Here are some great links to well-written articles for fine-tuning apache on VPS or Dedicated Servers (Plesk Servers Also)
Some general considerations,
KeepAlives are important for dynamic sites & most plesk servers. If you use a linux server to ONLY offer large files (MP3/Video) turn KeepAlives off.
If a server is running out of memory, be sure you have not set the MaxClients and ServerLimit to high. On High Volume Servers 256 or even 512 should be enough. Be SURE to tune this per your memory requirements.
If you have a high number of requests constantly hitting a server, be sure to Set StartServers to a high number (50 or more) and MinSpareServers to 20 or more (this means how many processes are in "standby" mode) With MaxSpareServers at the same # of StartServers (this is for servers that are serving content 24/7; for servers that "peak" it is better to set a low number of MinSpareServers)
If you have memory leaks (dynamic content sites/servers) then be SURE to set MaxRequestsPerChild to a "one day" limit, so that the process can be killed and memory regained. (This will take some experimentation, but 1000 to 4000 is typically good, though in cases of very little dynamic content and a lot of static content numbers as low as 20 are good)
If you have NO dynamic content (such as MP3 servers) set this to 0 so that processes are never killed and keep serving requests indefinitely. Just remember, if there is a memory leak, this will eat up all available RAM, so watch the server carefully.
If you have a high number of requests constantly hitting a server, be sure to Set StartServers to a high number (50 or more) and MinSpareServers to 20 or more (this means how many processes are in "standby" mode) With MaxSpareServers at the same # of StartServers (this is for servers that are serving content 24/7; for servers that "peak" it is better to set a low number of MinSpareServers)
