PHP tuning: Opcode Caching

When a PHP script is requested, PHP reads the script and compiles it into what's called Zend opcode, a binary representation of the code to be executed. This opcode is then executed by the PHP engine and thrown away. An opcode cache saves this compiled opcode and reuses it the next time the page is called. This saves a considerable amount of time. Several opcode caches are available; I've had a great deal of success with eAccelerator.

Installing eAccelerator requires the PHP development libraries on your computer. Because different Linux distributions place files in difference places, it's best to get the installation instructions directly from the eAccelerator Web site . It's also possible that your distribution has already packaged an opcode cache, and you just have to install it.

Regardless of how you get eAccelerator on your system, there are a few configuration options to look at. The configuration file is usually /etc/php.d/eaccelerator.ini. eaccelerator.shm_size defines the size of the shared memory cache, which is where the compiled scripts are stored. The value is in megabytes. Determining the proper size depends on your application. eAccelerator provides a script to show the status of the cache, which includes the memory usage; 64 megabytes is a good start (eaccelerator.shm_size="64"). You may also have to tweak your kernel's maximum shared memory size if the value you choose isn't accepted. Add kernel.shmmax=67108864 to /etc/sysctl.conf, and run sysctl -p to make the setting take effect. The value for kernel.shmmax is in bytes.

If the shared memory allocation is exceeded, eAccelerator must purge old scripts from memory. By default, this is disabled; eaccelerator.shm_ttl = "60" specifies that when eAccelerator runs out of shared memory, any script that hasn't been accessed in 60 seconds should be purged.

Another popular alternative to eAccelerator is the Alternative PHP Cache (APC). The makers of Zend also have a commercial opcode cache that includes an optimizer to further increase efficiency.