A few posts ago I was concerned about the apparent diminishing of available RAM as reported by checking "/proc/meminfo". But then I added this command to my midnight RAM test:
sudo /sbin/sysctl vm.drop_caches=3
Which causes the system to clear memory that is temporarily held -- just in case some data might be accessed again.
So, adding that command makes the MemFree number accurate, but probably lowers the efficiency of the OS slightly.
Duh. (but of course, the "/proc/meminfo" data is misleading)