apache prefork and workers

http://www.genericarticles.com/mediawiki/index.php?title=How_to_optimize_apache_web_server_for_maximum_concurrent_connections_or_increase_max_clients_in_apache

http://serverfault.com/questions/214448/what-is-the-best-way-to-configure-the-number-of-workers-in-apache

https://httpd.apache.org/docs/2.2/mod/worker.html