How to install Apache2 worker MPM, FCGI and APC on CentOS
Apache on CentOS uses Prefork by default with the PHP module. Under heavy loads, switching to Apache worker with PHP fcgi (PHP fastcgi) can reduce memory use.
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Apache worker.
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How to install Apache worker + PHP FastCGI and PHP-FPM on Ubuntu 12.04 LTS
Apache worker suits heavily loaded web servers with limited resources because it uses less memory than Prefork. Enabling PHP support requires additional configuration.