If you notice that server CPU utilization appears low, but client
requests for static pages take a long time to service, your server may be
running out of server threads to handle requests.
This situation results when you have more inbound requests than you have
Apache threads to handle those requests. New connections queue in the TCP/IP
stack listen queue wait for acceptance from an available thread. As a thread
becomes available, it accepts and handles a connection off of the listen queue.
Connections can take a long time to reach the top of the listen queue. When
this condition occurs, the following error message will appear in the error
log:
"Server
reached MaxClients setting, consider raising the MaxClients setting"
"Server ran out of threads to serve requests.
Consider raising the ThreadsPerChild setting"