THE AUSTRALIAN NATIONAL UNIVERSITY Practice Final Examination, October 2012

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1 THE AUSTRALIAN NATIONAL UNIVERSITY Practice Final Examination, October 2012 COMP2310 / COMP6310 (Concurrent and Distributed Systems ) Writing Period: 3 hours duration Study Period: 15 minutes duration Permitted Materials: One A4 page with notes on both sides. NO calculator permitted. Questions are NOT equally weighted. This exam will contribute 50% to your final assessment. Write your answers in blue or black pen only. Answers written in pencil will not be marked. The questions are followed by labelled, framed blank panels into which your answers are to be written. Additional answer panels are provided (at the end of the paper) should you wish to use more space for an answer than is provided in the associated labelled panels. If you use an additional panel, be sure to indicate clearly the question and part to which it refers to. The marking scheme will put a high value on clarity so, as a general guide, it is better to give fewer answers in a clear manner than to outline a greater number in a sketchy, half-answered fashion. Please write clearly if we cannot read your writing you may lose marks! Student Number (please write clearly): Enrollment (circle one): COMP2310 COMP6310 Official use only: Q1 (15) Q2 (26) Q3 (10) Q4 (14) Q5 (17) Q6 (18) Total (100) COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 1 of 18

2 QUESTION 1 [15 marks] General Concurrency (a) Detail what events might cause the following process transitions: i. from running state to blocked state ii. from running state to ready state iii. from blocked state to ready state QUESTION 1(a) [3 marks] (b) A concurrent language must provide some concept of a task/thread or other concurrent entity. List TWO other elements that relate to concurrency and might be provided by a concurrent language. QUESTION 1(b) (c) Give a precise definition of an atomic operation AND identify two different atomic operations that might be supported in hardware on a modern computer system. QUESTION 1(c) [3 marks] COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 2 of 18

3 Question 1 (continued) Student Number:... (d) Briefly describe the difference between a sequential and a concurrent program. QUESTION 1(d) (e) Describe the terms safety and liveness. Briefly explain how these relate to the correctness of a concurrent program. QUESTION 1(e) [3 marks] (f) Given an example of a defensive programming technique the is specific to concurrent programming. QUESTION 1(f) COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 3 of 18

4 QUESTION 2 [26 marks] Mutual Exclusion and Synchronization (a) In the context of concurrent programming explain what is meant by a race condition? Include in your answer 20 lines or less of pseudo code that shows a race condition. QUESTION 2(a) [4 marks] (b) When several threads are waiting on a condition variable, and that condition changes, is it ever necessary to release more than one of them? Explain with a simple example. QUESTION 2(b) COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 4 of 18

5 Question 2 (continued) Student Number:... (c) The following class attempts to implement a general semaphore in Java. It contains two different logical errors. Identify the two errors and correct them. For each logical error state what the effect would be if the code contained only that error (note: the fact that value can become negative should not be considered as one of the two error.). Ð Ñ Ô ÓÖ { ÔÖÓØ Ø ÒØ Ú ÐÙ ¼ ÔÖÓØ Ø Ñ Ô ÓÖ µ { Ú ÐÙ ¼ } ÔÖÓØ Ø Ñ Ô ÓÖ ÒØ Ò Ø Ð µ { Ú ÐÙ Ò Ø Ð } ÔÙ Ð ÝÒ ÖÓÒ Þ ÚÓ È µ {»» ÓÛÒ µ } Ú ÐÙ Ú ÐÙ ½ Ú ÐÙ < ¼µ ØÖÝ { Û Ø µ } Ø Á Ò Ø Ö Ö Ù Ô Ø Ü Ô Ø Ó Ò µ { } } ÔÙ Ð ÝÒ ÖÓÒ Þ ÚÓ Î µ {»» ÙÔ µ } Ú ÐÙ Ú ÐÙ ½ Ú ÐÙ < ¼µ ÒÓØ Ý ÐÐ µ QUESTION 2(c) [4 marks] COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 5 of 18

6 Question 2 (continued) (d) A museum allows at most N visitors to enter through the east entrance and leave through the west. When the museum opens, the janitor signals to a controller to accept both enter and leave actions; when the museum closes, only visitors already inside the museum may leave. (i) Write FSP processes to model the east door (generating enter actions, EAST), the west door (generating leave actions, WEST), the janitor (generating sequences of open and close actions, JANITOR) and the control of the above constraints (CONTROL). QUESTION 2(d)[i] [5 marks] (ii) Specify an FSP composition of the whole system. QUESTION 2(d)[ii] [1 mark] COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 6 of 18

7 Question 2 (continued) Student Number:... (iii) Implement this system as a concurrent program using shared objects in the programming language of your choice. pseudo-code is acceptable if the meaning is clear. Code to initialize the threads may be omitted. QUESTION 2(d)[iii] [10 marks] COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 7 of 18

8 QUESTION 3 [10 marks] Safety and Liveness (a) Give two examples of liveness properties in concurrent systems. QUESTION 3(a) (b) No-preemption is one of the necessary conditions for deadlock. Briefly explain the term. Discuss whether the condition is realistic in (distributed) computer systems. QUESTION 3(b) [4 marks] COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 8 of 18

9 Question 3 (continued) Student Number:... (c) Consider the following resource allocation graph involving processes P 1,P 2 and P 3 and resources R 1,R 2 and R 3. R 1 R 2 P P P R 3 The above diagram means that P 1 holds R 4 and is requesting R 1, P 2 holds R 1 and P 3 holds R 2. (i) Assuming all currently held resources are maintained, give two resource requests that together would result in an immediate deadlock. QUESTION 3(c)[i] [1 mark] (ii) Assuming all currently held resources are maintained, give two resource requests that together would not result in an immediate deadlock. QUESTION 3(c)[ii] [1 mark] (d) Why is the use of object-oriented programming particularly risky in conjunction with synchronization constructs (such as monitors). QUESTION 3(d) COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 9 of 18

10 QUESTION 4 [14 marks] Message Passing (a) Can the exclusive use of either of synchronous message passing or asynchronous message passing cause two process or two threads to deadlock. For each, if this is true, illustrate with a simple example. If not, explain why deadlock cannot occur. QUESTION 4(a) [4 marks] (b) Implement the museum system of Q2(d) using synchronous message passing. Only the parts implementing CONTROL and one of the other 3 components need be described in detail. You may use the following: ÔÙ Ð Ð Ë Ð Ø Ð { ÔÙ Ð ÚÓ Ù Ö ÓÓÐ Ò µ } ÔÙ Ð Ð ÒÒ Ð<Ì> ÜØ Ò Ë Ð Ø Ð { ÔÙ Ð ÝÒ ÖÓÒ Þ ÚÓ Ò Ì Ú µ ÔÙ Ð ÝÒ ÖÓÒ Þ Ì Ö Ú µ } ÔÙ Ð Ð Ë Ð Ø { ÔÙ Ð ÚÓ Ë Ð Ø Ð µ ÔÙ Ð ÝÒ ÖÓÒ Þ ÒØ ÓÓ µ } QUESTION 4(b) [10 marks] COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 10 of 18

11 Question 4 (continued) Student Number:... QUESTION 4(b) continued. COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 11 of 18

12 QUESTION 5 [17 marks] Architectures (a) Processes in Unix are created by the ÓÖ µ system call. Explain why this model could be problematic in terms of efficiency. Also briefly explain how this issue is resolved on current Unix implementations. QUESTION 5(a) (b) Describe the relationship between the creation of a Unix Ô Ô and the ÓÖ µ system call. QUESTION 5(b) [3 marks] COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 12 of 18

13 Question 5 (continued) Student Number:... (c) Consider a user wishing to connect via the Secure Shell remote login client ssh to a remote server at IP address The remote server has a Secure Shell daemon process (sshd) listening on port 22. Describe the series of socket system calls that both (i) the ssh process on the local host and (ii) the sshd on the remote host must perform in order to establish a connection. Hint: your answer is likely to include the system calls ÒØ Ó Ø ºººµ, ÓÒÒ Ø ÒØ Ó ÓÒ Ø ØÖÙØ Ó Ö ÖÚ Ö ºººµ Ò ÒØ Ó ÓÒ Ø ØÖÙØ Ó Ö ÑÝ Ö ºººµ, Ð Ø Ò ÒØ Ó ÒØ ÒÙÑ Ð ÒØ µ and ÔØ ÒØ Ó ØÖÙØ Ó Ö Ð ÒØ Ö ºººµ. Where ººº appears in the above, you are not expected to describe what values would be required for that parameter. QUESTION 5(c) [5 marks] COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 13 of 18

14 Question 5 (continued) (d) Both pipes and sockets provide a byte-stream abstraction for inter-process communication. Aside from possible differences in performance, briefly describe three major advantages (or disadvantages) of one over the other. QUESTION 5(d) [3 marks] (e) In the TCP/IP protocol, packets may arrive out-of-order or sometimes not arrive at their destination. How is this handled in order to make it a reliable protocol? QUESTION 5(e) (f) What is the internet transport addressing scheme and how can it be used for filtering? QUESTION 5(f) COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 14 of 18

15 Student Number:... QUESTION 6 [18 marks] Distributed Systems (a) This question addresses issues with virtual (logical) times in distributed systems. (i) If you find two logical times C(a) and C(b) associated to events a and b in different processes, what can you conclude if (1) C(a)= C(b), and (2) C(a) C(b). QUESTION 6(a)[i] (ii) Alternatively, if you know something about the the relationship between events a and b in a distributed system, what can you conclude about their virtual times if (1) a happened concurrently with b, and (2) a is the send event and b is the receive event for the same message. QUESTION 6(a)[ii] COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 15 of 18

16 Question 6 (continued) (iii) In any implementation of virtual times, the sequence of timestamps seen by any process should never decrease. Briefly explain what problem may arise if a later timestamp has a lower value than an earlier one. QUESTION 6(a)[iii] [1 mark] (iv) The three general strategies for dealing with deadlock are detection / recovery, avoidance and prevention. Consider these strategies in the context of transactions. (i) What property of transactions makes detection / recovery an acceptable strategy? QUESTION 6(a)[i] [1 mark] (ii) Which strategies are applicable to the two-phase locking, time-stamp ordering (TSO) and optimistic transaction schemes? QUESTION 6(a)[ii] [3 marks] COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 16 of 18

17 Question 6 (continued) Student Number:... (iii) Compare the strengths and weaknesses of the two-phase locking, time-stamp ordering (TSO) and optimistic schemes. QUESTION 6(a)[iii] [3 marks] COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 17 of 18

18 Question 6 (continued) (iv) Describe how virtual (logical) clocks may be used to implement either distributed critical regions or the consistent collection of distributed states (by an observer process). QUESTION 6(a)[iv] [6 marks] (overflow answer pages have been omitted) COMP2310 / COMP6310 Practice Final Exam, October 2012 Page 18 of 18

Concurrent Architectures - Unix: Sockets, Select & Signals

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