================================================================ I finally found the reason with oprofile. It was because the routing cache of Symfony. We have lots of pages with different urls and symfony caches them in one file (serialized data). So the cache file grows large and it needs more CPU to serialize and unserialize data.
Everything went well after I disabled the routing cache
Recently I updated my PHP program on my web server (Centos 5.3 + PHP 5.2.6), but it puts a heavy load on my server. So I decided to use oprofile to find what makes my server slow.
I've successfully installed oprofile but what's next? Does anyone have practical experience tuning an apache server?