I run a CentOS 5.6 (64bit) machine that has Nginx (latest version) running, with php-fpm (latest version). Things run very well, but since about 2 weeks I noticed in my Munin graphs that about every 2 hours the 'cache' usages drops. Before it used be a steady fully graph, that didn't seem to reset every so often.

PHP-FPM settings:

pm.max_children = 300
daemonize = yes
pm = static
listen = /tmp/fpm.sock
pm.max_requests = 1000

I have checked the php-fpm.log, and about once per 5 seconds a child process is killed, and restarted. But this is all the time, so this does not explain the sudden drops.

I only run Nginx, PHP (via fpm), Munin and vsftpd on this machine. No crons run at exactly the time of the drops.

My question: What could be causing these drops in cache usage?