What is a Page Fault?
A page fault is an interrupt that occurs when a program attempts to access a page of memory that is not currently in physical memory (RAM). When this happens, the operating system must retrieve the required page from secondary storage (such as a hard drive) and load it into RAM before allowing the program to continue executing. Page faults are a normal part of virtual memory management and are essential for allowing programs to use more memory than is physically available.
Causes of Page Faults
There are several reasons why a page fault may occur. One common cause is when a program tries to access a page of memory that has been swapped out to disk to make room for other pages. Another cause is when a program attempts to access memory that has not yet been allocated. Page faults can also occur when a program accesses memory that is protected by the operating system, such as kernel memory.
Impact of Page Faults on System Performance
Page faults can have a significant impact on system performance, as they require the operating system to perform disk I/O operations to retrieve the required page from secondary storage. This can result in delays and slowdowns in program execution, as the program must wait for the required page to be loaded into memory before it can continue executing. Excessive page faults can lead to thrashing, where the system spends more time swapping pages in and out of memory than actually executing programs.
How to Handle Page Faults
To handle page faults, the operating system typically uses a technique called demand paging. With demand paging, the operating system only loads pages into memory when they are actually needed by a program. When a page fault occurs, the operating system identifies the required page, retrieves it from secondary storage, and loads it into RAM. The program is then allowed to continue executing as if nothing had happened.
Difference between Hard Page Faults and Soft Page Faults
There are two types of page faults: hard page faults and soft page faults. Hard page faults occur when a program tries to access a page of memory that is not currently in physical memory and must be retrieved from secondary storage. Soft page faults, on the other hand, occur when a program tries to access a page of memory that is not currently in physical memory but is already present in secondary storage. Soft page faults are much less costly in terms of performance, as the required page is already available and just needs to be loaded into memory.
Common Misconceptions about Page Faults
One common misconception about page faults is that they are always a sign of a poorly written program or a system that is running out of memory. While excessive page faults can indicate performance issues, they are a normal part of virtual memory management and are essential for allowing programs to use more memory than is physically available. Another misconception is that all page faults are detrimental to system performance. In reality, soft page faults are relatively harmless and do not have a significant impact on performance.
