session_start seems to be very slow (but only sometimes)

Question

For some odd reason, just today our server decided to be very slow during the starting of sessions. For every session_start, the server either times out after 30 seconds, or it'll take about 20 seconds for it to start the session. This is very weird, seeing as it hasn't done this for a very long time (the last time our server did this was about 7 months ago). I've tried to change the session to run through a database instead, and that works fine, however, as our current website is built, it'd take days to go on every page and change the loading of sessions to include a new session handler. Therefore my question remains:

Why is it so slow, and why only sometimes?

We run on a dedicated hetzner server with 24GB's of ram, and a CPU fast enough to just run a simple webserver (a Xeon, I believe, but I'm not sure). We run debian on the server with an apache+fastcgi+php5 setup.

The server doesn't report much load, neither through server-status as well as the top command. Vnstat reports no problem whatsoever with our network link (again, that wouldn't result in a slow local session handling). IOtop reports no problem with processes taking over the entire harddrive. Writing to the tmp folder where the session files are located works fast if done through vim.

Again, to make this clear, my main concern here isn't whether or not we should switch to a DB or a memory-cached version of the sessions, it's simply to ask why this happens, because everything I take a look at seems to be working fine, except for the PHP itself.

EDIT: The maximum file in our PHP tmp directory is 2.9 MB, so nothing that should make an impact, I believe.

UPDATE: I did never figure out what was wrong and/or how to fix it, but the problem disappeared after we switched over to memcached/db sessions.

Answer 1

Have you tried session_write_close(); ? This will disable write-ability in session variables but you can still read data from them. And later when you need to write a session variable, reopen it.

I have also suffered from this problem but this thing worked like a charm. This is what i do:

session_start(); //starts the session $_SESSION['user']="Me"; session_write_close();   // close write capability echo $_SESSION['user']; // you can still access it 

Answer 2

I had the same problem: suddenly the server took 30 seconds to execute a request. I noticed it was because of session_start(). The first request was fast, but each next request took some 30 sec to be executed. I found that the session file in c:\wamp\tmp was locked by the first request for some 30 sec. During this time the second request was waiting for the file to be unlocked. I found out it had something to do with rewrite_mod and .htaccess. I disabled rewrite_mod and commented out every line in .htaccess and it works again like a charm. I don't know why this happend because I don't remember change any settings or conf on wamp.