The system balances RAM between kernel+application use and I/O buffers+cache. It changes the line dynamically to provide the best performance. If you'd like to stop that process or override it, you can change swappiness. At a setting of about 10, the system will try very hard not to swap, at the detriment of I/O. This is useful, for example, if you know that your I/O is of a highly serial nature and very rarely re-references data.
Last edited by macemoneta; 10-24-2011 at 10:34 AM.