I have a relatively small home Red Hat Linux server (about 8 GB RAM). I don't use it for much other than running some home grown apps to keep track of various things. The only real things running on the box are a database and a web server.
I've noticed that when checking system counters using tools like NMON and TOP that the total system free memory is relatively low (on the order of a few hundred MB), while the active memory for the database and web server is still low (only consuming a combined 3 GB). Even when including all other running processes the total consumed memory is less than 4 GB.
Why does Red Hat Linux report less free memory than the total memory minus the sum total of used memory of running processes?