Operating Systems (CS 340 D) Princess Nora University Faculty of Computer & Information Systems Computer science Department

(Chapter-9) Virtual Memory

Chapter 9: Virtual Memory 1.Background 2.Virtual Memory Implementation 3

OBJECTIVES :  To describe the benefits of a virtual memory system  To explain the concepts of demand paging  To explain the concepts of page-replacement algorithms 4

Background 5

In chapter of memory management,  various memory-management strategies used in computer systems were discussed and all of these strategies have the same goal: to keep many processes in memory simultaneously to allow multiprogramming.  However, they tend to require that an entire process be in memory before it can execute. Virtual memory is a technique that allows the execution of processes that are not completely in memory. Background 6

Advantages 1.programs can be larger than physical memory. 2.Map main memory into an extremely large, uniform array of storage, 3.separating logical memory as viewed by the user from physical memory. 4.This technique frees programmers from the concerns of memory- storage limitations. Background Virtual Memory That is Larger Than Physical Memory 7

 Advantages (cont) o Virtual memory also allows processes to share files easily and to implement shared memory. Background Shared Library Using Virtual Memory 8

 Disadvantage  Virtual memory is not easy to implement, however, and may substantially decrease performance if it is used carelessly Background 9

Virtual Memory Implementation Demand Paging 10

Virtual memory can be implemented via : 1. Demand paging 2. Demand segmentation 11

1-Demand Paging Consider how an executable program might be loaded from disk into memory.  One option is to load the entire program in physical memory at program execution time. However, a problem with this approach  is that we may not initially need the entire program in memory. Ex: Suppose a program starts with a list of available options from which the user is to select. Loading the entire program into memory results in loading the executable code for all options, regardless of whether an option is ultimately selected by the user or not.  An alternative strategy is to load pages only as they are needed during program excution. This technique is known as demand paging and is commonly used in virtual memory systems. 12

1-Demand Paging  A demand-paging system is similar to a paging system with swapping But rather than swapping the entire process into memory, we use a lazy swapper.  lazy swapper never swaps a page into memory unless that page will be needed.  Since we are now viewing a process as a sequence of pages, rather than as one large contiguous address space, use of the term swapper is technically incorrect.  A swapper manipulates entire processes, whereas a pager is concerned with the individual pages of a process. We thus use pager, rather than swapper, in connection with demand paging. 13

1-Demand Paging  Bring a page into memory only when it is needed  Less I/O needed  Less memory needed  Faster response  More users  Page is needed  reference to it  invalid reference  abort  not-in-memory  bring to memory 14

Transfer of a Paged Memory to Contiguous Disk Space 15

n With this scheme, we need some form of hardware support to distinguish between 1. the pages that are in memory 2. the pages that are on the disk. n The valid–invalid bit scheme can be used for this purpose.  “valid,” :  the associated page is both legal and in memory.  “invalid,”  the page either is not valid (that is, not in the logical address space of the process) or is valid but is currently on the disk. 1-Demand Paging (cont..) 16

Valid-Invalid Bit  With each page table entry a valid– invalid bit is associated (v  in-memory, i  not-in-memory)  Initially valid–invalid bit is set to i on all entries page table 1-Demand Paging(cont..) 17

Page Table When Some Pages Are Not in Main Memory 18

Page Fault  If there is a reference to a page, first reference to that page will trap to operating system: page fault 1. Operating system looks at another table to decide:  Invalid reference  abort  Just not in memory 2. Get empty frame 3. Swap page into frame 4. Reset tables 5. Set validation bit = v 6. Restart the instruction that caused the page fault 19

Steps in Handling a Page Fault 20

Performance of Demand Paging n Demand paging can significantly affect the performance of a computer system. why ? if 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. n Let p be the probability of a page fault (0 ≤ p ≤ 1). expect p to be close to only a few page faults. effective access time = (1 − p) × ma + p × page fault time. 21

Demand Paging Example  Memory access time = 200 nanoseconds  Average page-fault service time = 8 milliseconds  EAT = (1 – p) x p (8 milliseconds) = (1 – p x p x 8,000,000 = p x 7,999,800  If one access out of 1,000 causes a page fault, then EAT = 8.2 microseconds. This is a slowdown by a factor of 40!! 22

Page Replacement Algorithms 23

Page Replacement  If we increase our degree of multiprogramming, we are over- allocating memory.  Ex: If we run six processes, each of which is ten pages in size but actually uses only five pages, we have higher CPU utilization and throughput, with ten frames to spare.  It is possible, however, that each of these processes, for a particular data set, may suddenly try to use all ten of its pages, resulting in a need for sixty frames when only forty are available.  Further, consider that system memory is not used only for holding program pages. Buffers for I/O also consume a considerable amount of memory. This use can increase the strain on memory- placement algorithms. 24

Need For Page Replacement 25

Basic Page Replacement 1. Find the location of the desired page on disk 2. Find a free frame: - If there is a free frame, use it - If there is no free frame, use a page replacement algorithm to select a victim frame 3. Bring the desired page into the (newly) free frame; update the page and frame tables 4. Restart the process 26

Page Replacement 27

28 Page Replacement Algorithms 1. FIFO Page Replacement 2. Optimal Page Replacement 3. Least Recently Used (LRU) Algorithm

FIFO Page Replacement 29

n The simplest page-replacement algorithm is a first-in, first- out (FIFO) n FIFO  each page has memory load start time  or create FIFO queue to hold all pages in memory n When a page must be replaced, the oldest page is chosen. FIFO Page Replacement algorithm 30

FIFO Page Replacement 31

Optimal Page Replacement 32

Optimal Algorithm  Replace page that will not be used for longest period of time  4 frames example 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5  How do you know this?  Used for measuring how well your algorithm performs  Not easy to implement

Optimal Page Replacement 34

Least Recently Used (LRU) Page Replacement 35

Least Recently Used (LRU) Algorithm  Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5  Counter implementation o Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter o When a page needs to be changed, look at the counters to determine which are to change

LRU Page Replacement 37

LRU Algorithm (Cont.)  Stack implementation – keep a stack of page numbers in a double link form:  Page referenced:  move it to the top  requires 6 pointers to be changed  No search for replacement 38

Thank you End of Chapter 9 39