TDDD36 Säkra mobila System (Systemprogramvara)

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1 TENTAMEN TDDD36 Säkra mobila System (Systemprogramvara) Datum: Tid: 8-12 Lokal: TER1 Jourhavande-lärare: Jordi Cucurull Juan (jour runda, telefon: ) Hjälpmedel: Poängantal: Miniräknare, Lexikon 40p Betyg: 3, 4 and 5: U: - 19 p 3: p 4: p 5: p INSTRUCTIONS: Write your anonymous ID number on each sheet of paper that you hand in. Further, pages should only contain one answer per page (answers to sub-questions can be on the same page). You are asked to only answer on the front page of the paper. Sort all the sheets that you hand in with respect to the number of the question. Your answers can be presented in Swedish or English. Questions that are in English can be corrected by a non-swedish teacher. Make sure your answers include motivations and are presented precisely. A correct answer without any motivation will not be given any credits. Incorrect answers embedded in a partially correct one reduce the points given for that answer. Points will not be given to answers that cannot be read due to bad handwriting. Hints: Try to dispose of your time on each question in proportion of the assignment points. In those cases where you are in doubt about the question, write down your interpretation and your assumptions, and answer the question based on the interpretation. Figures can be of help when describing but should be accompanied by a text description. Results are reported no later than January 26, Good luck! Simin Nadjm-Tehrani Examiner TDDD36, Systems Software

2 Q1: Annika, Malin and Oscar are doing their school homework together. Each student has a different assignment. Each student owns 2 pencils, 1 book, and 1 ruler which they use until they have done their own assignment. In addition, there are 2 spare pencils, 1 spare calculator and 1 spare book. In order to make the assignments Annika requires 6 pencils, 1 calculator, 1 book, and 2 rulers. Malin requires 2 pencils, 1 calculator, 2 books, and 1 ruler, and Oscar requires 1 pencil and 4 books. Assume that the students will not release any tool (owned or acquired spare tool) until they have finished their assignment. We would like to know: can all the assignments be completed? Relate this problem to the Banker's algorithm for multiple resource allocation in computer systems, and use the concepts from that algorithm to show whether a sequence of assignments is possible. a) Take a stand on the following propositions (true or false) and motivate your answer! 1) A resource that is protected by a semaphore, can be accessed by one or more processes at a time, depending on the initial value of the semaphore. 2) Deadlock is only possible when at least one of the Coffman conditions holds. a) In Stockholm Tivoli there is a roller coaster called Jetline. It has a track with loops and curves with many cars where people take rides at fast speed. Along the track there are three cameras. Each camera takes pictures of the people when a car passes in front of it. The system is controlled by a single processor which runs the six processes, three devoted to capture the pictures, Ci (i: 1..3) and three devoted to process and store the pictures, Pi (i: 1..3). Since the pictures are stored in a shared database, mutual exclusion of access to the database must be guaranteed. Implement an algorithm using the database encapsulated in a monitor. In order to decouple the rate of capturing pictures and processing them, three buffers Bi (i: 1..3) are shared pair-wise (Each Bi shared by the pair Ci and Pi). You should describe your algorithm by writing the pseudo-code for all the processes, but are not expected to produce the details of the pseudo-code for operations on the buffers or the storage in database. Betrakta följande två processer som ska köras på samma processor och anta att tilldelning operationen är inte atomiskt implementerat i maskinspråk. Identifiera koden som ska skyddas som kritiska sektion i varje process. Du kan t.ex. skriva av koden och rita en cirkel kring delar av koden som ska betraktas som kritisk sektion. Den kod som du markerar ska leda till minst inverkan på parallellism, dvs inte tvinga att processerna körs sekventiellt i så stor utsträckning som möjligt. Motivera ditt val! Process A Process B while true do while true do {X = X+1; { display( Square of + X); Sq = X^2; display( is: + Sq); W = Z+3 C = C - 2 } }

3 Q2: b) Take a stand on the following propositions (true or false) and motivate your answer! 1) Live video broadcasting is an intolerant application in the QoS terminology. 2) Tele-conferencing is a non-interactive application in the QoS terminology. c) Consider a document sharing application where different users in a group can cooperate by reading and writing to a set of common documents. Decide which of the following properties is a functional property and which is an extra-functional property (also sometimes called a non-functional property), and motivate your answer! 1) Updates to the same document by different users should be time-stamped and version-controlled. 2) Data replication shall be used to ensure that no documents kept at the central repository can be lost due to a disk crash. 3) Each user can submit documents for storage up to a maximum of 2GB. 4) Only users that are authorised to access a document can access that document. Q3: a) Describe a variation of the client-server model that is an architectural model serving some extra-functional (non-functional) property. (2 poäng) b) What is the reason for having a fault model when designing a distributed system? Give an example fault model and the resulting work that has to be done during system design/implementation depending on that fault model. Q4: a) Using clear arguments identify a potential fault-error-failure chain of events in the following scenario. In Computer Sweden 2 nd December 2011, we read the background to the recent major data centre failure for the company Tieto. This company has several critical customers including a major Swedish Chemist (Apoteket), the SBAB bank, and a number of Swedish public authorities including the one in charge of social insurance. The data centre problem has resulted in payments to social insurance recipients being delayed, patient prescriptions not being available, bank accounts not reachable, etc., for around one week. The problem has been traced to a disfunctional double disk redundant subsystem, the so called fast cache, on which several other systems are dependent. Both the original disk and the mirrored disk had apparently crashed despite the fact that Tieto had service level agreements of 99,8% up time over a 3 month period with some of its customers. Recovery of the system has been delayed and customer data has apparently been lost in the process.

4 Glossary: English to Swedish atomic atomisk (oavbrytbar) request begäran availability tillgänglighet response time responstid avoid undvika safety säkerhet bandwidth bandbredd scheduler schemaläggare broker medlare security säkerhet clock drift klockdrift serialisable - serialiserbar concurrency samtidighet shaping - formning consistency konsistens shared memory gemensamt minne deadlock låsning (baklås) starvation svält delay fördröjning sufficient condition- tillräckligt villkor deliberate avsiktlig synchronisation synkronisering dependability pålitlighet system call systemanrop diversity mångfald thread - tråd error felyttring throughput genomströmning event händelse trace spår failure haveri transparency transparens fault felkälla trust tillit fault tolerance feltolerans validation validering forecast förutse verification - verifiering inheritance arv vulnerability - sårbarhet integrity dataintegritet interleavings sammanflätningar interoperability interoperabilitet intrusion intrång latency - latens maintainability reparerbarhet malicious - illvillig middleware - mellanvara mutual exclusion ömsesidig uteslutning necessary condition nödvändigt villkor non-functional property ickefunktionell egenskap omission utelämnande performance prestanda preemptible avbrytbar prevent förebygga quality of service tjänstekvalitet race condition kapplöpningstillstånd release - släppning reliability tillförlitlighet rendundancy redundans replica kopia replication replikering

5 Swedish English arv - inheritance atomisk (oavbrytbar) atomic avbrytbar - preemptible avsiktlig - deliberate bandbredd - bandwidth begäran - request dataintegritet - integrity felkälla - fault feltolerans - fault tolerance felyttring - error fördröjning - delay förebygga - prevent formning - shaping förutse - forecast gemensamt minne - shared memory genomströmning - throughput händelse - event haveri - failure ickefunktionell egenskap - non-functional property illvillig - malicious interoperabilitet - interoperability intrång intrusion kapplöpningstillstånd race condition klockdrift - clock drift konsistens - consistency kopia - replica låsning (baklås) - deadlock latens - latency mångfald - diversity medlare - broker mellanvara - middleware nödvändigt villkor - necessary condition ömsesidig uteslutning - mutual exclusion pålitlighet - dependability prestanda - performance redundans - rendundancy reparerbarhet - maintainability replikering - replication responstid - response time säkerhet - safety säkerhet - security sammanflätningar - interleavings samtidighet - concurrency sårbarhet - vulnerability schemaläggare - scheduler serialiserbar - serialisable släppning release spår - trace svält - starvation synkronisering - synchronisation systemanrop - system call tillförlitlighet - reliability tillgänglighet - availability tillit - trust tillräckligt villkor - sufficient condition tjänstekvalitet - quality of service tråd - thread transparens - transparency undvika - avoid utelämnande - omission validering - validation verifiering - verification