MID-II Examinations April 2018 Course: B. Tech Branch:CSE Year: II. Date of Exam: AN Max.Marks 30 TIME :02:00PM TO 03:00 PM

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1 MID-II Examinatins April 2018 Curse: B. Tech Branch:CSE Year: II Subject: OS Semester :II Date f Exam: AN Max.Marks 30 TIME :02:00PM TO 03:00 PM Answer ANY TWO f the fllwing 2 x 15 = 30 Marks 1.A) Define Pure Demand Paging?[2M] SCHEME OF EVALUATION Crrect Definitin Marks NO pages are swapped in fr a prcess until they are requested by page faults. This is knwn as pure demand paging. 1.B) Describe Virtual Address Space?[3marks] Crrect Descriptin Marks Virtual address space is the prgrammer s lgical view f prcess memry strage. The prcess s page table cntrls the actual physical layut. 1 C) State and Explain Belady s anmaly with example? [5 marks] Crrect Explanatin Marks Example Marks An interesting effect that can ccur with FIFO is Belady's anmaly, in which increasing the number f frames available can actually increase the number f page faults that ccur! Cnsider, fr example, the fllwing chart based n the page sequence ( 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 ) and a varying number f available frames. Obviusly the maximum number f faults is 12 ( every request generates a fault ), and the minimum number is 5 ( each page laded nly nce ), but in between there are sme interesting results:

2 Frame Pagefaults D) Assume that there are 3 page frames which are initially empty. If the page reference string is 1, 2, 3,4, 2, 1, 5, 3, 2, 4, 6. Calculate the number f page faults using the ptimal replacement plicy and Least Recently Used Plicy.? [5 marks] Crrect Prcess Marks Crrect Page faults Marks OPTIMAL Page Replacement N. f Page Faults: 7 Least Recently Used N. f Page Faults: 10

3 2 A) Define Bt disk? [2 marks] Crrect Definitin Marks A bt disk is a disk that a cmputer can start up r "bt" frm. The mst cmmn type f bt disk is an internal hard drive, which mst cmputers use t start up frm. 2 B) What are the typical peratins that can be perfrmed n directry? [3 marks] Identifying crrect Operatin Marks Search fr a file Create a file - add t the directry Delete a file - erase frm the directry List a directry - pssibly rdered in different ways. Rename a file - may change srting rder Traverse the file system. 2 C) Explain the cncept f file sharing? What are the criteria t be fllwed in systems which implement file sharing? [10 marks] Identifying crrect Implementatins Marks Multiple Users On a multi-user system, mre infrmatin needs t be stred fr each file: The wner ( user ) wh wns the file, and wh can cntrl its access. The grup f ther user IDs that may have sme special access t the file. What access rights are affrded t the wner ( User ), the Grup, and t the rest f the wrld ( the universe, a.k.a. Others. ) Sme systems have mre cmplicated access cntrl, allwing r denying specific accesses t specifically named users r grups. File Systems The advent f the Internet intrduces issues fr accessing files stred n remte cmputers The riginal methd was ftp, allwing individual files t be transprted acrss systems as needed. Ftp can be either accunt and passwrd cntrlled, r annymus, nt requiring any user name r passwrd. Varius frms f distributed file systems allw remte file systems t be munted nt a lcal directry structure, and accessed using nrmal file access cmmands. ( The actual files are still transprted acrss the netwrk as needed, pssibly using ftp as the underlying transprt mechanism. ) The WWW has made it easy nce again t access files n remte systems withut munting their filesystems, generally using ( annymus ) ftp as the underlying file transprt mechanism. The Client-Server Mdel When ne cmputer system remtely munts a filesystem that is physically lcated n anther system, the system which physically wns the files acts as a server, and the system which munts them is the client. User IDs and grup IDs must be cnsistent acrss bth systems fr the system t wrk prperly. ( I.e. this is mst applicable acrss multiple cmputers managed by the same rganizatin, shared by a cmmn grup f users. ) The same cmputer can be bth a client and a server. ( E.g. crss-linked file systems. ) There are a number f security cncerns invlved in this mdel: Servers cmmnly restrict munt permissin t certain trusted systems nly. Spfing ( a cmputer pretending t be a different cmputer ) is a ptential security risk. Servers may restrict remte access t read-nly.

4 Servers restrict which filesystems may be remtely munted. Generally the infrmatin within thse subsystems is limited, relatively public, and prtected by frequent backups. The NFS ( Netwrk File System ) is a classic example f such a system. Distributed Infrmatin Systems The Dmain Name System, DNS, prvides fr a unique naming system acrss all f the Internet. Dmain names are maintained by the Netwrk Infrmatin System, NIS, which unfrtunately has several security issues. NIS+ is a mre secure versin, but has nt yet gained the same widespread acceptance as NIS. Micrsft's Cmmn Internet File System, CIFS, establishes a netwrk lgin fr each user n a netwrked system with shared file access. Older Windws systems used dmains, and newer systems ( XP, 2000 ), use active directries. User names must match acrss the netwrk fr this system t be valid. A newer apprach is the Lightweight Directry-Access Prtcl, LDAP, which prvides a secure single sign-n fr all users t access all resurces n a netwrk. This is a secure system which is gaining in ppularity, and which has the maintenance advantage f cmbining authrizatin infrmatin in ne central lcatin. Failure Mdes When a lcal disk file is unavailable, the result is generally knwn immediately, and is generally nn-recverable. The nly reasnable respnse is fr the respnse t fail. Hwever when a remte file is unavailable, there are many pssible reasns, and whether r nt it is unrecverable is nt readily apparent. Hence mst remte access systems allw fr blcking r delayed respnse, in the hpes that the remte system ( r the netwrk ) will cme back up eventually. Cnsistency Semantics Cnsistency Semantics deals with the cnsistency between the views f shared files n a netwrked system. When ne user changes the file, when d ther users see the changes? 3 A) Describe the representatin f a resurce allcatin graph. [2 Marks] Descriptin Marks In sme cases deadlcks can be understd mre clearly thrugh the use f Resurce- Allcatin Graphs, having the fllwing prperties: A set f resurce categries, { R1, R2, R3,..., RN }, which appear as square ndes n the graph. Dts inside the resurce ndes indicate specific instances f the resurce. ( E.g. tw dts might represent tw laser printers. ) A set f prcesses, { P1, P2, P3,..., PN } Request Edges - A set f directed arcs frm Pi t Rj, indicating that prcess Pi has requested Rj, and is currently waiting fr that resurce t becme available. Assignment Edges - A set f directed arcs frm Rj t Pi indicating that resurce Rj has been allcated t prcess Pi, and that Pi is currently hlding resurce Rj.

5 3 B) List Gal f Prtectin. [3 marks] Each crrect Gal Mark 1x3=3 Marks Obviusly t prevent malicius misuse f the system by users r prgrams. T ensure that each shared resurce is used nly in accrdance with system plicies, which may be set either by system designers r by system administratrs. T ensure that errant prgrams cause the minimal amunt f damage pssible. Nte that prtectin systems nly prvide the mechanisms fr enfrcing plicies and ensuring reliable systems. It is up t administratrs and users t implement thse mechanisms effectively. 3 C) Explain Hw d system Recver frm Deadlck? [5 Marks] Identifying crrect appraches Marks Explanatin Marks There are three basic appraches t recvery frm deadlck: Prcess Terminatin 1. Infrm the system peratr, and allw him/her t take manual interventin. 2. Terminate ne r mre prcesses invlved in the deadlck 3. Preempt resurces. Tw basic appraches, bth f which recver resurces allcated t terminated prcesses: Terminate all prcesses invlved in the deadlck. This definitely slves the deadlck, but at the expense f terminating mre prcesses than wuld be abslutely necessary. Terminate prcesses ne by ne until the deadlck is brken. This is mre cnservative, but requires ding deadlck detectin after each step. Resurce Preemptin When preempting resurces t relieve deadlck, there are three imprtant issues t be addressed: 1. Selecting a victim - Deciding which resurces t preempt frm which prcesses invlves many f the same decisin criteria utlined abve.

6 2. Rllback - Ideally ne wuld like t rll back a preempted prcess t a safe state prir t the pint at which that resurce was riginally allcated t the prcess. Unfrtunately it can be difficult r impssible t determine what such a safe state is, and s the nly safe rllback is t rll back all the way back t the beginning. ( I.e. abrt the prcess and make it start ver. ) 3. Starvatin - Hw d yu guarantee that a prcess wn't starve because its resurces are cnstantly being preempted? One ptin wuld be t use a pririty system, and increase the pririty f a prcess every time its resurces get preempted. Eventually it shuld get a high enugh pririty that it wn't get preempted any mre. 3 D) Explain Varius Methds fr handling Deadlcks? [5 marks] Identifying crrect Methds Marks Explanatin Marks Generally speaking there are three ways f handling deadlcks: 1. Deadlck preventin r avidance - D nt allw the system t get int a deadlcked state. 2. Deadlck detectin and recvery - Abrt a prcess r preempt sme resurces when deadlcks are detected. 3. Ignre the prblem all tgether - If deadlcks nly ccur nce a year r s, it may be better t simply let them happen and rebt as necessary than t incur the cnstant verhead and system perfrmance penalties assciated with deadlck preventin r detectin. This is the apprach that bth Windws and UNIX take. 4A) Define Prtectin Dmain? [2 Marks] Definitin Marks A prtectin dmain specifies the resurces that a prcess may access.each dmain defines a set f bjects and the types f peratins that may be invked n each bject. 4 B) Describe the cnditin under which a deadlck situatin may arise?[ 3Marks] Crrect Cnditins Marks 1. Mutual Exclusin - At least ne resurce must be held in a nn-sharable mde; If any ther prcess requests this resurce, then that prcess must wait fr the resurce t be released. 2. Hld and Wait - A prcess must be simultaneusly hlding at least ne resurce and waiting fr at least ne resurce that is currently being held by sme ther prcess. 3. N preemptin - Once a prcess is hlding a resurce ( i.e. nce its request has been granted ), then that resurce cannt be taken away frm that prcess until the prcess vluntarily releases it. 4. Circular Wait - A set f prcesses { P0, P1, P2,..., PN } must exist such that every P[ i ] is waiting fr P[ ( i + 1 ) % ( N + 1 ) ].

7 4C) Write abut Swap Space Management. [5 marks] Explanatin Marks Example Marks Mdern systems typically swap ut pages as needed, rather than swapping ut entire prcesses. Hence the swapping system is part f the virtual memry management system. Managing swap space is bviusly an imprtant task fr mdern OSes. Swap-Space Use The amunt f swap space needed by an OS varies greatly accrding t hw it is used. Sme systems require an amunt equal t physical RAM; sme want a multiple f that; sme want an amunt equal t the amunt by which virtual memry exceeds physical RAM, and sme systems use little r nne at all! Sme systems supprt multiple swap spaces n separate disks in rder t speed up the virtual memry system. Swap-Space Lcatin Swap space can be physically lcated in ne f tw lcatins: As a large file which is part f the regular filesystem. This is easy t implement, but inefficient. Nt nly must the swap space be accessed thrugh the directry system, the file is als subject t fragmentatin issues. Caching the blck lcatin helps in finding the physical blcks, but that is nt a cmplete fix. As a raw partitin, pssibly n a separate r little-used disk. This allws the OS mre cntrl ver swap space management, which is usually faster and mre efficient. Fragmentatin f swap space is generally nt a big issue, as the space is re-initialized every time the system is rebted. The dwnside f keeping swap space n a raw partitin is that it can nly be grwn by repartitining the hard drive. Swap-Space Management: An Example Histrically OSes swapped ut entire prcesses as needed. Mdern systems swap ut nly individual pages, and nly as needed. ( Fr example prcess cde blcks and ther blcks that have nt been changed since they were riginally laded are nrmally just freed frm the virtual memry system rather than cpying them t swap space, because it is faster t g find them again in the filesystem and read them back in frm there than t write them ut t swap space and then read them back. ) In the mapping system shwn belw fr Linux systems, a map f swap space is kept in memry, where each entry crrespnds t a 4K blck in the swap space. Zers indicate free slts and nn-zers refer t hw many prcesses have a mapping t that particular blck ( >1 fr shared pages nly. ) The data structures fr swapping n Linux systems.

8 4D) Suppse that a disk drive has 5,000 cylinders, numbered 0 t The drive is currently serving a request at cylinder 143, and the previus request was at cylinder 125. The queue f pending requests, in FIFO rder, is: 86, 1470, 913, 1774, 948, 1509,1022, 1750, 130. Starting frm the current head psitin, what is the ttal distance (in cylinders) that the disk arm mves t satisfy all the pending requests fr each f the SSTF disk scheduling algrithms? [5 marks] Crrect Head Mvement Marks Calculatin Mark The SSTF schedule is 143, 130, 86, 913, 948, 1022, 1470, 1509, 1750, The ttal seek distance is 1745.