Virtual Memory ! Overview / Motivation ! Simple Approach: Overlays - PDF document

CPSC 410/611: Operating Systems Virtual Memory ! • Overview / Motivation ! • Simple Approach: Overlays ! • Locality of Reference ! • Demand Paging ! • Policies ! – Placement ! – Replacement ! – Allocation ! • Case Studies: Unix SystemV ! • Reading: Silberschatz, Chapter 9 ! Demand Paging ! 0 Process 2 ! page 1 AA table 0 AA 2 0 1 v 1 BB 3 1 9 v 2 CC 2 i 4 A AA BB CC DD 3 i 3 DD Process 1 ! 5 page 4 i A B 4 EE table 0 A 6 5 i 0 4 v C D E F 5 FF 6 i 1 B 7 1 i 7 i 6 GG 2 C 8 2 10 v F 7 HH 3 i logical 3 D 9 BB memory 4 i 4 E 10 C 5 8 v 5 F 11 6 i backing store 7 i 6 G 12 7 H physical memory logical memory • “Lazy Swapper”: only swap in pages that are needed. ! • Whenever CPU tries to access a page that is not swapped in, a page e fault occurs. ! fa Virtual Memory 1

CPSC 410/611: Operating Systems Mechanics of a Memory Reference ! OS reference 1 v CPU page table 3 2 complete access reference memory some frame physical memory Mechanics of a Page Fault ! page is on backing store 3 OS exception! 2 reference 1 v i CPU 6 restart instruction page table 4 frame free 5 frame used load page update page table physical memory Virtual Memory 2

CPSC 410/611: Operating Systems Locality of Reference ! • Page faults are expensive! ! • Th Thrash ashing ing: Process spends most of the time paging in and out instead of executing code. ! • Most programs display a pattern of behavior called the principle e of f locality of f refer eferen ence. ! Locality of f Referen ence ! A program that references a location n at some point in time is likely to reference the same location n and locations in the immediate vicinity of n in the near future. Memory Access Trace ! Virtual Memory 3

CPSC 410/611: Operating Systems Architectural Considerations ! • Must be able to restart any instruction after a page fault. ! • e.g. ! ADD A,B TO C • What about operations that modify several locations in memory? ! – e.g. block copy operations? ! • What about operations with side effects? ! – e.g. PDP-11, 80x86 auto-decrement, auto-increment operations? ! – Add mechanism for OS to “undo” instructions. ! Performance of Demand Paging ! • Effective Memory Access time ema ema: ! ema = (1-p) * ma ema ma + p * “p “page e fa fault time” me” ! • where ! – p = probability of a page fault ! – ma = memory access time ! • Operations during Page Fault: ! page is on backing store OS 1. service page fault trap interrupt reference 2. swap in page i CPU 3. restart process restart instruction page table free frame load page update page table Virtual Memory 4

CPSC 410/611: Operating Systems OS Policies for Virtual Memory ! • Fet etch Po Policy ! – How/when to get pages into physical memory. ! – demand paging vs . prepaging. ! • Pl Placemen ement Po Policy ! – Where in physical memory to put pages. ! – Only relevant in NUMA machines. ! • Rep eplacemen ement Po Policy ! – Physical memory is full. Which frame to page out? ! • Res esiden ent Set et Managemen ement Po Policy ! – How many frames to allocate to process? ! – Replace someone elses frame? ! • Clea eaning Po Policy ! – When to write a modified page to disk. ! • Load C Load Contr ontrol ol ! Configuring the Win2k Memory Manager ! • Registry Values that Affect the Memory Manager: ! ClearPageFileAtShutdown DisablePagingExecutive IoPageLockLimit LargePageMinimum LargeSystemCache NonPagedPoolQuota NonPagedPoolSize PagedPoolQuota PagedPoolSize SystemPages Virtual Memory 5

CPSC 410/611: Operating Systems Page Replacement ! • Virtual memory allows higher degrees of multiprogramming by over-allocating memory. ! 256kB 256kB 256kB 256kB 256kB 1024kB 0 K 0 2 v 1 L 1 4 v address ! 0 N 2 i 2 M space 1 ! 1 3 C 0 v 3 N M 2 K 3 B A 0 0 3 v A 1 i 4 L 1 B address ! 2 1 v 5 D 2 C space 2 ! 3 5 v 3 D frame table ! paging store ! page tables ! Mechanics of Page Replacement ! • Invoked whenever no free frame can be found. ! swap select out victim victim frame invalidate page vict.frame f v i d 3 2 entry for 1 victim page victim update nil f v i c 5 4 swap entry for in new new page page page table backing store physical memory • Problem: Need two page transfers Solution: Dirty bit . Virtual Memory 6

CPSC 410/611: Operating Systems Page Replacement Algorithms ! • Objective: Minimize page fault rate. ! • Why bother? ! i a • Example ! for(int i=0; i<10; i++) { x a = x * a; } • Evaluation: Sequence of memory references: refer eferen ence e string. ! FIFO Page Replacement ! enter frame in FIFO queue FIFO queue select 1 victim 6 swap out invalidate victim f v/i 3 2 entry for page victim page victim update nil/f i/v 5 4 swap entry for in new new page page page table backing store physical memory Virtual Memory 7

CPSC 410/611: Operating Systems FIFO Page Replacement (cont.) ! • Example: ! time 1 2 3 4 5 6 7 8 9 10 reference c a d b e b a b c d string frames a a a a a e e e e e d b b b b b b b a a a a c c c c c c c c b b b d d d d d d d d d c c ! ! ! ! ! • Advantage: simplicity • Disadvantage: Assumes that pages residing the longest in memory are the least likely to be referenced in the future (does not exploit principle of locality ). Optimal Replacement Algorithm ! • Algorithm with lowest page fault rate of all algorithms: ! Replace that page which will not be used ! for the longest period of time (in the future). ! • Example: ! time 1 2 3 4 5 6 7 8 9 10 reference c a d b e b a b c d string frames a a a a a a a a a a d b b b b b b b b b b b c c c c c c c c c c c d d d d d e e e e e e ! ! Virtual Memory 8

CPSC 410/611: Operating Systems Approximation to Optimal: LRU ! • Lea east Rec ecen ently Used ed: replace the page that has not been accessed for longest period of time (in the past). ! • Example: ! time 1 2 3 4 5 6 7 8 9 10 reference c a d b e b a b c d string frames a a a a a a a a a a a b b b b b b b b b b b c c c c c e e e e e d d d d d d d d d d c c ! ! ! LRU: Implementation ! • Need to keep chronological history of page references; need to be reordered upon each reference. ! • St Stack ack: ! stack ? c a d b e b a b c d ? ? c a d b e b a b c ? ? ? c a d d e e a b ? ? ? ? c a a d d e a • Cap Capacit acitors rs: Associate a capacitor with each memory frame. Capacitor is charged with every reference to the frame. The subsequent exponential decay of the charge can be directly converted into a time interval . ! • Aging reg egister ers: Associate aging register of n bits (R n-1 , ..., R 0 ) with each frame in memory. Set R n-1 to 1 for each reference. Periodically shift registers to the right. ! Virtual Memory 9

CPSC 410/611: Operating Systems Approximation to LRU: Clock Algorithm ! • Associate a use_bit with every frame in memory. ! – Upon each reference, set use_bit to 1. ! – Keep a pointer to first “victim candidate” page. ! – To select victim: If current frame’s use_bit is 0, select frame and increment pointer. Otherwise delete use_bit and increment pointer. ! time 1 2 3 4 5 6 7 8 9 10 reference c a d b e b a b c d string frames a/1 a/1 a/1 a/1 a/1 e/1 e/1 e/1 e/1 e/1 d/1 b/1 b/1 b/1 b/1 b/1 b/0 b/1 b/0 b/1 b/1 b/0 c/1 c/1 c/1 c/1 c/1 c/0 c/0 a/1 a/1 a/1 a/0 d/1 d/1 d/1 d/1 d/1 d/0 d/0 d/0 d/0 c/1 c/0 ! ! ! ! Improvement on Clock Algorithm ! (Second Chance Algorithm) ! • Consider read/write activity of page: dirty_bit (or modify_bit ) ! • Algorithm same as clock algorithm, except that we scan for frame with both use_bit and dirty_bit equal to 0. ! • Each time the pointer advances, the use_bit and dirty_bit are updated as follows: ! ud ud ! ud ud ! ud ud ! ud ! ud before ! be 11 ! 10 ! 01 ! 00 ! af after ! 01 ! 00 ! 00* ! (select) ! • Called Second Chance because a frame that has been written to is not removed until two full scans of the list later. ! • Note: Stallings describes a slightly different algorithm! ! Virtual Memory 10

CPSC 410/611: Operating Systems Improved Clock (cont) ! • Example: ! time 1 2 3 4 5 6 7 8 9 10 reference c a w d b w e b a w b c d string frames a/10 a/10 a/11 a/11 a/11 a/00 * a/00 * a/11 a/11 a/11 b/10 b/10 b/10 b/10 b/11 b/00 * b/10 * b/10 * b/10 * b/10 * c/10 c/10 c/10 c/10 c/10 e/10 e/10 e/10 e/10 e/10 d/10 d/10 d/10 d/10 d/10 d/00 d/00 d/00 d/00 c/10 ! ! ! The Macintosh VM Scheme (see Stallings) ! • Uses use_ e_bit and mo modify fy_bit. ! • Step ep 1: Scan the frame buffer. Select first frame with use_ e_bit and mo modify fy_bit cleared. ! • Step ep 2: If Step 1 fails, scan frame buffer for frame with use_ e_bit cleared and mo modify fy_bit set. During scan, clear use_ e_bit on each bypassed frame. ! • Now all use_ e_bit’s are cleared. Repeat Step 1 and, if necessary, Step 2. ! Virtual Memory 11