Chapter 5 CPU Scheduling Chapter 5 CPU Scheduling

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

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

Alternating Sequence of CPU And I/O Bursts Operating System Concepts – 7 th Edition,

Histogram of CPU-burst Times Operating System Concepts – 7 th Edition, Feb 2, 2005

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

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

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

Optimization Criteria n Max CPU utilization n Max throughput n Min turnaround time n

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

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

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

Example of Non-Preemptive SJF Process Arrival Time P 1 0. 0 7 P 2

Example of Preemptive SJF Process Arrival Time P 1 0. 0 7 P 2

Determining Length of Next CPU Burst n Can only estimate the length n Can

Prediction of the Length of the Next CPU Burst Operating System Concepts – 7

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

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

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

Example of RR with Time Quantum = 20 Process Burst Time P 1 53

Time Quantum and Context Switch Time Operating System Concepts – 7 th Edition, Feb

Turnaround Time Varies With The Time Quantum Operating System Concepts – 7 th Edition,

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

Multilevel Queue Scheduling Operating System Concepts – 7 th Edition, Feb 2, 2005 5.

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

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

Multilevel Feedback Queues Operating System Concepts – 7 th Edition, Feb 2, 2005 5.

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

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

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

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

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

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

Solaris 2 Scheduling Operating System Concepts – 7 th Edition, Feb 2, 2005 5.

Solaris Dispatch Table Operating System Concepts – 7 th Edition, Feb 2, 2005 5.

Windows XP Priorities Operating System Concepts – 7 th Edition, Feb 2, 2005 5.

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

The Relationship Between Priorities and Time-slice length Operating System Concepts – 7 th Edition,

List of Tasks Indexed According to Prorities Operating System Concepts – 7 th Edition,

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

5. 15 Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 41

5. 08 Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 43

In-5. 7 Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 44

In-5. 8 Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 45

In-5. 9 Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 46

Dispatch Latency Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 47

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

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

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

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

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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 Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 2 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 3 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 6 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 7 Silberschatz, Galvin and Gagne © 2005

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 time-sharing environment) Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 8 Silberschatz, Galvin and Gagne © 2005

Optimization Criteria n Max CPU utilization n Max throughput n Min turnaround time n Min waiting time n Min response time Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 9 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 10 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 11 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 12 Silberschatz, Galvin and Gagne © 2005

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 P 4 12 16 n Average waiting time = (0 + 6 + 3 + 7)/4 = 4 Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 13 Silberschatz, Galvin and Gagne © 2005

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 16 n Average waiting time = (9 + 1 + 0 +2)/4 = 3 Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 14 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 15 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 17 Silberschatz, Galvin and Gagne © 2005

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 Operating System Concepts – 7 th Edition, Feb 2, 2005 5. 18 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 19 Silberschatz, Galvin and Gagne © 2005

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 P 3 37 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 – 7 th Edition, Feb 2, 2005 5. 20 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 23 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 25 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 26 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 28 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 29 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 30 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 31 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 32 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 37 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 40 Silberschatz, Galvin and Gagne © 2005

End of Chapter 5

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 – 7 th Edition, Feb 2, 2005 5. 48 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 49 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 50 Silberschatz, Galvin and Gagne © 2005

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 – 7 th Edition, Feb 2, 2005 5. 51 Silberschatz, Galvin and Gagne © 2005