5.8 Consider a system implementing multilevel queue scheduling. What

strategy can a computer user employ to maximize the amount of CPU time allocated to the user’s process?

Answer: The program could maximize the CPU time allocated to it by not fully utilizing its time quantums. It could use a large fraction of its assigned quantum, but relinquish the CPU before the end of the quantum, thereby increasing the priority associated with the process.

5.9 Consider a preemptive priority scheduling algorithm based on dynami-cally changing priorities. Larger priority numbers imply higher priority.

When a process is waiting for theCPU(in the ready queue, but not run-ning), its priority changes at a rate ␣; when it is running, its priority changes at a rate␤. All processes are given a priority of 0 when they enter the ready queue. The parameters␣ and ␤ can be set to give many different scheduling algorithms.

a. What is the algorithm that results from␤ > ␣ > 0?

b. What is the algorithm that results from␣ < ␤ < 0?

Answer:

a. FCFS b. LIFO

5.10 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 Answer:

a. FCFS—discriminates against short jobs since any short jobs arriv-ing after long jobs will have a longer waitarriv-ing time.

b. RR—treats all jobs equally (giving them equal bursts ofCPUtime) so short jobs will be able to leave the system faster since they will finish first.

c. Multilevel feedback queues—work similar to theRRalgorithm—

they discriminate favorably toward short jobs.

5.11 Using the WindowsXPscheduling algorithm, what is the numeric pri-ority of a thread for the following scenarios?

a. A thread in the REALTIME PRIORITY CLASS with a relative pri-ority of HIGHEST.

b. A thread in the NORMAL PRIORITY CLASS with a relative pri-ority of NORMAL.

c. A thread in the HIGH PRIORITY CLASS with a relative priority of ABOVE NORMAL.

Answer:

a. 26 b. 8 c. 14

5.12 Consider the scheduling algorithm in the Solaris operating system for time sharing threads:

a. What is the time quantum (in milliseconds) for a thread with priority 10? With priority 55?

b. Assume a thread with priority 35 has used its entire time quantum without blocking. What new priority will the scheduler assign this thread?

c. Assume a thread with priority 35 blocks for I/O before its time quantum has expired. What new priority will the scheduler assign this thread?

Answer:

a. 160 and 40 b. 35

c. 54

5.13 The traditionalUNIXscheduler enforces an inverse relationship between priority numbers and priorities: The higher the number, the lower the priority. The scheduler recalculates process priorities once per second using the following function:

Priority = (RecentCPUusage / 2) + Base

where base = 60 and recentCPUusage refers to a value indicating how often a process has used theCPUsince priorities were last recalculated.

Assume that recentCPUusage for process P1 is 40, process P2 is 18, and process P3 is 10. What will be the new priorities for these three processes when priorities are recalculated? Based on this information, does the traditionalUNIXscheduler raise or lower the relative priority of aCPU-bound process?

Answer: The priorities assigned to the processes are 80, 69, and 65 respectively. The scheduler lowers the relative priority of CPU-bound processes.

6

C H A P T E R

Process

Synchronization

Chapter 6 is concerned with the topic of process synchronization among con-currently executing processes. Concurrency is generally very hard for students to deal with correctly, and so we have tried to introduce it and its problems with the classic process coordination problems: mutual exclusion, bounded-buffer, readers/writers, and so on. An understanding of these problems and their solutions is part of current operating-system theory and development.

We first use semaphores and monitors to introduce synchronization tech-niques. Next, Java synchronization is introduced to further demonstrate a language-based synchronization technique. We conclude with a discussion of how contemporary operating systems provide features for process synchro-nization and thread safety.

Exercises

6.1 The first known correct software solution to the critical-section problem for two processes was developed by Dekker. The two processes, P0and P1, share the following variables:

boolean flag[2]; /* initially false */

int turn;

The structure of process Pi(i== 0 or 1) is shown in Figure 6.25; the other process is Pj (j== 1 or 0). Prove that the algorithm satisfies all three requirements for the critical-section problem.

Answer: This algorithm satisfies the three conditions of mutual ex-clusion. (1) Mutual exclusion is ensured through the use of the flag and turn variables. If both processes set their flag to true, only one will succeed. Namely, the process whose turn it is. The waiting process can only enter its critical section when the other process updates the value of turn. (2) Progress is provided, again through the flag and turn variables. This algorithm does not provide strict alternation. Rather, if a 33

process wishes to access their critical section, it can set their flag vari-able to true and enter their critical section. It only sets turn to the value of the other process upon exiting its critical section. If this process wishes to enter its critical section again - before the other process - it repeats the process of entering its critical section and setting turn to the other process upon exiting. (3) Bounded waiting is preserved through the use of the TTturn variable. Assume two processes wish to enter their respec-tive critical sections. They both set their value of flag to true, however only the thread whose turn it is can proceed, the other thread waits.

If bounded waiting were not preserved, it would therefore be possible that the waiting process would have to wait indefinitely while the first process repeatedly entered - and exited - its critical section. However, Dekker’s algorithm has a process set the value of turn to the other pro-cess, thereby ensuring that the other process will enter its critical section next.

6.2 The first known correct software solution to the critical-section problem for n processes with a lower bound on waiting of n − 1 turns was presented by Eisenberg and McGuire. The processes share the following variables:

enum pstate{idle, want in, in cs};

pstate flag[n];

int turn;

All the elements offlagare initiallyidle;the initial value ofturnis immaterial (between 0 andn-1). The structure of process Piis shown in Figure 6.26. Prove that the algorithm satisfies all three requirements for the critical-section problem.

Answer: This algorithm satisfies the three conditions. Before we show that the three conditions are satisfied, we give a brief explanation of what the algorithm does to ensure mutual exclusion. When a process i requires access to critical section, it first sets itsflagvariable to want in to indicate its desire. It then performs the following steps: (1) It ensures that all processes whose index lies between turn and i are idle. (2) If so, it updates its flagto in cs and checks whether there is already some other process that has updated its flag to in cs. (3) If not and if it is this process’s turn to enter the critical section or if the process indicated by theturnvariable is idle, it enters the critical section. Given the above description, we can reason about how the algorithm satisfies the requirements in the following manner:

• Mutual exclusion is ensured: Notice that a process enters the critical section only if the following requirements is satisfied: no other process has itsflagvariable set to in cs. Since the process sets its ownflagvariable set to in cs before checking the status of other processes, we are guaranteed that no two processes will enter the critical section simultaneously.

• Progress requirement is satisfied: Consider the situation where multiple processes simultaneously set theirflagvariables to in cs and then check whether there is any other process has theflag

Exercises 35