The Alternative PHP Cache (APC) is a free and open opcode cache for PHP. While it works out-of-the-box, it's important to configure it correctly. This shows how to setup APC and increase its efficiency.
Use a package to install the latest version.
sudo add-apt-repository ppa:brianmercer/apc
sudo aptitude update
sudo aptitude install php-apc
sudo vi /etc/php5/conf.d/apc.ini
The shmsize setting suggests APC uses shared kernel memory, but most APC installs are complied with MMAP support. This is why we can increase shmsize above our kernel shmsize. With MMAP, APC only uses one shmsegment.
Increase shm_size to provide ample caching space and set TTLs so garbage collection works more efficiently. Also add a filter to bypass the APC stats page.
APC comes with a nifty stats page. Copy it to the web folder and unzip.
sudo cp /usr/share/doc/php-apc/apc.php.gz /srv/www/example.com/web
sudo gzip -d /srv/www/example.com/web/apc.php.gz
sudo vi /srv/www/example.com/web/apc.php
Change the password to enable web-based purging and advanced stats.
Reboot the server for APC to start and then test the page at http://example.com/apc.php.
Keep an eye on the 'cache full count' value. This is a count of the number of times APC runs out of memory and has to remove stale objects or purge memory completely. Also watch the fragmentation level. Now test using Apache Bench.