My Apache 2.2 (preform MPM) is hanging when MaxClients is reached, rather than queueing up requests and serving them when child processes become free. When this happens, the web server is totally unresponsive until it is manually restarted.
The server stack is Ubuntu 8, MySQL 5, PHP 5. Hardware is Dual Xeons (2.8) with 2GB of RAM. It serves 30,000 - 50,000 pageviews per day. Static images, CSS, and JS are offloaded to a separate server and PHP is cached using eAccelerator. The HTML output of many pages is cached to the filesystem.
Relevant Apache directives:
KeepAlive On MaxKeepAliveRequests 50 KeepAliveTimeout 2 StartServers 2 MaxClients 150 MinSpareThreads 25 MaxSpareThreads 75 ThreadsPerChild 25 MaxRequestsPerChild 2000