什么决定了Ubuntu在Rails和MySQL上的内存使用

Forgive the newbie type question but, what determines the RAM consumed by rails and MySQL (my server is Ubuntu)? With barely any requests coming in the server seems to be hovering around 1.5 of 2GB. MySQL has about 100MB of data stored in it. The site has about 3500 registered users and when trafic is high, the memory tends to peak around 1.8 GB. When the traffic is low or non-existent, it doesn't drop much though.Forgive the newbie type question but, what dete