Concept of page fault in memory management

Page fault is also known as the pf or #pf and can be thought of as a trap that the hardware raises for the software whenever the program tries to access a page that has been mapped to an address space in the virtual memory but has not been loaded in the main memory. 

In most cases, the page fault is handled by the operating system by helping in accessing the required page at an address space in the main or the physical memory or sometimes by terminating the program if it makes an illegal attempt to the access the page.

- Memory management unit is the hardware that is responsible for detecting the page faults and is located in the processor. 

- The software that helps the memory management unit in handling the page faults is the exception handling software and is seen as a part of the OS. 

- ‘Page fault’ is not always an error.

- These are often seen as a necessary role player in increasing the memory. 

- This can be made available to the software applications that makes use of the virtual memory of  the operating system for execution.

- Hard fault is the term used by the Microsoft instead of page fault in the resource monitor’s latest versions.

Classification of Page Faults

Page faults can be classified in to three categories namely:

1. Minor: 

- This type of fault is also called the soft page fault and is said to occur when the loading of the page in to the memory takes place at the time of the fault generation, but the memory management unit does not mark it as being loaded in the physical memory. 

- A page fault handler is included in the operating system whose duty is to make an entry for the page that is pointed to by the memory management unit. 

- After making the entry for it, its task is to give an indication that the page has been loaded. 

- However, it is not necessary that the page must be read in to the memory. 

- This is possible if the different programs share the memory and the page has been loaded in to the memory for the various applications. 

- In the operating systems that apply the technique of secondary page caching, the page can be removed from the working set of the process but not deleted or written to the disk.

2. Major: 

- Major fault is actually a fault that many operating systems use for increasing the memory for the program that must be available as demanded by the program. 

- The loading of the parts of the program is delayed by the operating system from the disk until an attempt is made by the program for using it and generating the page fault.

- In this case either a non – free page or a page in the memory has to be found by the page fault handler. 

- When the page is available, the data from it can be read by the operating system to the new page in the main memory, thus easily making an entry for the required page.

3. Invalid: 

- This type of fault occurs whenever a reference is made to an address that does not exists in the virtual address space and therefore it has no page corresponding to it in the memory. 

- Then the code by which the reference was made has to be terminated by the page fault handler and give an indication regarding the invalid reference. 

Define the Virtual Memory technique?

Modern operating systems come with multitasking kernels. These multitasking kernels often run in to the problems related to memory management. Physical memory does not suffice for them to execute the tasks assigned to them because of being fragmented. So they have to take some additional from the secondary memory. But they cannot use this memory directly. Virtual memory offers a solution to this problem. 

What is Virtual Memory technique?

- Using this technique makes the fragmented main memory available to the kernels as a contiguous main memory. 

- Since it is really not the main memory but just appears to be, it has been named as the virtual memory and this technique is called the virtual memory technique. 

- Since, it helps in managing the memory, it is essential a memory management technique. 

- The main storage gets fragmented because of many programming and processing problems. 

- The main memory available to the processes and the tasks is virtualized by the virtual memory technique and then it appears to the process as a contiguous memory location. 

- This memory is a global address space. 

- Virtual address spaces such as these are managed by the operating system. 

- The real memory is assigned to the virtual memory by the operating system itself. 

- The virtual addresses of the allocated virtual address spaces are translated in to the physical addresses automatically by the CPU. 

- It achieves this with the help of some memory management hardware specially designed for this purpose. 

- The processes continue to execute uninterrupted as long as this hardware properly translates the virtual addresses in to real memory addresses properly. 

- If it fails in doing so at any point of time, the execution comes to a halt and the control is transferred to the operating system. 

- The duty of the operating system now is to move the requested memory page to the main memory from the backing store. 

- Once done with this, it then returns the control again to the process that was interrupted. 

- It greatly simplifies the whole execution process. 

- Even if the application would require more data or code that would fit in real memory, it does not have to be moved to and fro between the backing store and the real memory. 

- Furthermore, this technique also offers protection to the processes that are provided distinct address spaces by the isolation of the memory allocate to them from other tasks.

- Application programming has been made a lot easier with the help of the virtual memory technique since it hides the fragmentation defects of the real memory. 

- The burden of memory hierarchy management is delegated to the kernel which eliminates the need for the explicit handling of the overlays by the program. 

- Thus each process can execute in an address space that is dedicated to it. 

- The need for relocating the code of the program is obviated along with using relative addressing for accessing the memory. 

- The concept of virtual memory was generalized and eventually named as memory virtualization. 

- Gradually, the virtual memory has become an inseparable part of the architecture of the modern computers. 

- For implementing it, dedicated hardware support is absolutely necessary. 

- This hardware is built in to the CPU in some sort of memory management hardware. - If required for boosting the performance of the virtual memory, some virtual machines and emulators may employ some additional hardware support. 

- The older mainframe computers did not have any support for the virtual memory concept. 

- In virtual memory technique, each program can solely access the virtual memory.

Introduction to Swapping

When the physical memory in the system runs out and a process needs to bring a page into memory then the operating system must decide what to do. It must fairly share the physical pages in the system between the processes running in the system, therefore it may need to remove one or more pages from the system to make room for the new page to be brought into memory. How virtual pages are selected for removal from physical memory affects the efficiency of the system.

Swapping is the one of the efficient regular and authentic approach of memory management. It is the process of swapping of higher priority process on the lower priority process.
Advantages of swapping are as follows :-
1. Higher degree of multi-programming.
2. Dynamic relocation.
3. Greater memory utilization.
4. Priority based scheduling.
5. Less wastage of CPU time.
6. Higher performance.

If the page to be removed came from an image or data file and has not been written to then the page does not need to be saved. Instead it can be discarded and if the process needs that page again it can be brought back into memory from the image or data file again. However, if the page has been written to then the operating system must preserve the contents of that page so that it can be accessed at a later time.

Linux uses a page aging technique to fairly choose pages which might be removed from the system. This scheme involves every page in the system having an age which changes as the page is accessed. The more that a page is accessed, the younger it is; the less that it is accessed the older it becomes. Old pages are good candidates for swapping.