Chapter 6: CPU Scheduling. Basic Concepts

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1 Chater 6: CPU Scheduling Basic Concets Scheduling Criteria Scheduling Algorithms Multile-Processor Scheduling Real-Time Scheduling Thread Scheduling Oerating Systems Examles Java Thread Scheduling Algorithm Evaluation 6.1 Silberschatz, Galvin and Gagne 2003 Basic Concets Maximum CPU utilization obtained with multirogramming CPU I/O Burst Cycle Process execution consists of a cycle of CPU execution and I/O wait CPU burst distribution 6.2 Silberschatz, Galvin and Gagne

2 Alternating Sequence of CPU And I/O Bursts 6.3 Silberschatz, Galvin and Gagne 2003 Histogram of CPU-burst Times 6.4 Silberschatz, Galvin and Gagne

3 CPU Scheduler Selects from among the rocesses in memory that are ready to execute, and allocates the CPU to one of them CPU scheduling decisions may take lace when a rocess: 1. Switches from running to waiting state 2. Switches from running to ready state 3. Switches from waiting to ready 4. Terminates Scheduling under 1 and 4 is nonreemtive All other scheduling is reemtive 6.5 Silberschatz, Galvin and Gagne 2003 Disatcher Disatcher module gives control of the CPU to the rocess selected by the short-term scheduler; this involves: switching context switching to user mode juming to the roer location in the user rogram to restart that rogram Disatch latency time it takes for the disatcher to sto one rocess and start another running 6.6 Silberschatz, Galvin and Gagne

4 Scheduling Criteria CPU utilization kee the CPU as busy as ossible Throughut # of rocesses that comlete their execution er time unit Turnaround time amount of time to execute a articular rocess Waiting time amount of time a rocess has been waiting in the ready queue Resonse time amount of time it takes from when a request was submitted until the first resonse is roduced, not outut (for time-sharing environment) 6.7 Silberschatz, Galvin and Gagne 2003 Otimization Criteria Max CPU utilization Max throughut Min turnaround time Min waiting time Min resonse time 6.8 Silberschatz, Galvin and Gagne

5 First-Come, First-Served (FCFS) Scheduling Process Burst Time P 1 24 P 2 3 P 3 3 Suose that the rocesses 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 = Silberschatz, Galvin and Gagne 2003 FCFS Scheduling (Cont.) Suose that the rocesses 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 revious case Convoy effect short rocess behind long rocess 6.10 Silberschatz, Galvin and Gagne

6 Shortest-Job-First (SJR) Scheduling Associate with each rocess the length of its next CPU burst. Use these lengths to schedule the rocess with the shortest time Two schemes: nonreemtive once CPU given to the rocess it cannot be reemted until comletes its CPU burst reemtive if a new rocess arrives with CPU burst length less than remaining time of current executing rocess, reemt. This scheme is know as the Shortest-Remaining-Time-First (SRTF) SJF is otimal gives minimum average waiting time for a given set of rocesses 6.11 Silberschatz, Galvin and Gagne 2003 Examle of Non-Preemtive SJF Process Arrival Time Burst Time P P P P SJF (non-reemtive) P 1 P 3 P 2 P Average waiting time = ( )/4 = Silberschatz, Galvin and Gagne

7 Examle of Preemtive SJF Process Arrival Time Burst Time P P P P SJF (reemtive) P 1 P 2 P 3 P 2 P 4 P Average waiting time = ( )/4 = Silberschatz, Galvin and Gagne 2003 Determining Length of Next CPU Burst Can only estimate the length Can be done by using the length of revious CPU bursts, using exonential averaging t n = actual lenght of n τ n + 1 = redicted value for the next CPU α, 0 α 1 Define : τ = α t + 1 α τ n= n th CPU burst ( ). 1 n burst 6.14 Silberschatz, Galvin and Gagne

8 Prediction of the Length of the Next CPU Burst 6.15 Silberschatz, Galvin and Gagne 2003 Examles of Exonential Averaging α =0 α =1 τ n+1 = τ n Recent history does not count τ n+1 = t n Only the actual last CPU burst counts If we exand the formula, we get: τ n+1 = α t n +(1 - α) α t n (1 - α ) j α t n (1 - α ) n+1 t n τ 0 Since both α and (1 - α) are less than or equal to 1, each successive term has less weight than its redecessor 6.16 Silberschatz, Galvin and Gagne

9 Priority Scheduling A riority number (integer) is associated with each rocess The CPU is allocated to the rocess with the highest riority (smallest integer highest riority) Preemtive nonreemtive SJF is a riority scheduling where riority is the redicted next CPU burst time Problem Starvation low riority rocesses may never execute Solution Aging as time rogresses increase the riority of the rocess 6.17 Silberschatz, Galvin and Gagne 2003 Round Robin (RR) Each rocess gets a small unit of CPU time (time quantum), usually milliseconds. After this time has elased, the rocess is reemted and added to the end of the ready queue. If there are n rocesses in the ready queue and the time quantum is q, then each rocess gets 1/n of the CPU time in chunks of at most q time units at once. No rocess waits more than (n-1)q time units. Performance q large FIFO q small q must be large with resect to context switch, otherwise overhead is too high 6.18 Silberschatz, Galvin and Gagne

10 Examle of RR with Time Quantum = 20 Process Burst Time P 1 53 P 2 17 P 3 68 P 4 24 The Gantt chart is: P 1 P 2 P 3 P 4 P 1 P 3 P 4 P 1 P 3 P Tyically, higher average turnaround than SJF, but better resonse 6.19 Silberschatz, Galvin and Gagne 2003 Time Quantum and Context Switch Time 6.20 Silberschatz, Galvin and Gagne

11 Turnaround Time Varies With The Time Quantum 6.21 Silberschatz, Galvin and Gagne 2003 Multilevel Queue Ready queue is artitioned into searate queues: foreground (interactive) background (batch) Each queue has its own scheduling algorithm foreground RR background FCFS Scheduling must be done between the queues Fixed riority 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 rocesses; i.e., 80% to foreground in RR 20% to background in FCFS 6.22 Silberschatz, Galvin and Gagne

12 Multilevel Queue Scheduling 6.23 Silberschatz, Galvin and Gagne 2003 Multilevel Feedback Queue A rocess can move between the various queues; aging can be imlemented this way Multilevel-feedback-queue scheduler defined by the following arameters: number of queues scheduling algorithms for each queue method used to determine when to ugrade a rocess method used to determine when to demote a rocess method used to determine which queue a rocess will enter when that rocess needs service 6.24 Silberschatz, Galvin and Gagne

13 Examle of Multilevel Feedback Queue Three queues: Q 0 time quantum 8 milliseconds Q 1 time quantum 16 milliseconds Q 2 FCFS 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 comlete, it is reemted and moved to queue Q Silberschatz, Galvin and Gagne 2003 Multilevel Feedback Queues 6.26 Silberschatz, Galvin and Gagne

14 Multile-Processor Scheduling CPU scheduling more comlex when multile CPUs are available Homogeneous rocessors within a multirocessor Load sharing Asymmetric multirocessing only one rocessor accesses the system data structures, alleviating the need for data sharing 6.27 Silberschatz, Galvin and Gagne 2003 Real-Time Scheduling Hard real-time systems required to comlete a critical task within a guaranteed amount of time Soft real-time comuting requires that critical rocesses receive riority over less fortunate ones 6.28 Silberschatz, Galvin and Gagne

15 Disatch Latency 6.29 Silberschatz, Galvin and Gagne 2003 Algorithm Evaluation Deterministic modeling takes a articular redetermined workload and defines the erformance of each algorithm for that workload Queueing models Imlementation 6.30 Silberschatz, Galvin and Gagne

16 Single Queue W T S customers waiting line T = W + S 6.31 Silberschatz, 2004 D.A. Galvin Menascé and Gagne 2003 Single server. Examle of An Analytic Model M/G/1 Queue Arrival rocess is Poisson (interarrival times are exonentially distributed). Service time is arbitrarily distributed. Where Second moment of the CPU time 2 2 λ E[ S ] ρe[ S](1 + Cs ) T = E[ S ] + = E[ S] + 2(1 ρ ) 2(1 ρ) ρ = λe[ S] < 1 Average CPU time (utilization) 6.32 Silberschatz, 2004 D.A. Galvin Menascé and Gagne

17 Little s Law N X R The average number of customers in a black box is equal to the average time sent in the box multilied by the throughut of the box. N = R x X 6.33 Silberschatz, 2004 D.A. Galvin Menascé and Gagne 2003 M/G/1 with Priorities P static riorities (= 1,, P). P is the highest riority. FCFS within each riority queue. λ 1 λ 2 λ P 6.34 Silberschatz, 2004 D.A. Galvin Menascé and Gagne

18 M/G/1 with Non-Preemtive Priorities W0 W = (1 Π )(1 Π ) W ρ = Π 0 1 = 2 i= = 1 = 1 = 1 = P P λ P i E[ S ρ λ E[ S 2 i ] = ] P + 1 ρ 6.35 Silberschatz, 2004 D.A. Galvin Menascé and Gagne M/G/1 with Non-Preemtive Priorities Average Waiting Time Total Utilization W1 W2 W Silberschatz, 2004 D.A. Galvin Menascé and Gagne

19 M/G/1 with Preemtive Resume Priorities T Π = = E[ S P i= ρ ](1 Π i (1 Π ) + P i= )(1 Π λ E[ S i + 1 ) 2 i ]/ Silberschatz, 2004 D.A. Galvin Menascé and Gagne 2003 Comaring Preemtive vs. Non- Preemtive M/G/1 Queues a b c d e f g=b*f Nonreemtive Preemtive Priority Lambda E[S] ρ Π E[S2] T T Silberschatz, 2004 D.A. Galvin Menascé and Gagne

20 Evaluation of CPU Schedulers by Simulation 6.39 Silberschatz, Galvin and Gagne 2003 Solaris 2 Scheduling 6.40 Silberschatz, Galvin and Gagne

21 Windows XP Priorities Priority Classes Relative Priority 6.41 Silberschatz, Galvin and Gagne 2003 Linux Scheduling Two algorithms: time-sharing and real-time Time-sharing Prioritized credit-based rocess with most credits is scheduled next Credit subtracted when timer interrut occurs When credit = 0, another rocess chosen When all rocesses have credit = 0, recrediting occurs Real-time Based on factors including riority and history Soft real-time Posix.1b comliant two classes FCFS and RR Highest riority rocess always runs first 6.42 Silberschatz, Galvin and Gagne

22 Thread Scheduling Local Scheduling How the threads library decides which thread to ut onto an available LWP Global Scheduling How the kernel decides which kernel thread to run next 6.43 Silberschatz, Galvin and Gagne 2003 Pthread Scheduling API #include <thread.h> #include <stdio.h> #define NUM THREADS 5 int main(int argc, char *argv[]) { int i; thread t tid[num THREADS]; thread attr t attr; /* get the default attributes */ thread attr init(&attr); /* set the scheduling algorithm to PROCESS or SYSTEM */ thread attrsetscoe(&attr, PTHREAD SCOPE SYSTEM); /* set the scheduling olicy - FIFO, RT, or OTHER */ thread attrsetschedolicy(&attr, SCHED OTHER); /* create the threads */ for (i = 0; i < NUM THREADS; i++) thread create(&tid[i],&attr,runner,null); 6.44 Silberschatz, Galvin and Gagne

23 Pthread Scheduling API /* now join on each thread */ for (i = 0; i < NUM THREADS; i++) thread join(tid[i], NULL); } /* Each thread will begin control in this function */ void *runner(void *aram ) { rintf("i am a thread\n"); thread exit(0); } 6.45 Silberschatz, Galvin and Gagne 2003 Java Thread Scheduling JVM Uses a Preemtive, Priority-Based Scheduling Algorithm FIFO Queue is Used if There Are Multile Threads With the Same Priority 6.46 Silberschatz, Galvin and Gagne

24 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 Secify Whether Threads are Time- Sliced or Not 6.47 Silberschatz, Galvin and Gagne 2003 Time-Slicing Since the JVM Doesn t Ensure Time-Slicing, the yield() Method May Be Used: while (true) { // erform CPU-intensive task... Thread.yield(); } This Yields Control to Another Thread of Equal Priority 6.48 Silberschatz, Galvin and Gagne

25 Thread Priorities Priority Thread.MIN_PRIORITY Thread.MAX_PRIORITY Thread.NORM_PRIORITY Comment Minimum Thread Priority Maximum Thread Priority Default Thread Priority Priorities May Be Set Using setpriority() method: setpriority(thread.norm_priority + 2); 6.49 Silberschatz, Galvin and Gagne

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