Repaging

A page fault is considered to be either a new page fault or a repage fault. A new page fault occurs when there is no record of the page having been referenced recently. A repage fault occurs when a page that is known to have been referenced recently is referenced again, and is not found in memory because the page has been replaced (and perhaps written to disk) since it was last accessed.

A perfect page-replacement policy would eliminate repage faults entirely (assuming adequate real memory) by always stealing frames from pages that are not going to be referenced again. Thus, the number of repage faults is an inverse measure of the effectiveness of the page-replacement algorithm in keeping frequently reused pages in memory, thereby reducing overall I/O demand and potentially improving system performance.

To classify a page fault as new or repage, the VMM maintains a repage history buffer that contains the page IDs of the N most recent page faults, where N is the number of frames that the memory can hold. For example, 512 MB memory requires a 128 KB repage history buffer. At page-in, if the page's ID is found in the repage history buffer, it is counted as a repage. Also, the VMM estimates the computational-memory repaging rate and the file-memory repaging rate separately by maintaining counts of repage faults for each type of memory. The repaging rates are multiplied by 0.9 each time the page-replacement algorithm runs, so that they reflect recent repaging activity more strongly than historical repaging activity.