Operating Systems 2: Tutorial 2 Questions:

Tutorial 2 Operating Systems 2
Faculty of Engineering Science and the Built Environment
Department of Electrical, Computer and Communications Engineering
Questions:
Operating Systems 2: Tutorial 2
1. Give several definitions of process. Why, do you suppose, is there no universally accepted
definition?
2. Sometimes the terms user and process are used interchangeably. Define each of these terms.
In what circumstances do they have similar meaning?
3. Describe the differences among short-term, medium-term, and long-term scheduling.
4. Describe the actions a kernel takes to context switch between processes.
5. What are two differences between user-level threads and kernel-level threads? Under what
circumstances is one type better than the other?
6. Describe the actions taken by a kernel to context switch between kernel-level threads.
7. Describe the actions taken by a thread library to context switch between user-level threads.
8. What resources are used when a thread is created? How do they differ from those used when
a process is created?
9. Define the difference between preemptive and nonpreemptive scheduling. State why strict
nonpreemptive scheduling is unlikely to be used in a computer center.
10. Consider a variant of the Round-Robin (RR) scheduling algorithm where the entries in the
ready queue are pointers to the PCBs.
a. What would be the effect of putting two pointers to the same process in the ready
queue?
b. What would be the major advantages and disadvantages of this scheme?
c. How would you modify the basic RR algorithm to achieve the same effect without the
duplicate pointers?
11. What advantage is there in having different time-quantum sizes on different levels of a
multilevel queueing system?
12. Consider the following preemptive priority-scheduling algorithm based on dynamically
changing priorities. Larger priority numbers imply higher priority. When a process is waiting
for the CPU (in the ready queue but not running), its priority changes at a rate α; when it is
running, its priority changes at a rate β. All processes are given a priority of 0 when they
enter the ready queue. The parameters α and β can be set to give many different scheduling
algorithms.
a. What is the algorithm that results from β > α >0?
b. What is the algorithm that results from α < β

Tutorial 2 Operating Systems 2
13. Many CPU scheduling algorithms are parameterized. For example, the RR algorithm requires
a parameter to indicate the time slice. Multilevel feedback queues require parameters to
define the number of queues, the scheduling algorithms for each queue, the criteria used to
move processes between queues, and so on. These algorithms are thus really sets of
algorithms (for example, the set of RR algorithms for all time slices, and so on). One set of
algorithms may include another (for example, the FCFS algorithm is the RR algorithm with
an infinite time quantum). What (if any) relation holds between the following pairs of sets of
algorithms?
a. Priority and SJF
b. Multilevel feedback queues and FCFS
c. Priority and FCFS
d. RR and SJF
14. Suppose that a scheduling algorithm (at the level of short-term CPU scheduling) favors those
processes that have used the least processor time in the recent past. Why will this algorithm
favour I/O-bound programs and yet not permanently starve CPU-bound programs?
15. Explain the differences in the degree to which the following scheduling algorithms
discriminate in favor of short processes:
a. FCFS
b. RR
c. Multilevel feedback queues
16. Give several reasons why the study of concurrency is appropriate and important for students
of operating systems.
17. Explain why the following statement is false: When several threats access shared information
in main memory, mutual exclusion must be enforced to prevent the production of
indeterminate results.
18. What is the real significance of Dekker’s algorithm?
19. Mutual exclusion primitives can be implemented with busy waiting or with blocking. Discuss
the applicability and relative merits of each approach.
20. Explain in detail how binary semaphores and binary semaphore operations can be
implemented in the kernel of an operating system.
21. Define deadlock and give an example of a deadlock involving only a single process and a
single resource.
22. Give an example of a simple resource deadlock involving three processes and three resources.
Draw the appropriate resource-allocation graph.
23. Is it possible to have a deadlock involving only one single process? Explain your answer.
24. State the four necessary conditions for a deadlock to exist. Give a brief intuitive argument for
the necessity of each individual condition.
25. Consider a system consisting of four resources of the same type that are shared by three
processes, each of which needs at most two resources. Show that the system is deadlock free.
26. Explain the intuitive appeal of deadlock avoidance over deadlock prevention.
Lecturer: S. Dimitriou Page 2 of 3

Tutorial 2 Operating Systems 2
27. We can obtain the banker’s algorithm for a single resource type from the general banker’s
algorithm simply by reducing the dimensionality of the various arrays by 1. Show through an
example that the multiple-resource-type banker’s scheme cannot be implemented by
individual application of the single-resource-type scheme to each resource type.
28. Can a system detect that some of its processes are starving? If you answer “yes,” explain how
it can. If you answer “no,” explain how the system can deal with the starvation problem.
29. Consider the following snapshot of a system:
Allocation Max Available
A B C D A B C D A B C D
P o 0 0 1 2 0 0 1 2 1 5 2 0
P 1 1 0 0 0 1 7 5 0
P 2 1 3 5 4 2 3 5 6
P 3 0 6 3 2 0 6 5 2
P 4 0 0 1 4 0 6 5 6
Answer the following questions using the banker’s algorithm:
a. What is the content of the matrix Need?
a. Is the system in a safe state?
b. If a request from process P1 arrives for (0,4,2,0), can the request be granted
immediately?
30. Consider the following resource-allocation policy. Requests and releases for resources are
allowed at any time. If a request for resources cannot be satisfied because the resources are
not available, then we check any processes that are blocked, waiting for resources. If they
have the desired resources, then these resources are taken away from them and are given to
the requesting process. The vector of resources for which the waiting process is waiting is
increased to include the resources that were taken away. For example, consider a system with
three resource types and the vector Available initialized to (4,2,2). If process P0 asks for
(2,2,1), it gets them. If P1 asks for (1,0,1), it gets them. Then, if P0 asks for (0,0,1), it is
blocked (resource not available). If P2 now asks for (2,0,0), it gets the available one (1,0,0)
and one that was allocated to P0 (since P0 is blocked). P0’s Allocation vector goes down to
(1,2,1), and its Need vector goes up to (1,0,1).
a. Can deadlock occur? If so, give an example. If not, which necessary condition cannot
occur?
b. Can indefinite blocking occur?
31. In a system in which it is possible for a deadlock to occur, under what circumstances would
you use a deadlock detection algorithm?
32. Why is deadlock recovery such a difficult problem and why is it difficult to choose which
processes to “flush” in deadlock recovery?
Lecturer: S. Dimitriou Page 3 of 3