The KeepAlive directive for Apache allows a single request to download multiple files. So on a typical page load, the client may need to download HTML, CSS, JS, and images. When KeepAlive is set to “On”, all of these files can be downloaded in a single request. If KeepAlive is set to “Off”, each file download would require it’s own request.
You can control how many files can be downloaded in a single request with the MaxKeepAliveRequests directive, which defaults to 100. If you have pages with a lot of different files, consider putting this higher so that your pages will load in a single request.
One thing to be cautious of when using KeepAlive, is the connections will remain open waiting for new requests once the connection is established. This can use up a lot of memory, as the processes sitting idly will be consuming RAM. You can help avoid this with the KeepAliveTimeout directive, which specifies how long the connections remain open. I generally put this below 5, depending on the average load times of my site.
An important factor when deciding to use KeepAlive is the CPU vs. RAM usage requirements for your server. Having KeepAlive On will consume less CPU as the files are served in a single request, but will use more RAM because the processes will sit idly. Here is an example of KeepAlive settings I use:
KeepAlive On MaxKeepAliveRequests 50 KeepAliveTimeOut 3
Once KeepAlive is on you can see the following header in your server’s response: