

The purpose of this question is that on our site, I want users to have long-term sessions (6 months). If 1% of all sessions trigger GC, then that effectively removes the purpose of having that long-term session, as GC will end up occurring every hour or two.

question from:https://stackoverflow.com/questions/7828975/php-garbage-collection-clarification

Answer 1

Every time a PHP script is executes and starts session there is a probability that it will sweep through the session folder killing off old session.

Cleanup will only delete sessions which were not accessed within a certain time. However PHP does not guarantee that the session WILL be destroyed within that time.

Your long-term session strategy should work just fine, but you might want to reduce 1% to something like 0.1%

Another thing to look out for is that operating system might clean up your /tmp folder during reboot so even if PHP won't do it.