APC is an Alternative PHP Cache or Opcode cache that speeds up performance dramatically by caching queries.
How to install APC?
apt-get install php5-apc
You will have to enble it in the php configuration. Add the following to your php.ini file
Alternatively, you could create a separate apc.ini file and put it in the conf.d directory.
If your error log shows a lot of messages about potential cache slam averted, it is a bug. Not much you can do about it but you can turn slam defense off so that it doesn’t spam your logs (or cause other fails)
Add after that line in your php.ini or apc.ini
apc.write_lock = 1 apc.slam_defense = 0