Feb 17th, 2012
Minor page fault
Minor page fault
If the page is loaded in memory at the time the fault is generated, but is not marked in the memory management unit as being loaded in memory, then it is called a minor or soft page fault.
The page fault handler in the operating system merely needs to make the entry for that page in the memory management unit point to the page in memory and indicate that the page is loaded in memory; it does not need to read the page into memory. This could happen if the memory is shared by different programs and the page is already brought into memory for other programs. The page could also have been removed from a process’s Working Set, but not yet written to disk or erased, such as in operating systems that use Secondary Page Caching. For example, an application software may remove a page that does not need to be written to disk (if it has remained unchanged since it was last read from disk, for example) and place it on a Free Page List if the working set is deemed too large.
However, the page contents are not overwritten until the page is assigned elsewhere, meaning it is still available if it is referenced by the original process before being allocated. Since these faults do not involve disk latency, they are faster and less expensive than major page faults.
