Magento Compilation

Many people are not aware of the Magento Compilation For Speeding up the Performance Magento Compilation tools is the best tools

The System → Tools → Compilation option allows you to compile all files of a Magento installation in order to create a single include path to increase performance.
In order to use this tool, the directory includes and the file includes/config.php must both be writeable.

Note: As it is important tools for Speed up the Performance but Some times it will break your website. You will be unable to view the frontend of backend of your magento website. you will need to turn this off through the magento core files. To do this you will need to:

disable and comment out the following in includes/config.php

#define(’COMPILER_INCLUDE_PATH’, dirname(__FILE__).DIRECTORY_SEPARATOR.’src’);
define(’COMPILER_COLLECT_PATH’, dirname(__FILE__).DIRECTORY_SEPARATOR.’stat’);

change to

#define(’COMPILER_INCLUDE_PATH’, dirname(__FILE__).DIRECTORY_SEPARATOR.’src’);
#define(’COMPILER_COLLECT_PATH’, dirname(__FILE__).DIRECTORY_SEPARATOR.’stat’);

it will fix Your issues