Download cs320fall2003test2

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CS 320, Test 2, Fall 2003
Name:____________________________
Score:______
40 questions, 2.5 points apiece, 100 points total. All of the questions are short answer
questions. Please make your answers short and clear. If you need more room than that
provided you are not focusing your answer enough. If only one out of a list of things is
requested, give only one in your answer. Do not try to cover your bases by including
several, hoping that one is correct. Muddy answers will not get full credit even if they
include some portion that is correct.
Part I. Ch. 6, CPU Scheduling.
1. What is the fundamental goal of multiprogramming as given by the author? (In
effect, this is just one of the several criteria for evaluating scheduling algorithms
given later.)
2. The author describes process execution as cyclical in nature. What are the two
elements of the cycle?
3. For the average process, does the ratio of short duration to long duration CPU
bursts tend to be high or low?
4. CPU scheduling can be done when a process switches from the running state to
the waiting state. Is such scheduling preemptive or nonpreemptive?
5. CPU scheduling can be done when a process switches from the waiting state to
the ready state. Is such scheduling preemptive or nonpreemptive?
6. A system may support preemption of user processes. A process may have
requested an operating system service with a system call. Under what
circumstances may such a user initiated system call not be interrupted?
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7. The scheduler implements algorithms for selecting a process to run. What
specific term is given to these three actions, as a group, which are applied to the
process: Switching context; switching to user mode; jumping to the execution
point in the user code and starting to run.
8. The author gives average waiting time and CPU utilization as criteria for
evaluating scheduling algorithms. He gave three more. Give any one of those
three.
9. Information about a set of processes is given below. Calculate the average
waiting time for these processes under FCFS scheduling. Give your answer as a
simple fraction.
Process
P1
P2
P3
P4
burst time
5
8
3
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10. Information about a set of processes is given below. Calculate the average
waiting time for these processes under SJF scheduling. Give your answer as a
simple fraction.
Process
P1
P2
P3
P4
burst time
5
8
3
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11. Information about a set of processes is given below. Calculate the average
waiting time for these processes under SJF scheduling. Give your answer as a
simple fraction.
Process
P1
P2
P3
P4
arrival time
0
4
6
8
burst time
5
8
3
4
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12. What is the theoretical claim for SJF scheduling, and what’s the corresponding
practical reality?
13. Let this formula be given: Τn+1 = αtn + (1 - α)Τn. If the estimate for T3 is fully
expanded and simplified, some power of the expression (1 - α) will be the
coefficient of T0. What is this power?
14. Information about a set of processes is given below. Calculate the average
waiting time for these processes under priority scheduling. (A low priority
number represents a high priority.) Give your answer as a simple fraction.
Process
P1
P2
P3
P4
priority
4
1
3
2
burst time
5
8
3
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15. Information about a set of processes is given below. Calculate the average
waiting time for these processes under RR scheduling. Let the time slice or
quantum be 3. Give your answer as a simple fraction.
Process
P1
P2
P3
P4
burst time
5
8
3
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16. Refer to the previous problem. If context switching takes 1 time unit on that
system, including the cost of switching in the initial process, give the ratio of
overhead to useful work in bringing the whole sequence of processes to
completion. Give your answer as a simple fraction.
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17. Suppose all process CPU bursts were shorter than the time slice for a round robin
scheduling algorithm. Effectively, what scheduling algorithm would result in this
situation?
18. In a multilevel queue scheduling system, is a process permanently assigned to a
single ready queue, or can it move from one to another?
19. Take the book’s example of a multilevel feedback queue for example: Queue 0,
quantum = 8; queue 1, quantum = 16; queue 2, FCFS scheduling. Under what
circumstances would a process be demoted from queue 1 to queue 2?
20. Take the book’s example of a multilevel feedback queue for example: Queue 0,
quantum = 8; queue 1, quantum = 16; queue 2, FCFS scheduling. Under what
circumstances would it be reasonable to promote a process from queue 1 to queue
0?
21. Take the book’s example of a multilevel feedback queue for example. What
statement can be made about the relative priorities of all processes in the
scheduling system, relative to their queue membership?
22. For homogeneous symmetric multiprocessing, from an O/S point of view, what
must exist in order to support load sharing by means of cooperative scheduling?
23. In a hard real time system a job can make a request for a system service and
include a maximum completion time. Under this scenario, what are the possible,
immediate system responses, and what do they signify?
24. From a scheduling algorithm point of view, the central characteristic of a soft real
time operating system can be described in a phrase of two words, or also less
specifically, by a single word. Give such a characterization.
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25. What does the phrase “a user process running in the kernel” mean?
Part II. Ch. 7, Process Synchronization, through section 7.5.
26. Let 2 unsynchronized threads have access to a shared variable, count. Let there
be a section of code, A: reg1 = count; reg1 = reg1 + 1; count = reg1. Let there be
another section of code, B: reg2 = count; reg2 = reg2 – 1; count = reg2. If one
thread runs A while the other thread runs B, and these code segments are nonatomic, what is the set of all possible outcomes for the variable count?
27. Give the 3 requirements for a correct solution to the critical section problem.
28. Suppose you try to manage concurrency of 2 threads in a critical section by means
of a single variable that keeps track of which thread’s turn it is. Let the turn be
arbitrarily set at the beginning to indicate that it’s one or the other thread’s turn,
and let it be reset to the other thread’s turn when a given thread leaves the critical
section. Which of the 3 requirements for a correct solution to the critical section
problem is violated by this solution, and how?
29. Suppose you try to manage concurrency of 2 threads in a critical section by means
of 2 flags, one for each thread. Let each thread set its flag to true when trying to
enter the critical section. Let successfully entering of the critical section depend
on the other thread’s flag not being true. Let each thread set its flag to false when
leaving the critical section. Which of the 3 requirements for a correct solution to
the critical section problem is violated by this solution, and how?
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30. A correct solution to the critical section problem is shown below. Under this
solution, if 2 threads both want to enter the critical section, state concisely, in
terms of the execution sequence of one of the lines shown in the solution, which
thread will yield to the other.
public void enteringCriticalSection(int t)
{
int other = 1 - t;
if (t == 0)
flag0 = true;
else
flag1 = true;
turn = other;
if (t == 0)
{
while ( (flag1 == true) && (turn == other) )
Thread.yield();
}
else
{
while ( (flag0 == true) && (turn == other) )
Thread.yield();
}
}
31. The functionality of a hardware testandset() instruction is roughly apparent from
its name. What characteristic does such an instruction have that makes it useful in
implementing synchronization?
32. From a hardware point of view, assuming it’s possible in the architecture, critical
sections can be protected by disabling interrupts for the duration of that section of
code. From the operating system point of view, what does this disabling of
interrupts accomplish?
33. Will disabling interrupts protect critical sections in all environments? If not, in
what environments does it work, and in what environments does it not work?
34. Various scenarios can be described as deadlock. What are the minimum number
of resources and the minimum number of processes needed to illustrate the
classical definition of deadlock?
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Let the following pseudo-code definition of semaphore operations, taken from the
book, be given for the next few questions.
P(S)
{
while(S <= 0)
{
;
S--;
}
V(S)
{
S++;
}
35. Although not indicated syntactically, what has to be true of these implementations
of P() and V() in order for this to be a correct solution to synchronization using
the semaphore S?
36. In user code, show how calls to P() and V() would be used to protect a critical
section.
37. Make a statement about the possibility or meaning of the following 3 situations
under the definitions given above.
a. S > 0
b. S == 0
c. S < 0
38. What is the disadvantage of the semaphore solution given above?
Don’t overlook the questions on the back of this page.
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Let the following pseudo-code definition of semaphore operations, taken from the
book, be given for the next few questions.
P(S)
{
value--;
if(value < 0)
{
add process to list;
block;
}
}
V(S)
{
value++;
if(value <= 0)
{
remove process P from list;
wakeup(P);
}
}
39. Make a statement about the possibility or meaning of the following 3 situations
under the definitions given above.
a. S > 0
b. S == 0
c. S < 0
40. There is a relationship between the queuing discipline for the list mentioned in the
definitions given above and one of the requirements for a correct solution to the
critical section problem. What is it?