ELEC 377 Operating Systems. Week 6 Class 3

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1 ELEC 377 Operatng Systems Week 6 Class 3

2 Last Class Memory Management Memory Pagng Pagng Structure ELEC 377 Operatng Systems

3 Today Pagng Szes Vrtual Memory Concept Demand Pagng ELEC 377 Operatng Systems

4 Page Table address generated by CPU s dvded nto two parts page number(p) - ndex nto page table page offset(d) locaton wthn the page logcal address physcal address CPU p d f d { f ELEC 377 Operatng Systems

5 Memory s mapped by Page Tables Process1 Logcal Address Space Process2 Logcal Address Space ELEC 377 Operatng Systems pagef page1 page pagef page1 page Memory (Physcal Address) P2pgF P1pgF P2pg P1pg1 P1pg P2pg1 Frame N Frame 2 Frame 1 Frame

6 Page Tables Each process can have ts own logcal address space excepton: Mac OS 7 had a sngle page table for all processes (no memory protecton between processes) Thus each process has ts own page table Thus each process has ts own mappng to physcal memory ELEC 377 Operatng Systems

7 Pagng frame table - keeps track of allocated and free frames I/O operatons have to know memory layout page table has to be swtched durng context swtch - ncrease n context swtch tme page tables are large, and usually kept n man memory - page table base regster, length regster Extra memory traffc Must frst use page number to fnd frame number, then access nstructon or data n memory Smple lnear table adds one memory access for each data access. ELEC 377 Operatng Systems

8 Pagng - TLB Want to mnmze extra memory traffc of page tables Small cache nsde of MMU TLB - translaton look-asde buffer assocatve memory Page # Frame # ELEC 377 Operatng Systems

9 TLB If address not n TLB, called a mss Requres one extra memory access cycle n lnear page model (one for table, one for memory) new address added to TLB performance very senstve to ht-rato Some entres n TLB are not modfable (kernel addresses) Some TLBs support multple processes by addng process IDs to the TLB. Ths allows more than one process n TLB at a tme. otherwse TLB must be flushed on each context swtch ELEC 377 Operatng Systems

10 Memory Protecton Snce memory s no longer contguous, base + lmt not suffcent for protecton could stll be used on logcal address sde memory belongng to other processes not n page table and thus not vsble!! OS may be n page table for quck access add vald-nvald bt to page table. Process can only access vald pages ELEC 377 Operatng Systems

11 Page Table Structure The page table can become very large. 32 bt address space, 12 bt page (4K) gve 4 Megabyte page table (allocated contguously) larger than some programs!! Want to reduce resources requred by page tables break up page table so not contguous reduce sze of page table not all process space may be vald!! Stack Data Code ELEC 377 Operatng Systems

12 Page Table Mddle of table s not used (Does not contan references to physcal memory frames) page the page table? logcal address physcal address CPU p d f d ELEC 377 Operatng Systems

13 Herarchcal Tables Multple Tables (forward-mapped page table) page number s splt nto one or more sectons Frst table gves address of next table endng at physcal memory *(*(*(T1+p1) + p2)+d) p1 p2 d f d { { ELEC 377 Operatng Systems

14 Herarchcal Tables Can be more than two levels (Pentum has 4 levels) Cost of mss(assume each lookup s one access) one extra access for each extra level of table p1 p2 p3 d f d { { {

15 Hashed Page Tables Store page entres n a hash table, hashed by page number Hash table has collsons (more than one page number mght hash to a gven locaton) Store as a lnked lst (varable extra cost!!) p d H f d p f ELEC 377 Operatng Systems

16 Inverted Pages prevous schemes have separate table(s) per process Just as the TLB has a combned representaton, so do page tables tables have empty space Inverted tables have one entry for each frame Stores process number and page number correspondng to the frame Search table for <pd,page#> hash table can shorten search tme Need to extend to handle shared memory ELEC 377 Operatng Systems

17 Page Structures - Summary Three ways of reducng the memory requrements of the page tables All of them ncrease the cost of convertng a logcal address to a physcal address TLB absorbs much of the cost Increase the cost of a TLB mss Effectveness of TLB s more mportant ELEC 377 Operatng Systems

18 Demand Pagng All pages of the process are swapped out swap each page n as t s needed lazy swapper f page s never referenced -> never loaded!! pager vs swapper Hardware needs to know whch pages are n memory ELEC 377 Operatng Systems

19 Logcal vs Physcal Address Sze Pages and Frames must be the same sze d same n both physcal and logcal spaces But there may be more frames than pages or more pages than frames not necessary that p = f CPU p d f d f ELEC 377 Operatng Systems

20 Logcal vs Physcal Address Sze d = 12, logcal = 32, physcal = 4 page/frame sze = 4K #pages = 2 = 124K pages = 1M pages - page table sze (1 word per entry) = 4M #frames = 28 = 256M frames d = 1, logcal = 32, physcal = 24 page/frame sze = 1K #pages = 22 = 4M pages #frames = 14 = 16K frames - total physcal memory = 16M ELEC 377 Operatng Systems

21 Logcal vs Physcal Address Sze how can the physcal address space be less than the logcal address space? Not all of the logcal address space s used 32 bts = 4Gb A process smple process may only have 1 pages (not ncludng shared lbrares) = 8K mddle s not mapped Stack Data Code ELEC 377 Operatng Systems

22 Example Translaton Logcal = 32, Physcal = 24, Page Sze = 4K CPU 2AF53 3F7 3EC 3F7 3EC th entry n page table ELEC 377 Operatng Systems

23 Herarchcal Tables Example Program wth 222k of Code and Data, 3K of Stack 32 bt address space, 1k pages, p1 s 12 bts - how much space s taken by the page tables?? - assume 4 bytes for each entry of p1 table and 4 bytes for each entry of each p2 table. p2 = 1 bts how many pages of code and data? -> 222 pages how many pages for stack? -> 3 pages How many p2 tables? -> 2 tables how many p1 tables? -> 1 table sze of p1 table = 2 12 * 4 = 2 14 sze of each p2 table = 2 1 * 4 = 2 12 Total table space = = =16k+8k=24k ELEC 377 Operatng Systems

24 Page Table Example Stack - 3 K Stack * * *111111* Code+Data 222K *111111* ** Data Code ELEC 377 Operatng Systems

25 Page Table Example entres Empty Stack Empty ELEC 377 Operatng Systems Empty 222 entres Data Code

26 Sharng Pages Shared Lbrares Multple nvocatons of a gven program (e.g. shell, edtor) Contguous memory allocaton makes sharng dffcult Shared code must be reentrant Must not modfy tself Must also be poston ndependent Most data s not shared - however IPC Shared Segments (lab3) now easy!! Page table entres for shared code and data n each process pont to the common frames Page table entres for prvate data pont to dfferent frames ELEC 377 Operatng Systems

27 Sharng Pages-Two Procs same Prog P ELEC 377 Operatng Systems P1

28 Sharng Pages -Two Progs, 1 Shared P ELEC 377 Operatng Systems P1

29 Today Pagng Szes Vrtual Memory <<<<<<< Concept Demand Pagng ELEC 377 Operatng Systems

30 Vrtual Memory Separaton of Logcal Address from Physcal Address Each process has t s own vrtual address space Thnks t has the machne to tself Not all of the program need be n memory at one tme dynamc loadng, overlays only map the needed pages to frames n memory store unused pages on the dsk (smlar to swap) more effcent to start a new process Logcal address space can be bgger than physcal address space!! process can more effectvely share avalable memory resources ELEC 377 Operatng Systems

31 Demand Pagng All pages of the process are swapped out swap each page n as t s needed lazy swapper f page s never referenced -> never loaded!! pager vs swapper localty of reference Hardware needs to know whch pages are n memory nvald bt -> now means not n memory when a process starts, all pages are nvald generate a page-fault when the page s not n memory. ELEC 377 Operatng Systems

32 Demand Pagng Vrtual Address Page Table Memory Backng Store Note: not all pages are vald! ELEC 377 Operatng Systems

33 Demand Pagng Vrtual Address Page Table Memory Backng Store Note: not all pages are vald! ELEC 377 Operatng Systems

34 Demand Pagng Vrtual Address Page Table Memory Backng Store ELEC 377 Operatng Systems

35 Demand Pagng Vrtual Address Page Table Memory Backng Store ELEC 377 Operatng Systems

36 Demand Pagng Vrtual Address Page Table Memory Backng Store ELEC 377 Operatng Systems Page

37 Demand Pagng Vrtual Address Page Table Memory Backng Store ELEC 377 Operatng Systems Page

38 Demand Pagng Vrtual Address Page Table Memory Backng Store ELEC 377 Operatng Systems Page

39 Demand Pagng Vrtual Address Page Table 4 v Memory Backng Store ELEC 377 Operatng Systems Page

40 Demand Pagng Vrtual Address Page Table 4 v Memory Backng Store ELEC 377 Operatng Systems

41 Demand Pagng Vrtual Address Page Table 4 v Memory Backng Store ELEC 377 Operatng Systems

42 Demand Pagng Vrtual Address Page Table Memory Backng Store v ELEC 377 Operatng Systems Page Fault

43 Demand Pagng Vrtual Address Page Table Memory Backng Store v ELEC 377 Operatng Systems Page Fault

44 Demand Pagng Vrtual Address Page Table Memory Backng Store v ELEC 377 Operatng Systems Page Fault Abort!!

45 Demand Pagng Pure demand pagng (no pages to start) In practce, we know that at least the frst code page wll be used Pages that are n memory are called memory resdent Restartng the nstructon may cause more page faults some nstructons can access a lot of memory multple ndrecton addressng modes memory copy (strng move nstructon on x86) text talks about restartng nstructons undong sde effects of nstructons some CPUs can suspend the nstructon save nternal regsters on stack ELEC 377 Operatng Systems

46 Copy on Wrte Unx fork() duplcate chld process - duplcate code - duplcate data - duplcate stack We already talked about sharng code pages, and separate data and stack pages On a fork(), we could share all pages only copy a page when t s modfed requres support from hardware (MMU) more effcent use of memory don t waste cpu cycles copyng memory ELEC 377 Operatng Systems

47 Copy on Wrte Process Memory ELEC 377 Operatng Systems

48 Copy on Wrte after fork() Process Memory Process ELEC 377 Operatng Systems

49 Copy on Wrte after P2 changes 2 Process Memory 2a Process ELEC 377 Operatng Systems

50 Memory Mapped Fles Tradtonal I/O lbrary accumulates output untl a buffer s flled and then calls O/S to wrte the buffer to the fle nput s read a buffer at a tme from O/S and then lbrary gves t n smaller amounts Pagng allows a dfferent approach Take unmapped pages of the process space and map them to the blocks of the fle. page fault brngs the block nto memory when the fle s closed, wrte all modfed blocks random access to data fle! fles can be shared wth wrte access! ELEC 377 Operatng Systems

51 Page Replacement What happens when we run out of memory? runnng more processes than memory (over allocaton) no spare frames select some other frame n physcal memory wrte ts contents to dsk nvaldate the MMU regsters that pont to the frame reuse the frame Two transfers (wrte old contents, read new contents) ELEC 377 Operatng Systems

52 Page Replacement drty bt? add another flag to the page table ndcates that the page has been changed (drty) only wrte drty pages (otherwse matches copy on the dsk) Code pages are mapped from the program executable snce code doesn t change (reentrant), never have to wrte code pages. Backng store only saves data and stack pages ELEC 377 Operatng Systems

53 Page Replacement Algorthms Smlar to schedulng algorthms want to mnmze page faults FIFO Not partcularly good Belady s Anomaly more memory more page faults Optmal smlar to Shortest Job Frst schedulng algorthm page that wll not be used for the longest tme future knowledge ELEC 377 Operatng Systems

54 Page Replacement Algorthms LRU - Least recently used past behavour predcts future behavour page referenced longest ago gets replaced hardware support(page counters, stack) Approxmaton reference bts (hstory of page references) second chance algorthm (FIFO wth 1 ref bt) Alternatves nclude modfed bt prefer clean pages to drty pages not as mportant as recently used reference bt, breaks tes ELEC 377 Operatng Systems

Virtual Memory. Background. No. 10. Virtual Memory: concept. Logical Memory Space (review) Demand Paging(1) Virtual Memory

Virtual Memory. Background. No. 10. Virtual Memory: concept. Logical Memory Space (review) Demand Paging(1) Virtual Memory Background EECS. Operatng System Fundamentals No. Vrtual Memory Prof. Hu Jang Department of Electrcal Engneerng and Computer Scence, York Unversty Memory-management methods normally requres the entre process