1. A method for classifying activity levels for memory pages of a virtual machine, the method comprising: maintaining a current activity level of each of the memory pages in a first data structure corresponding to a first time interval and in a second data structure corresponding to a second time interval that is less than the first time interval; receiving notification of an elapse of the first time interval; identifying the current activity level of a memory page of the virtual machine stored in the first data structure; retrieving first input data indicating the current activity level of the memory page stored in the second data structure, wherein the first input data is updated at the end of each of the second time intervals that elapsed within the first time interval, the first input data being updated at the end of an elapsed second time interval based on whether or not the memory page was accessed during the elapsed second time interval; determining a new activity level of the memory page based on the current activity level stored in the first data structure and the retrieved first input data; and storing the new activity level in an entry of the first data structure corresponding to the memory page as the current activity level of the memory page.