Adobe Scan 06-Jan-2023.pdf demand paging document
- 1. What is Demand Paging? Main memory Secondary Memory Processl A B C D E Process 2 1-___ F _ _ f_ G ~ H r I I Swap IN Swap Out A B C D E F G H I J IGuru99.com I K L M N 0 p_ Q' '>,;, R s1 Ti lJ' V A demand paging mechanism is very much similar to a paging system with swapping where processes stored in the secondary memory and pages are loaded only on demand, not in advance. So, when a context switch occurs, the OS never copy any of the old program's pages from the disk or any of the new program's pages into the main memory. Instead, it will start executing the new program after loading the first page and fetches the program's pages, which are referenced.
- 2. During the program execution, if the program references a page that may not be available in the main memory because it was swapped, then the processor considers it as an invalid memory reference. That's because the page fault and transfers send control back from the program to the OS, which demands to store page back into the memory. Types of Page Replacement Methods Here, are some important Page replacement methods • FIFO • Optimal Algorithm • LRU Page Replacement FIFO Page Replacement FlFO (First-in-first-out) is a simple implementation method. In this method, memory selects the page for a replacement that has been in the virtual
- 3. address of the memory for the longest time. Features: • Whenever a new page loaded, the page recently comes in the memory is removed. So, it is easy to decide which page requires to be removed as its identification number is always at the FlFO stack. • The oldest page in the main memory is one that should be selected for replacement first. Optimal Algorithm The optimal page replacement method selects that page for a replacement for which the time to the next reference is the longest. Features: • Optimal algorithm results in the fewest number of page faults. This algorithm is difficult to implement.
- 4. • An optimal page-replacement algorithm method has the lowest page-fault rate of all algorithms. This algorithm exists and which should be called MIN or OPT. • Replace the page which unlike to use for a longer period of time. It only uses the time when a page needs to be used. LRU Page Replacement The full form of LRU is the Least Recently Used page. This method helps OS to find page usage over a short period of time. This algorithm should be implemented by associating a counter with an even- page.
- 5. How does itwork? • Page, which has not been used for the longest time in the main memory, is the one that will be selected for replacement. • Easy to implement, keep a list, replace pages by looking back into time. Features: • The LRU replacement method has the highest count. This counter is also called aging registers, which specify their age and how much their associated pages should also be referenced. • The page which hasn't been used for the longest time in the main memory is the one that should be selected for replacement. • It also keeps a list and replaces pages by looking back into time.
- 6. Fault rate Fault rate is a frequency with which a designed system or component fails. It is expressed in failures per unit of time. It is denoted by the Greek letter? (lambda). Advantages ofVirtual Memory Here, are pros/benefits of using Virtual Memory: • Virtual memory helps to gain speed when only a particular segment of the program is required for the execution of the program. • It is very helpful in implementing a multiprogramming environment. • It allows you to run more applications at once. • It helps you to fit many large programs into smaller programs. • Common data or code may be shared between memor .
- 7. • Process may become even larger than all of the physical memory. • Data/ code should be read from disk whenever required. • The code can be placed anywhere in physical memory without requiring relocation. • More processes should be maintained in the main memory, which increases the effective use of CPU. • Each page is stored on a disk until it is required after that, it will be removed. • It allows more applications to be run at the same time. • There is no specific limit on the degree of multiprogramming. • Large programs should be written, as virtual address space available is more compared to physical memory. Disadvantages ofVirtual Memory
- 8. • Applications may run slower if the system is using virtual memory. • Likely takes more time to switch between applications. • Offers lesser hard drive space for your use. • It reduces system stability. • It allows larger applications to run in systems that don't offer enough physical RAM alone to run them. • It doesn't offer the same performance as RAM. • It negatively affects the overall performance of a system. • Occupy the storage space, which may be used otherwise for long term data storage.
- 9. How does demand paging work? When the request comes from the CPU for accessing any page, the page table Is referred to find where that page Is in the main memory. Suppose that page Is found In the main memory, then well and good. If not found then page fault occurs. So page fault Is a situation In which the page CPU wants to access ls not In the main memory. What to do In that case? In that case, that particular page Is brought(swapped) In. But from where It ls swapped In? The answer Is secondary memory. Swapped-In means removing a program from a hard disk and putting It back Into the main memory or RAM. But If the page Is there Is main memory, It Is fetched from It. Else pages are loaded from secondary memory. Swapping Is also done In that case. We also have valid and Invalid bits. A valid-Invalid bit Is associated with each page table entry. So when the CPU tries to find the page in the main memory, it first checks the valid-Invalid bit corresponding to that page. 1. Valid bits-It Is sometimes denoted by v.lf the bit Is valid, then the page is both legal and in memory. 2. Invalid bits-'!' denotes an invalid bit. If the bit is invalid, then that means the page Is either not valid or the page Is valid but Is currently not on the disk.
- 10. Let's understand with an example. 1 2 3 4 5 6 7 8 9 10 11 Frame ~ M I p Physical memory Valid-Invalid bit I 5 V I I I 4 V i I 7 V Page table Steps of demand paging 2 3 4 5 6 7 8 I J K L M N 0 p Logical memory Whenever a request for any page comes, the page table Is consulted first. There are some frames which have Invalid bit means either they are not In the main memory then- 1. In case of a page fault, an interrupt Is generated. 2. After Interrupting the current process, the operating system blocks the process and moves to the blocked state. 3. The requested process is searched in the secondary memory. 4. Check for the free frame, but if do not find any free frame, then page replacement algorithms are followed. 5. The page is replaced(from secondary memory to main memory) with the help of the page replacement algorithms. 6. The CPU is informed about that update. And asked to go ahead with the execution, and the process returns to its ready state. The page table is updated accordingly.
- 11. Whenever any page is referred for the first time in the main memory. then that page will be found in the secondary memory. After that. it may or may not be present in the main memory depending upon the page replacement algorithm which will be covered later in this tutorial. What Is a Page Fault? If the referred page is not present in the main memory then there will be a miss and the concept is called Page miss or page fault. The CPU has to access the missed page from the secondary memory. If the number of page fault is very high then the effective access time of the system wi11 become very high.
- 12. What is Thrashing? If the number of page faults is equal to the number of referred pages or the number of page faults are so high so that the CPU remains busy in just reading the pages from the secondary memory then the effective access time will be the time taken by the CPU to read one word from the secondary memory and it will be so high. The concept is called thrashing. If the page fault rate is PF %, the time taken in getting a page from the secondary memory and again restarting is S (service time) and the memory access time is ma then the effective access time can be given as;
- 13. Performance Demand Paging of Demand paging can significantly affect the performance of a computer system. To see why, let's compute the effective access time for a demand-paged memory. For most computer systems, the memory-access time, denoted ma, ranges from 10 to 200 nanoseconds. As long as we have no page faults, the effective access time is equal to the memory access time. If however a page fault occurs, we must first read the relevant page from disk and then access the desired word. Let p be the probability of a page fault. We would expect p to be close to zero-that is, we would expect to have only a few page faults. The effective access time is then Effective Access Time= (1- p) *ma+ p * page fault time. • A
- 14. Effective Access Time= (1 - p) * ma + p * page fault time. To compute the effective access time, we must know how much time is needed to service a page fault.