Please explain what OS X is doing when it uses a high amount of RAM, being idle. Not what you think that it does, facts what it does and why. Thank you.
Well when a program is opened, it immediately loads into the RAM. That's universal of all computers, not just Macs. However, UNIX systems use RAM slightly differently to Windows. They'll use as much physical RAM as possible. When you close an application on OS X it will still stay in the RAM for a certain amount of time. This means that if you reopen the program it will open much quicker and run far faster. This also means that the RAM usage might show as being larger than it 'should be' if you've recently used an application.
If you've had options like 'reopen windows when logging back in', this will reopen the application itself, not just the application window. Even if you Cmd+Q these applications on restart, the RAM usage will still show as being high for the reasons stated above. With high RAM usage when idling with Finder only, OS X is likely to be running maintenance such as caching/indexing. If you monitor the RAM usage you'll see that opening up an application won't have an immediate or detrimental impact on the RAM usage because OS X will mix and match in real-time (it's been like that since Tiger, maybe even a little earlier).
When the RAM is getting full, OS X will get rid of the RAM-cached applications to make way for the new application being opened. With the addition of memory compression in Mavericks you can put the RAM under heavy strain and it won't page as quickly because the RAM for idle applications is compressed in real-time.
'Paging' is when OS X runs out of all of its RAM, and the open applications require more RAM than OS X can physically give it. When this happens, it will write to the hard-drive as virtual memory, which briefly uses free hard-drive space to act as RAM. With an SSD this is virtually seamless due to the quick access speeds, and read/write speeds of an SSD compared to a conventional hard-drive. However with a standard hard-drive you'll see a severe impact in performance when this happens.
Windows, on the other hand, uses RAM differently. When you open an application in Windows, it will always page to the hard-drive and the RAM simultaneously, regardless of how much free RAM you have. That's often why Windows doesn't really sing, no matter what hardware you chuck at it -- although that part about Windows performance is just my opinion.
Hope this helps.
