AIX always tries to use ALL of your real memory. Every piece of RAM that is not used by your application is used for file caching. So "all" of the memory appears to be "in use" all the time. When you exit your app the memory is not cleared but the former content is kept for possible reuse later. This is normal behaviour and expected. No reason to worry as long as no paging takes place. Having no "free" memory does not equal shortage of memory. AIX is not Windoze.

If you want to sort of flush the server's cache for a short period of time you could copy a huge file somewhere within your server (from /blah to /tmp or vice versa) and delete it immediately after copying. Then you would see "free" memory