By the way, your problem is not a problem.
Cached memory is freed up when necessary, if no process needs new memory space its content will stay like that forever.
This doesn't make you server slower or anything; it just makes faster the next launch of the process that filled the cached memory to begin with.
You can remove manually the cache (read http://egloo.wordpress.com/2008/10/29/linux-cached-memory/) but it's just the dumbest thing to do.
That's all. Linux will have to read more from the disk instead of recovering the mmap()ed files and executables from the memory cache.
You're probably used to seeing in Windows XP and similar versions where all you'd have is Free Memory and Total Memory. All you had to do was look at the first value to tell whether things were running like you wanted. With Linux, it treats free memory as wasted memory. It caches all the data it can with the free memory, but will give up that cached memory to any programs that need it. It doesn't harm anything. It can only help. So when you see that you only have 30mb of free memory, don't freak. Check first to see how much of your total is actually cached.
