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
Best Answer
It seems that your server is swapping. Assuming a use of 16 MB for the apache processes, 150 x 16MB = 2400 MB, beyond the capacity of your server (2GB)
Check the memory used by Apache, I think you will need to reduce the MaxClients value.
You can find more information about this in the Hardware and OS Issues section from Apache Performance Tuning