Chapter 5: CPU Scheduling - Operating Systems | CAP332 … · Operating System Concepts – 8 th Edition 5.2 Silberschatz, Galvin and Gagne ©2009 Chapter 5: CPU Scheduling Basic

Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition,

Chapter 5: CPU Scheduling

5.2 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition

Chapter 5: CPU Scheduling

Basic Concepts Scheduling Criteria Scheduling Algorithms Thread Scheduling Multiple-Processor Scheduling Linux Example


Objectives

To introduce CPU scheduling, which is the basis for multiprogrammed operating systems

To describe various CPU-scheduling algorithms To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a

particular system


Basic Concepts

The objective of multiprogramming is to have some process running at all time, to Maximum CPU utilization.

CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait

Process execution begins with a CPU burst that is followed by an I/O burst, which is followed by another CPU burst , then another I/O burst , and so on,.. The final CPU burst ends the process.

CPU burst distribution large number of short CPU bursts and a small number of long

CPU bursts. An I/O –bound program has many short CPU bursts. A CPU –bound program has few long CPU bursts.


Histogram of CPU-burst Times


Alternating Sequence of CPU And I/O Bursts


CPU Scheduler

When the CPU becomes idle, the OS must Select from among the processes in memory that are ready to execute, and allocates the CPU to one of them.

The selection process is carried out by the short-term scheduler (CPU scheduler ).

CPU scheduling decisions may take place when a process:1. Switches from running state to the waiting state(result of I/o request or

wait for the termination of one of the child processes).2. Switches from running state to ready state(interrupt).3. Switches from waiting state to ready state(completion of I/O)4. Terminates

Scheduling under 1 and 4 is nonpreemptive or cooperative. All other scheduling is preemptive


Diagram of Process State


Preemptive scheduling

Under nonpreemptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating or by switching to the waiting state.

Windows 95 and all subsequent versions of windows OS have used preemptive scheduling.


Dispatcher

The Dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler; this involves: switching context switching to user mode jumping to the proper location in the user program to restart

that program It should be fast. Dispatch latency – the time it takes for the dispatcher to stop one

process and start another running


Scheduling Criteria

CPU utilization – keep the CPU as busy as possible. Throughput – # of processes that complete their execution per

time unit(10 processes/second) Turnaround time – amount of time to execute a particular

process(the interval from the time of submission of a process to the time of completion, waiting to get into memory, waiting in the ready queue, exciting on the CPU, doing I/O).

Waiting time – the amount of times a process has been waiting in the ready queue

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)


Scheduling Algorithm Optimization Criteria

Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time


First-Come, First-Served (FCFS) Scheduling Jobs are scheduled in order of arrival When a process enters the ready queue, its PCB is linked onto the tail

of the queue. When the CPU is free, it is allocated to the process at the head of the

queue (the running process is then removed from the queue). Disadvantages: Non-preemptive : once the CPU is allocated to a process, the process

keeps the CPU until it releases it, either by terminating or requesting I/O. The average waiting time is often quite long.


example

Process Burst TimeP1 24P2 3P3 3

Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is:

Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17

P1 P2 P3

24 27 300


FCFS Scheduling (Cont)

Suppose that the processes arrive in the orderP2 , P3 , P1

The Gantt chart for the schedule is:

Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case Convoy effect as short processes go behind long process lower

CPU and device utilization.

P1P3P2

63 300


Shortest-Job-First (SJF) Scheduling

This algorithm Associates with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time, if the next CPU bursts of two processes are the same, FCFS scheduling is used.

Two schemes: Nonpreemptive – once CPU given to the process it

cannot be preempted until completes 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 known as the Shortest-Remaining-Time-First (SRTF)


Examples of SJFExample1:

Process Arrival Time Burst TimeP1 0.0 6P2 2.0 8P3 4.0 7P4 5.0 3

SJF scheduling chart

Average waiting time = (3 + 16 + 9 + 0) / 4 = 7

Compare with FCFS AWT=(0+6+14+21)/4=10.25

P4 P3P1

3 160 9

P2

24



Example2: Process Arrival Time Burst TimeP1 0 7P2 2 4P3 4 1P4 5 4

Non preemptive SJF

P1 P3 P2

7P1(7)

160

P4

8 12

Average waiting time = (0 + 6 + 3 + 7)/4 = 4

2 4 5

P2(4)

P3(1)

P4(4)

P1‘s wating time = 0





Shortest-Job-First (SJF) Scheduling Example3:

Process Arrival Time Burst TimeP1 0 7

P2 2 4P3 4 1P4 5 4

Preemptive SJF(SRTF)

P1 P3P2

42 110

P4

5 7

P2 P1

16

Average waiting time = (9 + 1 + 0 +2)/4 = 3

P1(7)P2(4)

P3(1)

P4(4)





P1(5)P2(2)



SJF is optimal – gives minimum average waiting time for a given set of processes The difficulty is knowing the length of the next CPU

request.


Prediction of the Length of the Next CPU Burst


Priority Scheduling

A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest

integer highest priority in Unix but lowest in Java) Equal-priority processes are scheduled in FCFS order.

Preemptive: preempt the CPU if the priority of the newly arrived process is higher than the priority of the currently running process.

Nonpreemptive : put the new process at the head of the ready queue. SJF is a priority scheduling where priority is the predicted next CPU burst

time Problem Starvation – low priority processes may never execute Solution Aging – as time progresses increase the priority of the process(for example : 1 every 15 minutes)


Priority Scheduling

Example : Process Burst Time priorityP1 10 3P2 1 1P3 2 4P4 1 5p5 5 2

The AWT is (6 +0+ 16+18+1)/5 = 8.2

All arrived at time 0.The Gantt chart for the schedule is:

P2 P1P5

1 160 6

P3

1918

P4


Priority Scheduling

Example:Process arrival time Burst length PriorityP1 0 10 3P2 0 1 1P3 0 2 4P4 0 1 5P5 3 5 2

Gantt chart: Non-preemptive priority scheduling

0 1 11 16 18 19 Gantt chart: Preemptive priority scheduling

0 1 3 8 16 18 19

P4P3P5P1P2

P4P3P1P5P1P2


Round Robin (RR)

Is designed especially for time-sharing systems. Similar to FCFS, but it is Preemptive to enable

the system to switch between processes. Each process gets a small unit of CPU time (time

quantum or time slice), usually 10-100 milliseconds.

The Ready queue is FIFO (new processes are added to the tail of the queue.)

The CPU scheduler picks the first process from the ready queue ,set a timer to interrupt after 1 time quantum, and dispatch the process.


Round Robin (RR) One of two things will happen

The process may have a CPU burst of < 1 time quantum the process itself will release the CPU voluntarily.

The CPU burst of the currently running process > 1 time quantum the timer will go off and will cause an interrupt to the OS. a context switch will be executed, and the process will be put at the tail of the ready queue.

The CPU scheduler will then select the next process in the ready queue.

Typically, higher average turnaround than SJF, but better response


Round Robin (RR)Example1:Time quantum = 4

Process Burst TimeP1 24P2 3P3 3

The Gantt chart is:

AWT(6(10-4)+4+7)/3 = 5.66

P1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30


Round Robin (RR) Example2: Time quantum = 20

Process Burst Time Wait TimeP1 53 57 +24 = 81P2 17 20P3 68 37 + 40 + 17= 94P4 24 57 + 40 = 97

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

Average wait time = (81+20+94+97)/4 = 73

57

20

37

57

24

40

40

17

P1(53)

P2(17)

P3(68)P4(24)

P1(33) P1(13)

P3(48) P3(28) P3(8)

P4(4)


Round Robin (RR) 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 until its next time quantum. (Ex: 5 processes, TQ = 20 milliseconds, each process will get up to 20 milliseconds every 100 milliseconds.

The Performance of RR depends heavily on the size of the TQ. TQ large FCFS TQsmall TQ must be large (but not too large)with

respect to context switch time, otherwise overhead is too high


Time Quantum and Context Switch Time


Turnaround Time Varies With The Time Quantum

The average TurnAroundTime of a set of process does not necessarily improve as the TQ size increase. The AVG TAT can be improved if most process finish their next CPU burst in a single time quantum.


Multilevel Queue Processes are classified into different groups. Each group have different response-time requirements different scheduling

needs. A multilevel queue scheduling algorithm partitions the Ready queue 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 preemptive 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


Multilevel Queue Scheduling


Multilevel Feedback Queue Implement multiple ready queues

Different queues may be scheduled using different algorithms Just like multilevel queue scheduling, but assignments are not

static

Multilevel feedback queue-scheduling algorithm allows a process to 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 and downgrade a

process The most general CPU-scheduling algorithm. The most complex algorithm.


Example of Multilevel Feedback Queue

Three queues: Q0 – RR with time quantum 8 milliseconds Q1 – RR time quantum 16 milliseconds Q2 – FCFS

Scheduling A new job enters queue Q0 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 Q1.

At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.

AT Q2 job is served FCFS only when queue 0 and queue 1 are empty.


Multilevel Feedback Queues


Thread Scheduling Distinction between user-level and kernel-level threads On OSs that support them, it is the kernel-level threads-

not processes- that are being scheduled by OS. User-level threads are managed by a thread library and

the kernel is unaware of them. To run on CPU, the user level threads must be mapped to

an associated kernel-level thread. It may use a lightweight process(LWP).

contention scope: one distinction between user-level and kernel-level

threads lies in how they are scheduled.


Thread Scheduling

Many-to-one and many-to-many models, thread library schedules user-level threads to run on LWP. Known as process-contention scope (PCS) since scheduling competition takes place among threads belonging to the same process.

PCS is done according to preempt priority. PTHREAD SCOPE PROCESS schedules threads using PCS

scheduling. Kernel thread scheduled onto available CPU is system-contention

scope (SCS) – competition takes place among all threads in system Systems using the one-to-one model schedule threads using only

SCS. PTHREAD SCOPE SYSTEM schedules threads using SCS

scheduling.


Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Different rules for homogeneous processors (Identical processors in terms of

their functionality) or heterogeneous processors. Asymmetric multiprocessing: All scheduling decisions, I/O processing, and other system activities handled by

a single processor – the master server. The other processors execute only user code. Simple because only one processor accesses the system data structures,

reducing the need for data sharing. Symmetric multiprocessing (SMP): each processor is self-scheduling, all processes in common ready queue, or

each has its own private queue of ready processes Multiple processors try to access and update a common data structures. So,

scheduler must be programmed carefully. Must ensure that 2 processors don’t choose the same process.


Linux Scheduling

Linux Scheduler is a preemptive, priority-based algorithm with 2 separate priority ranges: Two priority ranges: time-sharing and real-time A real-time range from 0 to 99 Longer time quantum A nice value ranging from 100 to 140 Shorter time quantum


Linux Scheduling

The kernel maintains a list of all runnable tasks in a runqueue data structure.

Each runqueue contains two priority arrays : Active :

contains all tasks with time remaining in their time slices

Expired :contains all expired tasks.


List of Tasks Indexed According to Priorities

The scheduler chooses the task with the highest priority from the active array for execution on the CPU.

When the active array is empty the 2 arrays are exchanged (the expired array becomes the active array, and vice versa).


Algorithm Evaluation

More examples P: 214


Conclusion

We’ve looked at a number of different scheduling algorithms.

Which one works the best is application dependent. General purpose OS will use priority based, round

robin, preemptive Real Time OS will use priority, no preemption.

Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition,

End of Chapter 5

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Chapter 5: CPU Scheduling - Operating Systems | CAP332 … · Operating System Concepts – 8 th Edition 5.2 Silberschatz, Galvin and Gagne ©2009 Chapter 5: CPU Scheduling Basic