CPU Scheduling Algorithms

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1 CPU Scheduling Algorithms Notice: The slides for this lecture have been largely based on those from the course text Operating Systems Concepts, 9th ed., by Silberschatz, Galvin, and Gagne. Many, if not all, the illustrations contained in this presentation come from this source. Revised by X.M. from notes by Perrone. CSCI 315 Operating Systems Design 1

2 Scheduling Algorithms CSCI 315 Operating Systems Design 2

3 First-Come, First-Served (FCFS) Process Burst Time P 1 24 P 2 3 P 3 3 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 P Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = 17 CSCI 315 Operating Systems Design 3

4 Issues with FCFS Suppose that the processes arrive in the order P 2, P 3, P 1 The Gantt chart for the schedule is: P 2 P 3 P Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3 Average waiting time: ( )/3 = 3 Much better than previous case. Convoy effect: all process are stuck waiting until a long process terminates. CSCI 315 Operating Systems Design 4

5 Shortest-Job-First (SJF) Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time. Two schemes: Nonpreemptive once CPU given to a process it cannot be preempted until completing its CPU burst. 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). SJF is optimal gives minimum average waiting time for a given set of processes. Question: Is this practical? How can one determine the length of a CPU-burst? CSCI 315 Operating Systems Design 5

6 Non-Preemptive SJF Process Arrival Time Burst Time P P P P SJF (non-preemptive) P 1 P 3 P 2 P Average waiting time = ( )/4 = 4 CSCI 315 Operating Systems Design 6

7 Preemptive SJF Process Arrival Time Burst Time P P P P SJF (preemptive) P 1 P 2 P 3 P 4 P 2 P Average waiting time = ( )/4 = 3 CSCI 315 Operating Systems Design 7

8 Determining Length of Next CPU-Burst We can only estimate the length. This can be done by using the length of previous CPU bursts, using exponential averaging: ( ) n τ = α t + 1 α τ n + 1 n th 1. tn = actual lenght of n CPU burst 2. τ = predicted value for the CPU burst at time n 3. 0 α 1 4. The effect of the value of α? n CSCI 315 Operating Systems Design 8

9 Prediction of the Length of the Next CPU-Burst The graph is shown when α is 0.5 CSCI 315 Operating Systems Design 9

10 Class Exercise Given the actual CPU bursts are 6, 4, 6, 4, 13, 13, 13, and the initial estimate of τ is 10 as in previous slide, show the first three predictions when α takes the value of When α is 0.2, estimates are 9.2, 8.16, 7.73 When α is 0.7, estimates are 7.2, 4.96, 5.69 CSCI 315 Operating Systems Design 10

11 Priority Scheduling A priority number (integer) is associated with each process. The CPU is allocated to the process with the highest priority (typically, smallest integer highest priority) Preemptive Nonpreemptive SJF is a priority scheduling where priority is the predicted next CPUburst time. Problem: Starvation low priority processes may never execute. Solution: Aging as time progresses increase the priority of the process. CSCI 315 Operating Systems Design 11

12 Process Priority in Linux Priority scheduling is commonly used in production OSes such as Linux In Linux, the priority values range from -20 (most favorite) to 20 (least favorite) Try ps al command on a Linux terminal We can run a CPU intensive job and use the nice command to set its priority, or renice command to change its priority. CSCI 315 Operating Systems Design 12

13 practice]\$./a.out & lectures]\$ ps l F UID PID PPID PRI NI VSZ RSS WCHAN STAT TTY TIME COMMAND n_tty_ Ss+ pts/1 0:01 -bin/tcsh R+ pts/0 0:00 ps l hrtime S pts/1 0:00./a.out lectures]\$ renice : old priority 0, new priority 10 lectures]\$ ps l hrtime SN pts/1 0:00./a.out CSCI 315 Operating Systems Design 13

14 Round Robin (RR) Each process gets a small unit of CPU time (time quantum), usually milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. 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. CSCI 315 Operating Systems Design 14

15 RR with Time Quantum = 20 The Gantt chart is: Process Burst Time P 1 53 P 2 17 P 3 68 P 4 24 P 1 P 2 P 3 P 4 P 1 P 3 P 4 P 1 P 3 P Typically, higher average turnaround than SJF, but better response. CSCI 315 Operating Systems Design 15

16 Time Quantum and Context Switch Time Question: What influences the choice of value for the quantum? CSCI 315 Operating Systems Design 16

17 Turnaround Time Varies with the Time Quantum CSCI 315 Operating Systems Design 17

18 Performance of RR Effects of the quantum length q: q large FIFO. q small q must be large with respect to context switch, otherwise overhead is too high. If q is extremely small, and we ignore the context switch cost, the result is processor sharing. CSCI 315 Operating Systems Design 18

19 Multilevel Queue Ready queue is partitioned into separate queues: foreground (interactive) background (batch) Each queue has its own scheduling algorithm. foreground: RR background: FCFS Scheduling must be done between the queues: Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation. Time slice each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR. 20% to background in FCFS. CSCI 315 Operating Systems Design 19

20 Multilevel Queue Scheduling CSCI 315 Operating Systems Design 20

21 Multilevel Feedback Queue A process can move between the various queues; aging can be implemented this way. Multilevel-feedback-queue scheduler defined by the following parameters: number of queues, scheduling algorithms for each queue, method used to determine when to upgrade a process, method used to determine when to demote a process, method used to determine which queue a process will enter when that process needs service. CSCI 315 Operating Systems Design 21

22 Three queues: Example of Multilevel Feedback Queue Q 0 time quantum 8 milliseconds (most favorite queue) Q 1 time quantum 16 milliseconds Q 2 FCFS (least favorite queue) Scheduling A new job enters queue Q 0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1. At Q 1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2. CSCI 315 Operating Systems Design 22

23 Multilevel Feedback Queues CSCI 315 Operating Systems Design 23

24 Linux Scheduling Linux maintains 140 run queues, one for each priority level Two sets of queues, queues 0-99 for real time processes queues for regular processes The priority values can be set by the system call nice (2) only the super users can decrement the nice values Different priority gives different CPU quanta CSCI 315 Operating Systems Design 24

25 Determine Quantum Values Calculate quantum q = (140 SP) 20 if SP < 120 q = (140 SP) 5 if SP 120 where SP is the static priority Higher priority process get longer quanta Basic idea: important processes should run longer Other mechanisms used for quick interactive response CSCI 315 Operating Systems Design 25

26 Typical Quantum Values Static Pri Nice Quantum Highest static ms High ms Normal ms Low static ms Lowest static ms CSCI 315 Operating Systems Design 26

27 POSIX Thread Scheduling A process could contain multiple threads or a single thread. POSIX thread scheduling defines scope of thread scheduling. PTHREAD_SCOPE_SYSTEM : a thread contends for CPU as if it were a process PTHREAD_SCOPE_PROCESS : all threads in a process are grouped together to contend for CPU CSCI 315 Operating Systems Design 27

29 Other Scheduling Parameters Other scheduling parameters can be set or examined (get) using the pthread library calls pthread_getschedparam() and pthread_setschedparam(). The priority and scheduling policy are meaningful only within the threads that are in the same scope. CSCI 315 Operating Systems Design 29

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