11.1. Setting the MaxClients Directive

It's important to specify MaxClients on the basis of the resources your machine has. The MaxClients directive sets the limit on the number of simultaneous requests that can be supported. No more than this number of child server processes will be created. To configure more than 256 clients, you must edit the HARD_SERVER_LIMIT entry in httpd.h and recompile Apache.

With a plain Apache server, it doesn't matter much if you run many child processes—the processes are about 1 MB each (most of it shared), so they don't eat a lot of RAM. The situation is different with mod_perl, where the processes can easily grow to 10 MB and more. For example, if you have MaxClients set to 50, the memory usage becomes 50 × 10 MB = 500 MB.^[1] Do you have 500 MB of RAM dedicated to the mod_perl server?

^[1] Of course, you also have to take into account the shared memory usage, as described in Chapter 10.