Chapter 5 CPU Scheduling Operating System Concepts with

Chapter 5: CPU Scheduling Operating System Concepts with Java – 8 th Edition 5. 1 Silberschatz, Galvin and Gagne © 2009

Chapter 5: CPU Scheduling n Basic Concepts n Scheduling Criteria n Scheduling Algorithms n Thread Scheduling n Multiple-Processor Scheduling n Operating Systems Examples n Algorithm Evaluation Operating System Concepts with Java – 8 th Edition 5. 2 Silberschatz, Galvin and Gagne © 2009

Objectives n To introduce CPU scheduling, which is the basis for multiprogrammed operating systems n To describe various CPU-scheduling algorithms n To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a particular system Operating System Concepts with Java – 8 th Edition 5. 3 Silberschatz, Galvin and Gagne © 2009

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 with Java – 8 th Edition 5. 4 Silberschatz, Galvin and Gagne © 2009

Histogram of CPU-burst Times Operating System Concepts with Java – 8 th Edition 5. 5 Silberschatz, Galvin and Gagne © 2009

Alternating Sequence of CPU And I/O Bursts Operating System Concepts with Java – 8 th Edition 5. 6 Silberschatz, Galvin and Gagne © 2009

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 n All other scheduling is preemptive Operating System Concepts with Java – 8 th Edition 5. 7 Silberschatz, Galvin and Gagne © 2009

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 with Java – 8 th Edition 5. 8 Silberschatz, Galvin and Gagne © 2009

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 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 timesharing environment) Operating System Concepts with Java – 8 th Edition 5. 9 Silberschatz, Galvin and Gagne © 2009

Scheduling Algorithm Optimization Criteria n Max CPU utilization n Max throughput n Min turnaround time n Min waiting time n Min response time Operating System Concepts with Java – 8 th Edition 5. 10 Silberschatz, Galvin and Gagne © 2009

First-Come, First-Served (FCFS) Scheduling Process CPU Burst Time (msecs) 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 (AWT): (0 + 24 + 27)/3 = 17 msecs Operating System Concepts with Java – 8 th Edition 5. 11 Silberschatz, Galvin and Gagne © 2009

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 P 3 P 1 0 3 6 n Waiting time for P 1 = 6; P 2 = 0; P 3 = 3 30 n Average waiting time: (6 + 0 + 3)/3 = 3 n Much better than previous case n Thus Average Waiting Time under FCFS is not minimal and may vary substantially if the CPU-burst times vary greatly. Operating System Concepts with Java – 8 th Edition 5. 12 Silberschatz, Galvin and Gagne © 2009

FCFS Scheduling (Cont. ) n The FCFS algorithm is non-preemptive (once the CPU has been allocated to a process, the process keeps the CPU till it terminates or requests I/O). n This algorithm is not well suited to a time-sharing system where it is important that each user get a share of the CPU at regular intervals. Operating System Concepts with Java – 8 th Edition 5. 13 Silberschatz, Galvin and Gagne © 2009

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 SJF is optimal – gives minimum average waiting time for a given set of processes l The difficulty is knowing the length of the next CPU request Operating System Concepts with Java – 8 th Edition 5. 14 Silberschatz, Galvin and Gagne © 2009

Example of SJF Process Arrival Time Burst Time P 1 0. 0 6 P 2 2. 0 8 P 3 4. 0 7 P 4 5. 0 3 n SJF Gantt chart: P 1 P 4 0 3 P 3 9 P 2 16 24 n Average waiting time (AWT) = (3 + 16 + 9 + 0) / 4 = 7 msecs Operating System Concepts with Java – 8 th Edition 5. 15 Silberschatz, Galvin and Gagne © 2009

With FCFS Process Arrival Time Burst Time P 1 0. 0 6 P 2 2. 0 8 P 3 4. 0 7 P 4 5. 0 3 n FCFS Gannt chart: P 2 P 1 0 6 P 3 14 P 4 21 24 n Average waiting time (AWT) = (0 + 6 + 14 + 21) / 4 = 10. 25 msecs Operating System Concepts with Java – 8 th Edition 5. 16 Silberschatz, Galvin and Gagne © 2009

SJF n SJF provides minimum average waiting time for a given set of processes. n It decreases the waiting time of a short process more than it increases the waiting time of a long process by moving a short process ahead of a long process. n Drawback: It is not possible to know exactly the length of next CPU burst and so SFJ is not used much as a short-term scheduler. However, estimates are made of the next CPU burst based on past history using a formula. Operating System Concepts with Java – 8 th Edition 5. 17 Silberschatz, Galvin and Gagne © 2009

Determining Length of Next CPU Burst n Can only estimate the length n Can be done by using the length of previous CPU bursts, using exponential averaging Operating System Concepts with Java – 8 th Edition 5. 18 Silberschatz, Galvin and Gagne © 2009

Prediction of the Length of the Next CPU Burst Operating System Concepts with Java – 8 th Edition 5. 19 Silberschatz, Galvin and Gagne © 2009

Examples of Exponential Averaging n =0 l n+1 = n l Recent history does not count n =1 l l n+1 = tn 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 + … +(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 with Java – 8 th Edition 5. 20 Silberschatz, Galvin and Gagne © 2009

Preemptive SJF n Suppose a new process arrives at the ready queue while a previous process is executing. The new process could have a shorter next CPU burst than what is left of the currently executing process. The preemptive SJF will preempt the currently executing process. n Note: non-preemptive SJF will allow the currently executing process to complete its CPU burst and then reevaluate the scheduling of the new process along with the others still in the ready queue. Operating System Concepts with Java – 8 th Edition 5. 21 Silberschatz, Galvin and Gagne © 2009

Example of Preemptive SJF Process. Arrival Time Burst Time P 1 0 8 P 2 21 4 P 3 4. 2 9 P 4 5. 3 5 n SJF Gantt chart: P 2 P 1 P 4 P 1 P 3 17 26 10 5 n Average waiting time (AWT) = ((10 – 1) + (1 - 1) + (17 – 2) + (5 – 3)) / 4 = 26 / 4 = 6. 5 msecs 0 – 1 Operating System Concepts with Java – 8 th Edition 5. 22 Silberschatz, Galvin and Gagne © 2009

Compare with Non-Preemptive SJF Process. Arrival Time Burst Time P 1 0 8 P 2 21 4 P 3 4. 2 9 P 4 5. 3 5 n SJF Gantt chart: P 1 P 2 P 4 P 3 17 26 12 8 n Average waiting time (AWT) = ((0 + (8 - 1) + (17 – 2) + (12 – 3)) / 4 = 31 / 4 = 7. 75 msecs 0 – 1 Operating System Concepts with Java – 8 th Edition 5. 23 Silberschatz, Galvin and Gagne © 2009

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). Equal priority processes are scheduled in FCFS order l Preemptive l Nonpreemptive n SJF is a priority scheduling where priority is the predicted next CPU burst time n Problem Starvation – low priority process may never execute because a steady stream of high-priority processes will prevent it from ever getting the CPU n Solution Aging – as time progresses increase the priority of the process (e. g. increase priority by 1 every 15 minutes) Operating System Concepts with Java – 8 th Edition 5. 24 Silberschatz, Galvin and Gagne © 2009

Example of Priority Scheduling Assume the following processes arrive at time 0 in the order P 1, P 2, . . P 5 and priority 1 is the highest. Process. Arrival Priority Burst Time P 1 3 10 P 2 21 1 P 3 4. 3 2 P 4 5. 4 1 P 5 2 5 n SJF Gantt chart: P 5 P 2 n 0 1 P 3 P 1 16 6 P 4 18 19 n Average waiting time (AWT) = (6 + 0 + 16 + 18 + 1) / 4 = 41 / 5 = 8. 2 msecs Operating System Concepts with Java – 8 th Edition 5. 25 Silberschatz, Galvin and Gagne © 2009

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. 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 with Java – 8 th Edition 5. 26 Silberschatz, Galvin and Gagne © 2009

Example of RR with Time Quantum = 4 Process Burst Time P 1 P 2 P 3 24 3 3 n The Gantt chart is: P 1 0 P 2 4 P 3 7 P 1 10 P 1 14 P 1 18 22 P 1 26 P 1 30 n AWT = ((10 – 4) + 4 + 7) / 3 = 17 / 3 = 5. 66 msecs n Typically, higher average turnaround than SJF, but better response n If a process’ CPU burst exceeds 1 time quantum, it is preempted and put back in ready queue. Thus RR is preemptive. Operating System Concepts with Java – 8 th Edition 5. 27 Silberschatz, Galvin and Gagne © 2009

Round Robin (RR) n Performance depends on the time quantum. If time quantum is too short, then there will be too many context switches. n If time quantum is too large, then RR degenerates into FCFS. n Time quanta ranges from 10 – 100 millisecs and context switch is generally < 10 microseconds. n The time quanta should be large with respect to context switch time but not too large. Operating System Concepts with Java – 8 th Edition 5. 28 Silberschatz, Galvin and Gagne © 2009

Time Quantum and Context Switch Time Operating System Concepts with Java – 8 th Edition 5. 29 Silberschatz, Galvin and Gagne © 2009

Turnaround Time Varies With The Time Quantum Operating System Concepts with Java – 8 th Edition 5. 30 Silberschatz, Galvin and Gagne © 2009

Multilevel Queue n Ready queue is partitioned into separate queues: foreground (interactive) 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; i. e. , 80% to foreground in RR l 20% to background in FCFS Operating System Concepts with Java – 8 th Edition 5. 31 Silberschatz, Galvin and Gagne © 2009

Multilevel Queue Scheduling Operating System Concepts with Java – 8 th Edition 5. 32 Silberschatz, Galvin and Gagne © 2009

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 with Java – 8 th Edition 5. 33 Silberschatz, Galvin and Gagne © 2009

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 FCFS. 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 FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2. Operating System Concepts with Java – 8 th Edition 5. 34 Silberschatz, Galvin and Gagne © 2009

Multilevel Feedback Queues Operating System Concepts with Java – 8 th Edition 5. 35 Silberschatz, Galvin and Gagne © 2009

Thread Scheduling n Distinction between user-level and kernel-level threads n Many-to-one and many-to-many models, thread library schedules user- level threads to run on LWP l Known as process-contention scope (PCS) since scheduling competition is within the process n Kernel thread scheduled onto available CPU is system-contention scope (SCS) – competition among all threads in system Operating System Concepts with Java – 8 th Edition 5. 36 Silberschatz, Galvin and Gagne © 2009

Algorithm Evaluation – An Example n 5 processes arrive at time 0, in the order given. Consider FCFS, SJF, RR (quantum = 10 msecs) scheduling algorithms. Which algorithm would give the minimum average waiting time? Process Arrival P 1 Burst Time 10 P 2 2 29 P 3 4. 3 P 4 5. 7 P 5 12 FCFS P 2 P 1 0 10 P 3 39 P 5 P 4 42 49 61 AWT = (0 + 10 + 39 + 42 + 49) / 5 = 140 / 5 = 28 msecs Operating System Concepts with Java – 8 th Edition 5. 37 Silberschatz, Galvin and Gagne © 2009

Algorithm Evaluation – An Example SJF (non-preemptive) P 3 P 4 P 1 P 5 P 2 3 32 20 0 10 AWT = (10 + 32 + 0 + 3 + 20) / 5 = 65 / 5 = 13 msecs 61 RR (time quantum = 10 msecs) P 1 P 2 P 3 P 4 P 5 P 2 23 20 30 40 50 52 10 AWT = (0 + (52 – 10) + 20 + 23 + (50 – 10)) / 5 = 115 / 5 = 23 msecs 0 61 Result: SJF gives the least average waiting time. RR yields intermediate result. Operating System Concepts with Java – 8 th Edition 5. 38 Silberschatz, Galvin and Gagne © 2009

Pthread Scheduling n API allows specifying either PCS or SCS during thread creation l PTHREAD SCOPE PROCESS schedules threads using PCS scheduling l PTHREAD SCOPE SYSTEM schedules threads using SCS scheduling. Operating System Concepts with Java – 8 th Edition 5. 39 Silberschatz, Galvin and Gagne © 2009

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 with Java – 8 th Edition 5. 40 Silberschatz, Galvin and Gagne © 2009

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 with Java – 8 th Edition 5. 41 Silberschatz, Galvin and Gagne © 2009

Multiple-Processor Scheduling n CPU scheduling more complex when multiple CPUs are available n Homogeneous processors within a multiprocessor n Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing n Symmetric multiprocessing (SMP) – each processor is self- scheduling, all processes in common ready queue, or each has its own private queue of ready processes n Processor affinity – process has affinity for processor on which it is currently running l soft affinity l hard affinity Operating System Concepts with Java – 8 th Edition 5. 42 Silberschatz, Galvin and Gagne © 2009

NUMA and CPU Scheduling Operating System Concepts with Java – 8 th Edition 5. 43 Silberschatz, Galvin and Gagne © 2009

Multicore Processors n Recent trend to place multiple processor cores on same physical chip n Faster and consume less power n Multiple threads per core also growing l Takes advantage of memory stall to make progress on another thread while memory retrieve happens Operating System Concepts with Java – 8 th Edition 5. 44 Silberschatz, Galvin and Gagne © 2009

Multithreaded Multicore System Operating System Concepts with Java – 8 th Edition 5. 45 Silberschatz, Galvin and Gagne © 2009

Operating System Examples n Solaris scheduling n Windows XP scheduling n Linux scheduling Operating System Concepts with Java – 8 th Edition 5. 46 Silberschatz, Galvin and Gagne © 2009

Solaris Dispatch Table Operating System Concepts with Java – 8 th Edition 5. 47 Silberschatz, Galvin and Gagne © 2009

Solaris Scheduling Operating System Concepts with Java – 8 th Edition 5. 48 Silberschatz, Galvin and Gagne © 2009

Windows XP Priorities Operating System Concepts with Java – 8 th Edition 5. 49 Silberschatz, Galvin and Gagne © 2009

Linux Scheduling n Constant order O(1) scheduling time n Two priority ranges: time-sharing and real-time n Real-time range from 0 to 99 and nice value from 100 to 140 n (figure 5. 15) Operating System Concepts with Java – 8 th Edition 5. 50 Silberschatz, Galvin and Gagne © 2009

Priorities and Time-slice length Operating System Concepts with Java – 8 th Edition 5. 51 Silberschatz, Galvin and Gagne © 2009

List of Tasks Indexed According to Priorities Operating System Concepts with Java – 8 th Edition 5. 52 Silberschatz, Galvin and Gagne © 2009

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 with Java – 8 th Edition 5. 53 Silberschatz, Galvin and Gagne © 2009

Evaluation of CPU schedulers by Simulation Operating System Concepts with Java – 8 th Edition 5. 54 Silberschatz, Galvin and Gagne © 2009

End of Chapter 5 Operating System Concepts with Java – 8 th Edition 5. 55 Silberschatz, Galvin and Gagne © 2009

5. 08 Operating System Concepts with Java – 8 th Edition 5. 56 Silberschatz, Galvin and Gagne © 2009

In-5. 7 Operating System Concepts with Java – 8 th Edition 5. 57 Silberschatz, Galvin and Gagne © 2009

In-5. 8 Operating System Concepts with Java – 8 th Edition 5. 58 Silberschatz, Galvin and Gagne © 2009

In-5. 9 Operating System Concepts with Java – 8 th Edition 5. 59 Silberschatz, Galvin and Gagne © 2009

Dispatch Latency Operating System Concepts with Java – 8 th Edition 5. 60 Silberschatz, Galvin and Gagne © 2009

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 with Java – 8 th Edition 5. 61 Silberschatz, Galvin and Gagne © 2009

Java Thread Scheduling (Cont. ) n 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 with Java – 8 th Edition 5. 62 Silberschatz, Galvin and Gagne © 2009

Time-Slicing n Since the JVM Doesn’t Ensure Time-Slicing, the yield() Method May Be Used: while (true) { // perform CPU-intensive task. . . Thread. yield(); } n This Yields Control to Another Thread of Equal Priority Operating System Concepts with Java – 8 th Edition 5. 63 Silberschatz, Galvin and Gagne © 2009

Thread Priorities Priority Comment Thread. MIN_PRIORITY Minimum Thread Priority Thread. MAX_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 with Java – 8 th Edition 5. 64 Silberschatz, Galvin and Gagne © 2009

Solaris 2 Scheduling Operating System Concepts with Java – 8 th Edition 5. 65 Silberschatz, Galvin and Gagne © 2009