Chapter 5 CPU Scheduling Chapter 5 CPU Scheduling

Chapter 5: CPU Scheduling

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 CSCI 380 - Operating System 5. 2

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 CSCI 380 - Operating System 5. 3

Alternating Sequence of CPU And I/O Bursts CSCI 380 - Operating System 5. 4

Histogram of CPU-burst Times CSCI 380 - Operating System 5. 5

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 l What does that mean? CSCI 380 - Operating System 5. 6

Dispatcher n Dispatcher module does the actual giving of 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 CSCI 380 - Operating System 5. 7

Scheduling Criteria n Must consider how to schedule, not as simple as you might think 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 (human nature to want this approach) 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) CSCI 380 - Operating System 5. 8

Optimization Criteria n Max CPU utilization n Max throughput n Min turnaround time n Min waiting time n Min response time CSCI 380 - Operating System 5. 9

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 n Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 n Average waiting time: (0 + 24 + 27)/3 = 17 CSCI 380 - Operating System 5. 10 P 3 27 30

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 CSCI 380 - Operating System 5. 11

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 (if average waiting time was your only criteria) CSCI 380 - Operating System 5. 12

Example of Non-Preemptive SJF Process Arrival Time P 1 0. 0 7 P 2 2. 0 4 P 3 4. 0 1 P 4 5. 0 4 Burst Time n SJF (non-preemptive) P 1 0 3 P 3 7 P 2 8 n Average waiting time = (0 + 6 + 3 + 7)/4 = 4 CSCI 380 - Operating System 5. 13 P 4 12 16

Example of Preemptive SJF Process Arrival Time P 1 0. 0 7 P 2 2. 0 4 P 3 4. 0 1 P 4 5. 0 4 Burst Time n SJF (preemptive) P 1 0 P 2 2 P 3 4 P 2 5 P 4 P 1 11 7 n Average waiting time = (9 + 1 + 0 +2)/4 = 3 CSCI 380 - Operating System 5. 14 16

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 CSCI 380 - Operating System 5. 15

Prediction of the Length of the Next CPU Burst CSCI 380 - Operating System 5. 16

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 n Problem Starvation – low priority processes may never execute n Solution Aging – as time progresses increase the priority of the process CSCI 380 - Operating System 5. 18

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 CSCI 380 - Operating System 5. 19

Example of RR with Time Quantum = 20 Process Burst Time P 1 53 P 2 17 P 3 68 P 4 24 n The Gantt chart is: P 1 0 P 2 20 37 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 CSCI 380 - Operating System 5. 20

Time Quantum and Context Switch Time CSCI 380 - Operating System 5. 21

Turnaround Time Varies With The Time Quantum CSCI 380 - Operating System 5. 22

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 CSCI 380 - Operating System 5. 23

Multilevel Queue Scheduling CSCI 380 - Operating System 5. 24

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 CSCI 380 - Operating System 5. 25

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. CSCI 380 - Operating System 5. 26

Multilevel Feedback Queues CSCI 380 - Operating System 5. 27

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 n Symmetric multiprocessing (SMP) – each processor has it’s own scheduler. May or may not share ready que. l Current OS’s implement this n What happens with multiple processors and their corresponding caches? Isn’t it inefficient to have processes move? CSCI 380 - Operating System 5. 28

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 CSCI 380 - Operating System 5. 30

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); CSCI 380 - Operating System 5. 31

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); } CSCI 380 - Operating System 5. 32

Operating System Examples n Windows XP scheduling n Linux scheduling n Solaris scheduling CSCI 380 - Operating System 5. 33

Windows XP Priorities Reference p. 177 & 178 What’s it really doing? Lowest Number gets the CPU CSCI 380 - Operating System mixture of previous algorithms 5. 36

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 CSCI 380 - Operating System 5. 37

The Relationship Between Priorities and Time-slice length CSCI 380 - Operating System 5. 38

List of Tasks Indexed According to Prorities CSCI 380 - Operating System 5. 39

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 CSCI 380 - Operating System 5. 40

5. 15 CSCI 380 - Operating System 5. 41

End of Chapter 5

5. 08 CSCI 380 - Operating System 5. 43

In-5. 7 CSCI 380 - Operating System 5. 44

In-5. 8 CSCI 380 - Operating System 5. 45

In-5. 9 CSCI 380 - Operating System 5. 46

Dispatch Latency CSCI 380 - Operating System 5. 47

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 CSCI 380 - Operating System 5. 48

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 CSCI 380 - Operating System 5. 49

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 CSCI 380 - Operating System 5. 50

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); CSCI 380 - Operating System 5. 51