Operating Systems Sample Exam Questions

1. Describe the two general roles of an operating system, and elaborate why these roles are important.

2. Using a simple system call as an example (e.g. getpid, or uptime), describe what is generally involved in providing the result, from the point of calling the function in the C library to the point where that function returns.

3.  Why must the operating system be more careful when accessing input to a system call (or producing the result) when the data is in memory instead of registers?

4. Is putting security checks in the C library a good or a bad idea? Why?

5. Describe the three state process model, describe what transitions are valid between the three states, and describe an event that might cause such a transition.

6. Multi-programming (or multi-tasking) enables more than a single process to apparently execute simultaneously. How is this achieved on a uniprocoessor?

7. What is a process? What are attributes of a process?

8. What is the function of the ready queue?

9. What is the relationship between threads and processes?

10. Describe how a multi-threaded application can be supported by a user-level threads package. It may be helpful to consider (and draw) the components of such a package, and the function they perform.

11. Name some advantages and disadvantages of user-level threads.

12. Why are user-level threads packages generally cooperatively scheduled?

13. Enumerate the advantages and disadvantages of supporting multi-threaded applications with kernel-level threads.

14. Describe a sequence the sequence of step that occur when a timer interrupt occurs that eventually results in a context switch to another application.

15. Context switching between two threads of execution within the operating system is usually performed by a small assembly language function. In general terms, what does this small function do internally?

16. What is a race condition? Give an example.

17. What is a critical region?  How do they relate to controlling access to shared resources?

18.  What are three requirements of any solution to the critical sections problem? Why are the requirements needed?

19. Why is turn passing a porr solution to the critical sections problem?

20. Interrupt disabling and enabling is a common approach to implementing mutual exclusion, what are its advantages and disadvantages?

21. What is a test-and-set instruction? How can it be used to implement mutual exclusion? Consider using a fragment of psuedo-assembly language aid you explanation.

22. What is the producer consumer problem? Give an example of its occurence in operating systems.

23. A semaphore is a blocking synchronisation primitive. Describe how they work with the aid of pseudo-code.  You can assume the existance of a thread_block() and a thread_wakeup() function.

24. Describe how to implement a lock using semaphores.

25. What are monitors and condition variables?

26. What is deadlock? What is startvation? How do they differ from each other?

27.  What are the four condtions required for deadlock to occur?

28.  Describe four general strategies for dealing with deadlocks.

29.  For single unit resources, we can model resource allocation and requests as a directed graph connecting processes and resources. Given such a graph, what is involved in deadlock detection.

30.  Is the following system of four processes with 2 resources deadlocked?

Current allocation matrix

P1 1 3
P2 4 1
P3 1 2
P4 2 0

Current request matrix
P1 1 2
P2 4 3
P3 1 7
P4 5 1

Availability Vector
1 4

If the availability vector is as below, is the system above still deadlocked?
2 3

Is the system deadlocked if the availability is
2 4

31.  Assuming the operating system detects the system is deadlocked, what can the operating system do to recover from deadlock?

32.  What must the banker's algorithm know a priori in order to prevent deadlock?

33.  Describe the general strategy behind dealock prevention, and give an example of a practical deadlock prevention method.

34. Filesystems can support sparse files, what does this mean? Give an example of  an application's file organisation that might benefit from a file system's sparse file support.

35.  Give an example of a scenario that might benefit from a file system supporting an append-only access write.

36. Give a scenario where choosing a large filesystem block size might be a benefit; give an example where it might be a hinderance.

37.  Give an example where contiguous allocation of file blocks on disks can be used in practice.

38. What file access pattern is particularly suited to chained file allocation on disk?

39. What file allocation strategy is most appropriate for random access files?

40. Compare bitmap-based allocation of blocks on disk with a free block list.

41. How can the block count in an inode differ from the (file size / block size) rounded up to the nearest integer. Can the block count be greater, smaller, or both.

42. Why might the direct blocks  be stored in the inode itself?

43.  Given that the maximum file size of combination of direct, single indirection, double indirection, and triple indirection in an inode-based filesystem is approximately the same as a filesystem soley using triple indirection, why not simply use only triple indirection to locate all file blocks?

44.  What is the maximum file size supported by a file system with 16 direct blocks, single, double, and triple indirection? The block size is 512 bytes. Disk block numbers can be stored in 4 bytes.

45.  The berkely fast filesystem (and Linux Ext2fs) use the idea of block groups. Describe what this idea is and what improvements block groups have over the simple filesystem layout of  the System V file system (s5fs).

46.  What is the reference count field in the inode? You should consider its relationship to directory entries in you answer.

47. The filesystem buffer cache does both buffering and caching. Describe why buffering is needed. Describe how buffering can improve performance (potentially to the detriment of file system robustness). Describe how the caching component of the buffer cache improves performance.

48. What does flushd do on a UNIX system?

49. Why might filesystems managing external storage devices do write-through caching (avoid buffering writes) even though there is a detrimental affect on performance.

50. Describe the difference between external and internal fragmentation. Indicate which of the two are most likely to be an issues on a) a simple memory memory mangement machine using base limit registers and static partitioning, and b) a similar machine using dynamic partitioning.

51. List and describe the four memory allocation algorithms covered in lectures. Which two of the four are more commonly used in practice?

52. Base-limit MMUs can support swapping. What is swapping? Can swapping permit an application requiring 16M memory to run on a machine with 8M of RAM?

53. Describe page-based virtual memory. You should consider pages, frames, page tables, and Memory Management Units in your  answer.

54. Give some advantages of a system with page-based virtual memory compared to a simply system with base-limit registers that implements swapping.

55. Describe segmentation-based virtual memory.  You should consider the components of a memory address, the segment table and its contents, and how the final physical address is formed in your answer.

56. What is a translation look-aside buffer? What is contained in each entry it contains?

57. Some TLBs support address space identifiers (ASIDS), why?

58. Describe a two-level page table? How does it compare to a simple page table array?

59. What is an inverted page table? How does it compare to a two-level page table?

60.  What are temporal locality and spatial locality?

61. What is the working set of a process?

62. How does page size of a particular achitecture affect working set size?

63. What is thrashing? How might it be detected? How might one recover from it once detected?

64.  Enumerate some pros and cons for increasing the page size.

65. Describe two virtual memory page fetch policies. Which is less common in practice? Why?

66. What operating system event might we observe and use as input to an algorithm that decides how many  frames an application receives (i.e. an algorithm that determines the application's resident set size)?

67. Name and describe four page replacement algorithms. Critically compare them with each other.
68. Describe buffering in the I/O subsystem of an operating system. Give reasons why it is required, and give a case where it is an advantage, and a case where it is a disadvantage.

69. Device controllers are generally becoming more complex in the functionality they provide (e.g. think about the difference between implementing a serial port with a flip-flop controlled by the CPU and a multi-gigabit network adapter with the TCP/IP stack on the card itself). What effect might this have on the operating system and system performance?

70.  Compare I/O based on polling with interrupt-driven I/O. In what situation would you favour one technique over the other?

71.  Explain how the producer-consumer problem is relevant to operating system I/O.

72. What is disk interleaving? What problem is it trying to solve?

73. What is cylinder skew? What problem is it trying to solve?

74. Name four disk-arm scheduling algorithms. Outline the basic algorithm for each.

90. Why is it generally correct to favour I/O bound processes over CPU-bound processes?

91. What is the difference between preemptive scheduling and non-preemptive scheduling? What is the issue with the latter?

93. Describe round robin scheduling. What is the parameter associated with the scheduler? What is the issue in chosing the parameter?

94. The traditional UNIX scheduler is a priority-based  round robin scheduler (also called a multi-level round robin schduler). How does the scheduler go about favouring I/O bound jobs over long-running CPU-bound jobs?

97. In a real-time system with a periodic task set,, how are priorities assigned to each of the periodic tasks?

98. What is an EDF scheduler? What is its advantage over a rate monotic scheduler?

99.  Describe why the use of spinlocks might be more appropriate than blocking locks on a multiprocessor when compared to a uniprocessor. Is there a trade-off between spinning and blocking on a multiprocessor? Discuss.

100. Is a single ready queue on a multiprocessor a good idea? Why?

101. What is thread afinity. Why might it improve performance?