I am not an expert on this but I remember looking into it 2 or so years ago... from what I understand it stores the executed PHP code in memory saving time next time it needs the same code. The problem we ran into is that we have so many accounts \ php scripts being executed that even with a ridiculous amount of memory it would not be able to cache it all.
Again, I'm not an expert on this and may be wrong. If I am I'd be happy to try it out. Does anyone have a better idea of how the software above works?
I've actually used eAccelerator on my Linux router here at my home back when it used to be my web server for my website (2 years ago, way before I moved to x10Hosting). Now, because it was just my site, eAccelerator didn't really show much improvement in terms of speed, especially since it was running on a Pentium III box with 512MB of RAM which was acting as a router as well, but it did in fact cache every PHP script I ran on the server so that the hard drive was free to do other things. If you're going to try it, I'd say take a measure of the amount of accounts that run PHP scripts frequently and how much memory each user is using when their scripts are running, then add up the total amount of RAM that the scripts in a given time used, compare them with the free server memory and then decide on how much memory to allow eAccelerator to cache with. I'd say for a rough estimate, if your servers are running 8GB of RAM, Linux should at least be left with 700MB to run with, Apache, PHP, MySQL their own Gigabyte or so of RAM, whichever they will run the quickest with, and then for the caching, whatever is left of the RAM. Anything else would go to a separate swap drive or clear out the cache temporarily. I have no idea of how much RAM the servers are using for what so this is more or less an estimate.
But if I'm remembering correctly, when it comes down to memory usage, back then my site was probably caching no more than 20 megabytes or so of scripts (SMF forum, MediaWiki, another SMF forum which I hosted for someone temporarily). In total, the Linux box out of it's 512MB of RAM was using about half of that at any given time for the OS and web software. I pretty much had Linux using 192MB of RAM with about 40 or so megabytes of data of Linux in the swap, and then I had Apache, MySQL and PHP all running in a 64MB area (though they typically sat in a combined total of 18MB of RAM used). Anything else on the system such as my routing software and filtering software was using whatever RAM and swap they needed, based on network load. The machine and software could remain online for months and it'd be running smoothly. That machine is still in service to me today, acting as a network router, file server and web filter, it's just no longer serving any content to the outside world, so Apache, PHP and MySQL aren't installed onto it.
Just as an FYI though, eAccelerator back when I ran it didn't like certain scripts but I'm sure this is fixed, but I can add that it did in fact cache the entire PHP file to RAM. Whether or not it still does that to this day, I don't know.
But in general, I'd say go ahead and install it on a server, and see how it holds up and how the server resources look compared to one without caching.
EDIT: May I also note, that I believe you need to have a script that supports eAccelerator to really get the most out of it. I know that SMF does in the deep part of it's admin panel and can be turned on if PHP is set up with that accelerator. Below is a screenshot of the caching page of SMF 2.0 RC1-1, running from my website on Lotus.
