On Oct 20, David gave a presentation of the Lullabot topic on Drupal optimization. Opcode Cache was mentioned as a way to improve performance.
I found this information:
Opcode Cache for Dummies
http://devbee.com/opcode_cache_for_dummies
PHP is an interpreted language. This means that each time a PHP generated page is requested, the server must read in the various files needed and "compile" them into something the machine can understand (opcode). A typical Drupal page requires more than a dozen of these bits of code be compiled.
Opcode cache mechanisms preserve this generated code in cache so that it need only be generated a single time to server hundreds or millions of subsequent requests.
Enabling opcode cache will reduce the time it takes to generate a page by up to 90%.
Detecting Opcode Cache with PHPinfo()
Look for APC or eAccelerator (a fork of Turck MMCache)
Example:
eAccelerator
eAccelerator support enabled
Version 0.9.5
Caching Enabled true
Optimizer Enabled true
Memory Size 16,777,180 Bytes
Memory Available 16,770,416 Bytes
Memory Allocated 6,764 Bytes
Cached Scripts 1
Removed Scripts 0
Cached Keys 0
APC issues with Drupal
http://us3.php.net/apc
http://drupal.org/node/48946
Boost performance, throughout with opcode cache software
http://groups.drupal.org/node/2929
PHP caches
http://drupal.org/node/2603
* PHP Accelerator
* After Burner
* Zend Accelerator
* APC : Alternative PHP Cache
* eAccelerator
* Turck MMCache
A beginner's guide to caching data
http://www.lullabot.com/articles/a_beginners_guide_to_caching_data
All of the above is only scratching the surface of how to use caching.
For more reading see "Pro Drupal Development." (Apress)
John