Chapter 5 CPU Scheduling Chapter 5 CPU Scheduling

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Chapter 5: CPU Scheduling

Chapter 5: CPU Scheduling

Chapter 5: CPU Scheduling n Basic Concepts n Scheduling Criteria n Scheduling Algorithms n

Chapter 5: CPU Scheduling n Basic Concepts n Scheduling Criteria n Scheduling Algorithms n Multiple-Processor Scheduling n Real-Time Scheduling n Thread Scheduling n Operating Systems Examples n Java Thread Scheduling n Algorithm Evaluation Operating System Concepts 5. 2 Silberschatz, Galvin and Gagne © 2005

Basic Concepts n Maximum CPU utilization obtained with multiprogramming n CPU–I/O Burst Cycle –

Basic Concepts n Maximum CPU utilization obtained with multiprogramming n CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait n CPU burst distribution Operating System Concepts 5. 3 Silberschatz, Galvin and Gagne © 2005

Alternating Sequence of CPU And I/O Bursts Operating System Concepts 5. 4 Silberschatz, Galvin

Alternating Sequence of CPU And I/O Bursts Operating System Concepts 5. 4 Silberschatz, Galvin and Gagne © 2005

Histogram of CPU-burst Times Operating System Concepts 5. 5 Silberschatz, Galvin and Gagne ©

Histogram of CPU-burst Times Operating System Concepts 5. 5 Silberschatz, Galvin and Gagne © 2005

Bursts of CPU usage alternate with periods of waiting for I/O (a) A CPU-bound

Bursts of CPU usage alternate with periods of waiting for I/O (a) A CPU-bound process. (b) An I/O-bound process. Operating System Concepts 5. 6 Silberschatz, Galvin and Gagne © 2005

CPU Scheduler n Selects from among the processes in memory that are ready to

CPU Scheduler n Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them n CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state 2. Switches from running to ready state 3. Switches from waiting to ready 4. Terminates n Scheduling under 1 and 4 is nonpreemptive (non harming scheduling) n All other scheduling is preemptive (harming scheduling) Operating System Concepts 5. 7 Silberschatz, Galvin and Gagne © 2005

Dispatcher n Dispatcher module gives control of the CPU to the process selected by

Dispatcher n Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: l switching context l switching to user mode l jumping to the proper location in the user program to restart that program n Dispatch latency – time it takes for the dispatcher to stop one process and start another running Operating System Concepts 5. 8 Silberschatz, Galvin and Gagne © 2005

Scheduling Criteria n CPU utilization – keep the CPU as busy as possible n

Scheduling Criteria n CPU utilization – keep the CPU as busy as possible n Throughput – # of processes that complete their execution per time unit n Turnaround time – amount of time to execute a particular process (texec+twait = tturnaraound) n Waiting time – amount of time a process has been waiting in the ready queue n Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) Operating System Concepts 5. 9 Silberschatz, Galvin and Gagne © 2005

Optimization Criteria n Max CPU utilization : l 100% cpu utilization is perfect n

Optimization Criteria n Max CPU utilization : l 100% cpu utilization is perfect n Max throughput : l Max number of processes completed / time unit n Min turnaround time : l min average time elapsed from when process is submitted to when it has completed. n Min waiting time : l min average time a process spends in the run queue. n Min response time : l min Average time elapsed from when process is submitted until useful output is obtained. Operating System Concepts 5. 10 Silberschatz, Galvin and Gagne © 2005

Types of Schedulers n First-Come, First-Served (FCFS) n Round-Robin (RR) n Shortest-Job-First (SJF) n

Types of Schedulers n First-Come, First-Served (FCFS) n Round-Robin (RR) n Shortest-Job-First (SJF) n Priority Scheduling (PS) Operating System Concepts 5. 11 Silberschatz, Galvin and Gagne © 2005

First-Come, First-Served (FCFS) Scheduling Process Burst Time P 1 24 P 2 3 P

First-Come, First-Served (FCFS) Scheduling Process Burst Time P 1 24 P 2 3 P 3 3 n Suppose that the processes arrive in the order: P 1 , P 2 , P 3 The Gantt Chart for the schedule is: P 1 P 2 0 24 P 3 27 30 n Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 n Average waiting time: (0 + 24 + 27)/3 = 17 Operating System Concepts 5. 12 Silberschatz, Galvin and Gagne © 2005

FCFS Scheduling (Cont. ) Suppose that the processes arrive in the order P 2

FCFS Scheduling (Cont. ) Suppose that the processes arrive in the order P 2 , P 3 , P 1 n The Gantt chart for the schedule is: P 2 0 P 3 3 P 1 6 30 n Waiting time for P 1 = 6; P 2 = 0; P 3 = 3 n Average waiting time: (6 + 0 + 3)/3 = 3 n Much better than previous case n Convoy effect short process behind long process Operating System Concepts 5. 13 Silberschatz, Galvin and Gagne © 2005

Shortest-Job-First (SJF) Scheduling n Associate with each process the length of its next CPU

Shortest-Job-First (SJF) Scheduling n Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time n Two schemes: l nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst l preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF) n SJF is optimal – gives minimum average waiting time for a given set of processes Operating System Concepts 5. 14 Silberschatz, Galvin and Gagne © 2005

Example of Non-Preemptive SJF Process Arrival Time Burst Time P 1 0. 0 7

Example of Non-Preemptive SJF Process Arrival Time Burst Time P 1 0. 0 7 P 2 2. 0 4 P 3 4. 0 1 P 4 5. 0 4 n SJF (non-preemptive) P 1 0 3 P 3 7 P 2 8 P 4 12 16 n Average waiting time = (0 + 6 + 3 + 7)/4 = 4 Operating System Concepts 5. 15 Silberschatz, Galvin and Gagne © 2005

Example of Preemptive SJF Process Arrival Time Burst Time P 1 0. 0 7

Example of Preemptive SJF Process Arrival Time Burst Time P 1 0. 0 7 P 2 2. 0 4 P 3 4. 0 1 P 4 5. 0 4 n SJF (preemptive) P 1 0 P 2 2 P 3 4 P 2 5 P 4 P 1 11 7 16 n Average waiting time = (9 + 1 + 0 +2)/4 = 3 Operating System Concepts 5. 16 Silberschatz, Galvin and Gagne © 2005

Determining Length of Next CPU Burst (Exponential Average) n Can only estimate the length

Determining Length of Next CPU Burst (Exponential Average) n Can only estimate the length n Can be done by using the length of previous CPU bursts, using exponential averaging Operating System Concepts 5. 17 Silberschatz, Galvin and Gagne © 2005

Prediction of the Length of the Next CPU Burst (exponential average) dg alpha 0.

Prediction of the Length of the Next CPU Burst (exponential average) dg alpha 0. 5 Operating System Concepts 5. 18 Silberschatz, Galvin and Gagne © 2005

Examples of Exponential Averaging n =0 n+1 = n l Recent history does not

Examples of Exponential Averaging n =0 n+1 = n l Recent history does not count n =1 l n+1 = tn l Only the actual last CPU burst counts n If we expand the formula, we get: n+1 = tn+(1 - ) tn -1 + … +(1 - )j tn -j + … l +(1 - )n +1 0 n Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor Operating System Concepts 5. 19 Silberschatz, Galvin and Gagne © 2005

Priority Scheduling n A priority number (integer) is associated with each process n The

Priority Scheduling n A priority number (integer) is associated with each process n The CPU is allocated to the process with the highest priority (smallest integer highest priority) l Preemptive l nonpreemptive n SJF is a priority scheduling where priority is the predicted next CPU burst time priority 1 n Same / static priority assignment cause problem called : starvation priority 2 priority M Operating System Concepts 5. 20 Silberschatz, Galvin and Gagne © 2005

Priority Queuing Operating System Concepts 5. 21 21 Silberschatz, Galvin and Gagne © 2005

Priority Queuing Operating System Concepts 5. 21 21 Silberschatz, Galvin and Gagne © 2005

Starvation n Problem: l Lower-priority may suffer starvation if there is a steady supply

Starvation n Problem: l Lower-priority may suffer starvation if there is a steady supply of high priority processes. n Solution l Allow a process to change its priority based on its age or execution history (aging) Operating System Concepts 5. 22 Silberschatz, Galvin and Gagne © 2005

Round Robin (RR) n Each process gets a small unit of CPU time (time

Round Robin (RR) n Each process gets a small unit of CPU time (time quantum), usually 10 -100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. n If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units. l E. g, with five processes and a time quantum of 20 milliseconds, each process will get up to 20 milliseconds every 100 milliseconds. n Performance l q large FIFO l q small q must be large with respect to context switch, otherwise overhead is too high Operating System Concepts 5. 23 Silberschatz, Galvin and Gagne © 2005

Example of RR with Time Quantum = 20 Process Burst Time P 1 P

Example of RR with Time Quantum = 20 Process Burst Time P 1 P 2 P 3 P 4 n The Gantt chart is: P 1 0 P 2 20 37 53 17 68 24 P 3 P 4 57 P 1 77 P 3 P 4 P 1 P 3 97 117 121 134 154 162 n Typically, higher average turnaround than SJF, but better response Operating System Concepts 5. 24 Silberschatz, Galvin and Gagne © 2005

Time Quantum and Context Switch Time The way in which a smaller time quantum

Time Quantum and Context Switch Time The way in which a smaller time quantum increases context switches. Operating System Concepts 5. 25 Silberschatz, Galvin and Gagne © 2005

Turnaround Time Varies With The Time Quantum Operating System Concepts 5. 26 Silberschatz, Galvin

Turnaround Time Varies With The Time Quantum Operating System Concepts 5. 26 Silberschatz, Galvin and Gagne © 2005

Multilevel Queue n Ready queue is partitioned into separate queues: l foreground (interactive) l

Multilevel Queue n Ready queue is partitioned into separate queues: l foreground (interactive) l background (batch) n Each queue has its own scheduling algorithm l foreground – RR l background – FCFS n Scheduling must be done between the queues l Fixed priority scheduling; (i. e. , serve all from foreground then from background). Possibility of starvation. l Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; 4 i. e. , Operating System Concepts 80% to foreground in RR, 20% to background in FCFS 5. 27 Silberschatz, Galvin and Gagne © 2005

Multilevel Queue Scheduling Operating System Concepts 5. 28 Silberschatz, Galvin and Gagne © 2005

Multilevel Queue Scheduling Operating System Concepts 5. 28 Silberschatz, Galvin and Gagne © 2005

Multilevel Feedback Queue n A process can move between the various queues; aging can

Multilevel Feedback Queue n A process can move between the various queues; aging can be implemented this way n Multilevel-feedback-queue scheduler defined by the following parameters: l number of queues l scheduling algorithms for each queue l method used to determine when to upgrade a process l method used to determine when to demote a process l method used to determine which queue a process will enter when that process needs service Operating System Concepts 5. 29 Silberschatz, Galvin and Gagne © 2005

Example of Multilevel Feedback Queue n Three queues: l Q 0 – RR with

Example of Multilevel Feedback Queue n Three queues: l Q 0 – RR with time quantum 8 milliseconds l Q 1 – RR time quantum 16 milliseconds l Q 2 – FCFS n Scheduling l A new job enters queue Q 0 which is served RR. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1. l At Q 1 job is again served RR and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2. Operating System Concepts 5. 30 Silberschatz, Galvin and Gagne © 2005

Multilevel Feedback Queues Operating System Concepts 5. 31 Silberschatz, Galvin and Gagne © 2005

Multilevel Feedback Queues Operating System Concepts 5. 31 Silberschatz, Galvin and Gagne © 2005

Multiple-Processor Scheduling n CPU scheduling more complex when multiple CPUs are available n Homogeneous

Multiple-Processor Scheduling n CPU scheduling more complex when multiple CPUs are available n Homogeneous processors within a multiprocessor n Load sharing n Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing Operating System Concepts 5. 32 Silberschatz, Galvin and Gagne © 2005

Real-Time Scheduling n Hard real-time systems – required to complete a critical task within

Real-Time Scheduling n Hard real-time systems – required to complete a critical task within a guaranteed amount of time n Soft real-time computing – requires that critical processes receive priority over less fortunate ones Operating System Concepts 5. 33 Silberschatz, Galvin and Gagne © 2005

Thread Scheduling n Local Scheduling – How the threads library decides which thread to

Thread Scheduling n Local Scheduling – How the threads library decides which thread to put onto an available LWP n Global Scheduling – How the kernel decides which kernel thread to run next Operating System Concepts 5. 34 Silberschatz, Galvin and Gagne © 2005

Pthread Scheduling API #include <pthread. h> #include <stdio. h> #define NUM THREADS 5 int

Pthread Scheduling API #include <pthread. h> #include <stdio. h> #define NUM THREADS 5 int main(int argc, char *argv[]) { int i; pthread t tid[NUM THREADS]; pthread attr t attr; /* get the default attributes */ pthread attr init(&attr); /* set the scheduling algorithm to PROCESS or SYSTEM */ pthread attr setscope(&attr, PTHREAD SCOPE SYSTEM); /* set the scheduling policy - FIFO, RT, or OTHER */ pthread attr setschedpolicy(&attr, SCHED OTHER); /* create threads */ for (i = 0; i < NUM THREADS; i++) pthread create(&tid[i], &attr, runner, NULL); Operating System Concepts 5. 35 Silberschatz, Galvin and Gagne © 2005

Pthread Scheduling API /* now join on each thread */ for (i = 0;

Pthread Scheduling API /* now join on each thread */ for (i = 0; i < NUM THREADS; i++) pthread join(tid[i], NULL); } /* Each thread will begin control in this function */ void *runner(void *param) { printf("I am a threadn"); pthread exit(0); } Operating System Concepts 5. 36 Silberschatz, Galvin and Gagne © 2005

Operating System Examples n Solaris scheduling n Windows XP scheduling n Linux scheduling Operating

Operating System Examples n Solaris scheduling n Windows XP scheduling n Linux scheduling Operating System Concepts 5. 37 Silberschatz, Galvin and Gagne © 2005

Solaris 2 Scheduling Operating System Concepts 5. 38 Silberschatz, Galvin and Gagne © 2005

Solaris 2 Scheduling Operating System Concepts 5. 38 Silberschatz, Galvin and Gagne © 2005

Solaris Dispatch Table Operating System Concepts 5. 39 Silberschatz, Galvin and Gagne © 2005

Solaris Dispatch Table Operating System Concepts 5. 39 Silberschatz, Galvin and Gagne © 2005

Windows XP Priorities Operating System Concepts 5. 40 Silberschatz, Galvin and Gagne © 2005

Windows XP Priorities Operating System Concepts 5. 40 Silberschatz, Galvin and Gagne © 2005

Linux Scheduling n Two algorithms: time-sharing and real-time n Time-sharing Prioritized credit-based – process

Linux Scheduling n Two algorithms: time-sharing and real-time n Time-sharing Prioritized credit-based – process with most credits is scheduled next l Credit subtracted when timer interrupt occurs l When credit = 0, another process chosen l When all processes have credit = 0, recrediting occurs 4 Based on factors including priority and history n Real-time l Soft real-time l l Posix. 1 b compliant – two classes 4 FCFS and RR 4 Highest priority process always runs first Operating System Concepts 5. 41 Silberschatz, Galvin and Gagne © 2005

The Relationship Between Priorities and Time-slice length Operating System Concepts 5. 42 Silberschatz, Galvin

The Relationship Between Priorities and Time-slice length Operating System Concepts 5. 42 Silberschatz, Galvin and Gagne © 2005

List of Tasks Indexed According to Prorities Operating System Concepts 5. 43 Silberschatz, Galvin

List of Tasks Indexed According to Prorities Operating System Concepts 5. 43 Silberschatz, Galvin and Gagne © 2005

Algorithm Evaluation n Deterministic modeling – takes a particular predetermined workload and defines the

Algorithm Evaluation n Deterministic modeling – takes a particular predetermined workload and defines the performance of each algorithm for that workload n Queueing models n Implementation Operating System Concepts 5. 44 Silberschatz, Galvin and Gagne © 2005

5. 15 Operating System Concepts 5. 45 Silberschatz, Galvin and Gagne © 2005

5. 15 Operating System Concepts 5. 45 Silberschatz, Galvin and Gagne © 2005

Tugas n Kerjakan dengan SJF (preeptive dan non Preemptive), FCFS, RR (gunakan time quantum

Tugas n Kerjakan dengan SJF (preeptive dan non Preemptive), FCFS, RR (gunakan time quantum = 2) n Gambar Gantt Chart dan hitung waiting time perproses dan average waiting time Process Arrival Time Burst Time P 1 0. 0 8 P 2 2 4 P 3 1. 0 1 P 4 2 3 P 5 3 2 Operating System Concepts 5. 46 Silberschatz, Galvin and Gagne © 2005

n Kerjakan dengan SJF (preeptive dan non Preemptive), FCFS, RR (gunakan time quantum =

n Kerjakan dengan SJF (preeptive dan non Preemptive), FCFS, RR (gunakan time quantum = 3) l Process Arrival Time Burst Tim P 1 0. 0 8 P 2 2 4 P 3 1. 0 1 P 4 l 1. 0 5 Gambar Gantt Chart dan hitung waiting time perproses dan average waiting time Operating System Concepts 5. 47 Silberschatz, Galvin and Gagne © 2005

End of Chapter 5

End of Chapter 5

5. 08 Operating System Concepts 5. 49 Silberschatz, Galvin and Gagne © 2005

5. 08 Operating System Concepts 5. 49 Silberschatz, Galvin and Gagne © 2005

In-5. 7 Operating System Concepts 5. 50 Silberschatz, Galvin and Gagne © 2005

In-5. 7 Operating System Concepts 5. 50 Silberschatz, Galvin and Gagne © 2005

In-5. 8 Operating System Concepts 5. 51 Silberschatz, Galvin and Gagne © 2005

In-5. 8 Operating System Concepts 5. 51 Silberschatz, Galvin and Gagne © 2005

In-5. 9 Operating System Concepts 5. 52 Silberschatz, Galvin and Gagne © 2005

In-5. 9 Operating System Concepts 5. 52 Silberschatz, Galvin and Gagne © 2005

Dispatch Latency Operating System Concepts 5. 53 Silberschatz, Galvin and Gagne © 2005

Dispatch Latency Operating System Concepts 5. 53 Silberschatz, Galvin and Gagne © 2005

Java Thread Scheduling n JVM Uses a Preemptive, Priority-Based Scheduling Algorithm n FIFO Queue

Java Thread Scheduling n JVM Uses a Preemptive, Priority-Based Scheduling Algorithm n FIFO Queue is Used if There Are Multiple Threads With the Same Priority Operating System Concepts 5. 54 Silberschatz, Galvin and Gagne © 2005

Java Thread Scheduling (cont) JVM Schedules a Thread to Run When: 1. The Currently

Java Thread Scheduling (cont) JVM Schedules a Thread to Run When: 1. The Currently Running Thread Exits the Runnable State 2. A Higher Priority Thread Enters the Runnable State * Note – the JVM Does Not Specify Whether Threads are Time-Sliced or Not Operating System Concepts 5. 55 Silberschatz, Galvin and Gagne © 2005

Time-Slicing Since the JVM Doesn’t Ensure Time-Slicing, the yield() Method May Be Used: while

Time-Slicing Since the JVM Doesn’t Ensure Time-Slicing, the yield() Method May Be Used: while (true) { // perform CPU-intensive task. . . Thread. yield(); } This Yields Control to Another Thread of Equal Priority Operating System Concepts 5. 56 Silberschatz, Galvin and Gagne © 2005

Thread Priorities Priority Comment Thread. MIN_PRIORITY Thread. MAX_PRIORITY Minimum Thread Priority Maximum Thread Priority

Thread Priorities Priority Comment Thread. MIN_PRIORITY Thread. MAX_PRIORITY Minimum Thread Priority Maximum Thread Priority Thread. NORM_PRIORITY Default Thread Priority Priorities May Be Set Using set. Priority() method: set. Priority(Thread. NORM_PRIORITY + 2); Operating System Concepts 5. 57 Silberschatz, Galvin and Gagne © 2005

5. 1 A CPU scheduling algorithm determines an order for the execution of its

5. 1 A CPU scheduling algorithm determines an order for the execution of its scheduled processes. Given n processes to be scheduled on one processor, how many possible different schedules are there? Give a formula in terms of n. 5. 2 Define the difference between preemptive and nonpreemptive scheduling. Operating System Concepts 5. 58 Silberschatz, Galvin and Gagne © 2005

5. 3 Suppose that the following processes arrive for execution at the times indicated.

5. 3 Suppose that the following processes arrive for execution at the times indicated. Each process will run the listed amount of time. In answering the questions, use nonpreemptive scheduling and base all decisions on the information you have at the time the decision must be made. l Process Arrival Time Burst Time P 1 0. 0 8 P 2 0. 4 4 P 3 1. 0 1 a. What is the average turnaround time for these processes with the FCFS scheduling algorithm? b. What is the average turnaround time for these processes with the SJF scheduling algorithm? Operating System Concepts 5. 59 Silberschatz, Galvin and Gagne © 2005

Process Arrival Time Burst Time P 1 0. 0 8 P 2 2 4

Process Arrival Time Burst Time P 1 0. 0 8 P 2 2 4 P 3 1. 0 1 P 4 2 3 P 5 3 2 Operating System Concepts 5. 60 Silberschatz, Galvin and Gagne © 2005

c. The SJF algorithm is supposed to improve performance, but notice that we chose

c. The SJF algorithm is supposed to improve performance, but notice that we chose to run process P 1 at time 0 because we did not know that two shorter processes would arrive soon. Compute what the average turnaround timewill be if the CPU is left idle for the first 1 unit and then SJF scheduling is used. Remember that processes P 1 and P 2 are waiting during this idle time, so their waiting time may increase. This algorithm could be known as future -knowledge scheduling. 5. 4 What advantage is there in having different timequantum sizes on different levels of a multilevel queueing system? Operating System Concepts 5. 61 Silberschatz, Galvin and Gagne © 2005

5. 5 Many CPU-scheduling algorithms are parameterized. For example, : n the RR algorithm

5. 5 Many CPU-scheduling algorithms are parameterized. For example, : n the RR algorithm requires a parameter to indicate the time slice. n 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. n 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). Operating System Concepts 5. 62 Silberschatz, Galvin and Gagne © 2005

n What (if any) relation holds between the following pairs of sets of algorithms?

n What (if any) relation holds between the following pairs of sets of algorithms? l a. Priority and SJF l b. Multilevel feedback queues and FCFS l c. Priority and FCFS l d. RR and SJF 5. 6 Suppose that a scheduling algorithm (at the level of short-term CPU scheduling) favors those 9 Operating System Concepts 5. 63 Silberschatz, Galvin and Gagne © 2005

n http: //www. cs. uic. edu/~jbell/Course. Notes/Operating. Sys tems/5_CPU_Scheduling. html n http: //siber. cankaya.

n http: //www. cs. uic. edu/~jbell/Course. Notes/Operating. Sys tems/5_CPU_Scheduling. html n http: //siber. cankaya. edu. tr/Operating. Systems/ceng 328/ node 122. html n http: //groups. engin. umd. umich. edu/CIS/course. des/cis 4 50/mcfadyen/chapter 5. htm n http: //siber. cankaya. edu. tr/Operating. Systems/week 5/no de 21. html Operating System Concepts 5. 64 Silberschatz, Galvin and Gagne © 2005