Chapter 6 Virtual Memory Virtual Memory Background Demand

Chapter 6 Virtual Memory

Virtual Memory • • Background Demand Paging Performance of Demand Paging Page Replacement Algorithms Allocation of Frames Thrashing 2

Background • Virtual memory – separation of user logical memory from physical memory. – Only part of the program needs to be in memory for execution. – Logical address space can therefore be much larger than physical address space. – Allows address spaces to be shared by several processes. – Allows for more efficient process creation. • Virtual memory can be implemented via: – Demand paging – Demand segmentation 3

Virtual Memory That is Larger Than Physical Memory 4

Demand Paging • Demand paging system is similar to a paging system with swapping processes reside on secondary memory (Usually a disk). • When we want execute a process, we swap it into memory. Rather than swapping the entire process into memory. • A lazy swapper never swaps a page into memory unless that page will be needed. • A swapper manipulates entire processes, whereas a pager is concerned with the individual pages of a process. 5

Demand Paging (Contd…) • The term pager is use frequently rather than swapper, in connection with demand paging. • Benefits of Demand Paging; – Less I/O needed – Less memory needed – Faster response – More users 6

Transfer of a Paged Memory to Contiguous Disk Space 7

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. • While the process executes and accesses pages that are memory resident, execution proceeds normally. • During address translation, if valid–invalid bit in page table entry is i = Page fault. 8

Valid-Invalid Bit Frame # Note here that the page table requires a "resident" bit showing that page is/isn't in memory. (Book uses "valid" bit to indicate residency. An "invalid" page is that way because a legal page isn't resident or because the address is illegal. It makes more sense to have two bits one indicating that the page is legal (valid) and a second to show that the page is in memory. valid-invalid bit 1 1 0 page table 0 Frame # Reside nt 1 0 validinvalid bit 1 0 9

Page Table When Some Pages Are Not in Main Memory 10

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 11

Page Fault (Cont. ) • Restart instruction – block move – auto increment/decrement location 12

Steps in Handling a Page Fault 13

What happens if there is no free frame? • Page replacement – find some page in memory, but not really in use, swap it out. – algorithm – performance – want an algorithm which will result in minimum number of page faults. • Same page may be brought into memory several times. 14

Performance of Demand Paging • Page Fault Rate 0 p 1. 0 – if p = 0 no page faults – if p = 1, every reference is a fault EAT = (1 -p) x ma + p x page fault time. • Effective Access Time (EAT) EAT = (1 – p) x memory access + p (page fault overhead + [swap page out ] + swap page in + restart overhead) 15

Demand Paging Example • If we take an average page fault service time of 25 milliseconds and a memory access time of 100 nanoseconds, then the effective access time in nanoseconds is: – EAT = (1 -p) x (100) + p (25 miliseconds) = (1 -p) x 100 + p x 25, 000 = 100 + 24, 999, 900 x p. 16

Process Creation • Virtual memory allows other benefits during process creation: - Copy-on-Write - Memory-Mapped Files 17

Copy-on-Write • Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory. If either process modifies a shared page, only then is the page copied. • COW allows more efficient process creation as only modified pages are copied. • Free pages are allocated from a pool of zeroed-out pages. 18

Memory-Mapped Files • Memory-mapped file I/O allows file I/O to be treated as routine memory access by mapping a disk block to a page in memory. • A file is initially read using demand paging. A page-sized portion of the file is read from the file system into a physical page. Subsequent reads/writes to/from the file are treated as ordinary memory accesses. • Simplifies file access by treating file I/O through memory rather than read() write() system calls. • Also allows several processes to map the same file allowing the pages in memory to be shared. 19

Memory Mapped Files 20

Hardware Support for Demand Paging • Page Table – Has the ability to mark an entry invalid through a validinvalid bit or special value of protection bits. • Secondary memory – Holds those pages that are not present in main memory. – Usually a high speed disk. – Known as swap device, and the section of disk used for this purpose is known as swap space or backing store. 21

Page Replacement • Prevent over-allocation of memory by modifying page-fault service routine to include page replacement. • Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk. • Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory. 22

Need For Page Replacement 23

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. - write the victim page to the disk; change the page and frame tables accordingly. 3. Read the desired page into the (newly) free frame. Update the page and frame tables. 4. Restart the user process. 24

Page Replacement 25

Page Replacement Algorithms • Many different page-replacement algorithms, every Os has its own unique replacement scheme. • Want lowest page-fault rate. • The string of memory reference is called reference string. • Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string. 26

Graph of Page Faults Versus The Number of Frames 27

FIFO Page Replacement 28

First-In-First-Out (FIFO) Algorithm • Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 • 3 frames (3 pages can be in memory at a time per process) • 4 frames 1 1 4 5 2 2 1 3 3 3 2 4 1 1 5 4 2 2 1 5 3 3 2 4 4 3 9 page faults 10 page faults • FIFO Replacement – Belady’s Anomaly – more frames less page faults 29

FIFO Illustrating Belady’s Anamoly 30

Optimal Algorithm • Replace the page that will not be used for longest period of time. 31

Least Recently Used (LRU) Algorithm • Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 1 5 2 3 5 4 3 4 • Counter implementation – Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter. – When a page needs to be changed, look at the counters to determine which are to change. 32

LRU Page Replacement 33

LRU Algorithm (Cont. ) • Time stamp on pages - records when the page is last touched. • Stack implementation – keep a stack of page numbers in a double link form: • Page stack - pull out touched page and put on top 34

Use Of A Stack to Record The Most Recent Page References 35

LRU Approximation Algorithms • Additional-Reference-bits Algorithm – With each page associate a bit, initially = 0. – When page is referenced bit set to 1. – Replace the one which is 0 (if one exists). We do not know the order, however. 36

LRU Approximation Algorithms • Second-chance Algorithm – Pick a page based on FIFO. – Need reference bit. – Clock replacement. – If page to be replaced (in clock order) has reference bit = 1. then: • set reference bit 0. • leave page in memory. • replace next page (in clock order), subject to same rules. 37

Second-Chance (clock) Page-Replacement Algorithm 38

Counting Algorithms • Keep a counter of the number of references that have been made to each page. • LFU Algorithm: The least frequently used page-replacement algorithm replaces page with smallest count. • MFU Algorithm: Most frequently used based on the argument that the page with the smallest count was probably just brought in and has yet to be used. 39

Allocation of Frames • Each process needs minimum number of pages. • Example: IBM 370 – 6 pages to handle SS MOVE instruction: – instruction is 6 bytes, might span 2 pages. – 2 pages to handle from. – 2 pages to handle to. • Two major allocation schemes. – fixed allocation – priority allocation 40

Allocation Algorithms • Equal allocation – e. g. , if 100 frames and 5 processes, give each 20 pages. • Easiest way to split m frames among n processes is to give everyone an equal share, m / n frames. • If there are 93 frames and five processes, each will get 18 frames. • Leftover three frames could be used as a free-frame buffer pool. This scheme is called equal allocation. 41

Allocation Algorithms • Proportional allocation – Allocate available memory according to the size of process. • Let the size of the virtual memory 42

Priority Allocation • Use a proportional allocation scheme using priorities rather than size. • If process Pi generates a page fault, – select for replacement one of its frames. – select for replacement a frame from a process with lower priority number. 43

Global vs. Local Allocation • Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another. • Local replacement – each process selects from only its own set of allocated frames. 44

Thrashing • Thrashing = a process is busy swapping pages in and out. • The number of processes that are in the memory determines the multiprogramming (MP) level. The effectiveness of virtual memory management is closely related to the MP level. • When there are just a few processes in memory, the possibility of processes being blocked and thus swapped out is higher. 45

Thrashing (Contd…) • When there are far too many processes (i. e. , memory is over committed), the resident set of each process is smaller. This leads to higher page fault frequency, causing the system to exhibit a behavior known as thrashing. • In other words, the system is spending its time moving pages in and out of memory and hardly doing anything useful. • A process is thrashing if it is spending more time paging than executing. 46

Thrashing • Why does paging work? Locality model – Process migrates from one locality to another. – Localities may overlap. • Why does thrashing occur? 47

Thrashing 48

Working-Set Model • = working-set window = a fixed number of page references Example: 10, 000 instruction • WSSi (working set of Process Pi) = total number of pages referenced in the most recent (varies in time) – if too small will not encompass entire locality. – if too large will encompass several localities. – if = will encompass entire program. • D = WSSi total demand frames • if D > m Thrashing • Policy if D > m, then suspend one of the processes. 49

Working-set model 50

Keeping Track of the Working Set • Approximate with interval timer + a reference bit • Example: = 10, 000 – Timer interrupts after every 5000 time units. – Keep in memory 2 bits for each page. – Whenever a timer interrupts copy and sets the values of all reference bits to 0. – If one of the bits in memory = 1 page in working set. • Why is this not completely accurate? • Improvement = 10 bits and interrupt every 1000 time units. 51

Page-Fault Frequency Scheme • Establish “acceptable” page-fault rate. – If actual rate too low, process loses frame. – If actual rate too high, process gains frame. 52

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