Mac OS X has fairly good memory management but it’s not perfect, and sometimes RAM can be held unnecessarily in the “inactive” state despite the contents no longer being needed. If you’ve been participating in memory heavy activities or you just need to free up some available RAM you can actually force Mac OS X to clear out inactive memory.
Launch Terminal, found in /Applications/Utilities/ and enter the following command
Give OS X a minute or two to complete the process
Open Activity Monitor to see the before and after results yourself, you’ll find dramatic changes at the “Free”, “Used”, and “Inactive” meters under System Memory.
The purge command forces disk and memory caches to be emptied, offering a ‘cold disk buffer cache’ which is similar to the state of the operating system after a reboot. Of course, the benefit of using purge rather than rebooting is that you don’t have to restart the machine and you can maintain currently active applications while still freeing up memory