P - Operating Systems

Chapter 5: CPU Scheduling

Operating System Concepts – 8th Edition,

Silberschatz, Galvin and Gagne ©2009

Chapter 5: CPU Scheduling  Basic Concepts  Scheduling Criteria  Scheduling Algorithms  Thread Scheduling  Multiple-Processor Scheduling  Linux Example

Operating System Concepts – 8th Edition

5.2


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


5.3


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.


5.4


Histogram of CPU-burst Times


5.5


Alternating Sequence of CPU And I/O Bursts


5.6


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


5.7


Diagram of Process State


5.8


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.


5.9


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


5.10


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 timesharing environment)


5.11


Scheduling Algorithm Optimization Criteria  Max CPU utilization  Max throughput  Min turnaround time  Min waiting time  Min response time


5.12


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.


5.13


example Process Burst Time P1 24 P2 3 P3 3  Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: P1 0

P2 24

P3 27

30

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


5.14


FCFS Scheduling (Cont) Suppose that the processes arrive in the order P2 , P3 , P1  The Gantt chart for the schedule is:

P2 0

P3 3

P1 6

30

 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.


5.15


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)


5.16


Examples of SJF Example1: Process

Arrival Time

Burst Time

P1

0.0

6

P2

2.0

8

P3

4.0

7

P4

5.0

3

 SJF scheduling chart

P4 0

P3

P1 3

9

P2 16

24

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



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


5.17


Shortest-Job-First (SJF) Scheduling Example2:

Process P1 P2 P3 P4  Non preemptive SJF

Arrival Time Burst Time 0 7 2 4 4 1 5 4 Average waiting time = (0 + 6 + 3 + 7)/4 = 4

P1 0

P3 4 5

2

7

P4

P2 8

12

P1(7)

16

P1‘s wating time = 0 P2‘s wating time = 6

P2(4)

P3‘s wating time = 3

P3(1)


P4(4) Operating System Concepts – 8th Edition

5.18


Shortest-Job-First (SJF) Scheduling Example3: Process Arrival Time Burst Time P1

0

7

P2

2

4

P3

4

1

P4

5

4

 Preemptive SJF(SRTF)

P1(7)

P1

P2 P3

0 2 P1(5)

4

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

P4

P2 5

11

7

16

P1‘s wating time = 9 P2‘s wating time = 1

P2(4) P2(2)


P3(1)


P4(4) Operating System Concepts – 8th Edition

P1

5.19


Shortest-Job-First (SJF) Scheduling  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.


5.20


Prediction of the Length of the Next CPU Burst


5.21


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)


5.22


Priority Scheduling Example :

Process P1 P2 P3 P4 p5

Burst Time priority 10 3 1 1 2 4 1 5 5 2

All arrived at time 0. The Gantt chart for the schedule is: P2 0

P1

P5 1

6

P3 16

18

P4 19

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


5.23


Priority Scheduling Example: Process

arrival time Burst length Priority

P1

0

10

3

P2

0

1

1

P3

0

2

4

P4

0

1

5

P5

3

5

2

 Gantt chart: Non-preemptive priority scheduling

P2 P1 0

P5

1

11

P3

P4

16

18

19

 Gantt chart: Preemptive priority scheduling

P2 0

P1 1

P5

P1

3

8


P3 16

5.24

P4 18

19


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. Operating System Concepts – 8th Edition

5.25


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


5.26


Round Robin (RR) Example1: Time quantum = 4 Process P1 P2 P3  The Gantt chart is: P1

P2

P3

Burst Time 24 3 3

P1

0 10 14 4 7  AWT(6(10-4)+4+7)/3 = 5.66


P1

P1 18 22

5.27

P1 26

P1 30


Round Robin (RR)  Example2:  Time quantum = 20

Process P1 P2 P3 P4

Burst Time 53 17 68 24

Wait Time 57 +24 = 81 20 37 + 40 + 17= 94 57 + 40 = 97

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 P1(33) 24P1(13) P1(53) 57 P2(17) 20 P3(68) P4(24)

37 57

P3(48)

40

40

17

P3(28) P3(8)

P4(4)

Average wait time = (81+20+94+97)/4 = 73 Operating System Concepts – 8th Edition

5.28


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


5.29


Time Quantum and Context Switch Time


5.30


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.


5.31


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


5.32


Multilevel Queue Scheduling


5.33


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.


5.34


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.


5.35


Multilevel Feedback Queues


5.36


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.


5.37


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.


5.38


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.


5.39


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


5.40


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.


5.41


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).


5.42


Algorithm Evaluation  More examples P: 214


5.43


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.


5.44


End of Chapter 5

Operating System Concepts – 8th Edition,


P - Operating Systems

P - Operating Systems

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