Apache maximum request number 256

ServerLimit 256
MaxClients 256
StartServers 19
ThreadsPerChild 10
MinSpareThreads 160
MaxSpareThreads 220
MaxRequestsPerChild 10000

This will:

• Start 190 threads when httpd starts
• Serve up to 256 simultaneous requests
• Ensure a min of 160 spare threads
• Limit a max of 220 spare threads
• Limit each httpd child process to serving 10,000 requests before replacing it with a new child process. This helps prevent hanging up the apache server in case there are memory leaks in the underlying application.

You're running out of swap because you're using all your RAM and then some. You have a serious problem that you need to rectify right now.

You have two choices: ignore cause and just add more RAM, or target the problem of what's actually munching on your memory.

Adding RAM is fairly cheap and fairly easy if it's your server but it's a temporary fix and if it's a VPS or a rented server, it's not so cheap. Let's have a go at fixing the root problem instead. What's sucking in that much memory? Here are a few tips:

• Turn off InnoDB (unless you need it) in MySQL
• Beat Apache (and MySQL) with the Stick of Configuration +5
• Consider a smaller httpd like cherokee, lighttpd or nginx (they're really fast and eat almost no RAM). Main downside is you can't use .htaccess files but you can hard-code in their functionality.
• Are you using an OP-code cache for PHP? Try turning it off or switching to another, more efficient one.

In terms of just getting swap items back into real RAM, you can do that by:

swapoff -a && swapon -a

But don't try then when you've got less free memory than you have things in swap. It'll crash your server.