Apache MPM Directives Explained: The Ultimate Performance Tuning Guide

The StartServers directive should mirror that of MinSpareServers.

ServerLimit

The ServerLimit directive represents the upper limit of MaxRequestWorkers. This setting is generally used as a safeguard or ceiling against input errors when modifying MaxRequestWorkers.

It becomes necessary to adjusted ServerLimit when the server is expected to handle more than the default of 256 requests simultaneously.

ServerLimit ties in directly with the thrashing point. The thrashing point is the maximum number of children Apache can run before memory usage tips the scale into perpetual swap. Match the ServerLimit to the calculated thrashing point to safeguard the server.

Note that increasing ServerLimit is not recommended with MPM Prefork. Running more than 256 simultaneous requests is hardware intensive when using the MPM Prefork module.

MaxConnectionsPerChild / MaxRequestsPerChild

This directive equals the number of requests a single Apache child server can handle.

This directive is a stop-gap for accidental memory leaks. Code executed through Apache may contain faults which leak memory. These leaks add up over time making less and less of the shared memory pool of the child usable. The way to recover from leaked memory is to recycle the affected Apache child process. Setting a MaxConnectionsPerChild limit will protect from this type of memory leakage.

Remember never to set this too low. If the server encounters memory leak issues start with 10,000 and reduce incrementally.