Ch05 - Cpu Scheduling.ppt

8/12/2019 Ch05 - Cpu Scheduling.ppt

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Silberschatz, Galvin and Gagne 2009Operating System Concepts 8thEdition,

CPU Scheduling

Modified by M.Rebaudengo - 2010


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5.2 Silberschatz, Galvin and Gagne 2009Operating System Concepts 8thEdition

Schedulers

!Process migrates among several queues

" Device queue, job queue, ready queue

! Scheduler selects a process to run from these queues

! Long-term scheduler:

" load a job in memory

" Runs infrequently

! Short-term scheduler:" Select ready process to run on CPU

" Should be fast

! Middle-term scheduler (or swapper)

" Reduce multiprogramming or memory consumption


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Classification of Scheduling Activity

! Long-term: which process to admit?! Medium-term: which process to swap in or out?! Short-term: which ready process to execute next?


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Queuing Diagram for Scheduling


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Long-Term Scheduling

! Determines which programs are admitted to the system forprocessing

! Controls the degree of multiprogramming! Attempts to keep a balanced mix of processor-bound and I/O-

bound processes" CPU usage

" System performance


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Medium-Term Scheduling

! Makes swapping decisions based on the current degree ofmultiprogramming" Controls which remains resident in memory and which jobs must be

swapped out to reduce degree of multiprogramming


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Short-Term Scheduling

! Selects from among ready processes in memory which one isto execute next" The selected process is allocated the CPU

! It is invoked on events that may lead to choose another processfor execution:" Clock interrupts

" I/O interrupts

" Operating system calls and traps

" Signals


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The Scheduler

!Programs execute concurrently

! Schedulerdetermines which thread executes next

! Scheduling policydetermines

" when thread is removed from CPU

" which thread allocated the CPU next

! Scheduling mechanismdetermines

" how the process manager can determine that it is time to multiplex theCPU

" how a thread can be allocated to/removed from the CPU


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The Scheduler

!Scheduling mechanism" Provides the tools and environment for controlling the flow of the thread

through the various queues

!Context Switching between Processes

!Queueing Capabilities

! Scheduling policy" Defines which ready thread the scheduler chooses from the ready list

" Uses the scheduling mechanism


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Basic Concepts

!Processes require alternate use of processor and I/O in a repetitivefashion

! Each cycle consist of a CPU burst followed by an I/O burst

! CPU-bound processes have longer CPU bursts than I/O-boundprocesses


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CPU and I/O bound processes

!Bursts of CPU usage alternate with periods of I/O wait

" a CPU-bound process

" an I/O bound process


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Histogram of CPU-burst Times

! The distribution of CPU cycle times shows a greater number of jobsrequesting short CPU cycles, and fewer jobs requesting long CPU cycles.


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Alternating Sequence of CPU And I/O Bursts


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Example

{

printf("Enter the first integer: \n");scanf("%d", &a);

printf("Enter the second integer: \n");

scanf("%d", &a);

c = a+b;

d = (a*b) c ;

e = a b;

f = d/e;

printf("\n a+b = %d", c);

printf("\n (a*b) c = %d", d);printf("\n a - b = %d", e);

printf("\n d/e = %d", f);

}

I/O Cycle

CPU Cycle

I/O Cycle


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Remember

timeout


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Scheduling Mechanisms

! Software components

" Enqueuer!Places a pointer to the process descriptor in the ready list

!places the process into Ready state and decides its priority

" Context switcher!Saves the contents of all CPU registers and other state

!Loads the state of the dispatcher

" Dispatcher

!Selects a thread from the ready list and allocates the CPU to it byperforming a context switch to it


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The Scheduler

ReadyProcess

Enqueuer Ready

List

DispatcherContext

Switcher

Process

Descriptor

CPU

From

Other

States

RunningProcess


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Process/Thread Context

! Context Switch" Saves all current registers (general purpose and status)

" Loads all registers for new process

" Context saved in a processs descriptor.


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Process/Thread Context

R1

R2

Rn

. . .

Status

Registers

Functional Unit

Left Operand

Right Operand

Result ALU

PC

IR

Ctl Unit


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Context Switching

CPU

New Thread Descriptor

Old Thread

Descriptor


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! Context switching: time

" Save all registers

!32 or more 32- or 64-bit registers + status registers

!Uses load/store operations

!Takes (n + m) b x Ktime units

ngeneral registers, m status registers

bstore operations required

Each store requires Ktime units


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! Context switching: time

" A 1GHz machine can run a regular instruction (no memory access) inabout 2 ns

" During the 4 microseconds of context switch, CPU could have executed2,000 instructions


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Performance

! Time to run a process depends on

" CPU time needed by process

" I/O time needed by process

" Context switch time (significant)

" Scheduler choices

!The longer you wait in the ready list, the longer your process takesto execute

!If you never get chosen, you starve


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Choosing a Process to Run

! Mechanismnever changes

! Strategy = policythe dispatcher uses to select a process from the

ready list! Different policies for different requirements

ReadyProcess

EnqueueReady

List

DispatchContext

Switch

Process

Descriptor

CPU

RunningProcess


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CPU Scheduling (cont.)

! CPU scheduling decisions may take place when a process:

1. Switches from running to waiting state (e.g., I/O request)

2. Switches from running to ready state (e.g., the CPU receives aninterrupt)

3. Switches from waiting to ready (e.g., completion of an I/O operation)

4. Terminates

! Scheduling under 1 and 4 is nonpreemptive! All other scheduling is preemptive


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Preemptive vs. nonpreemptive scheduling

! Many CPU scheduling algorithms have both preemptive and nonpreemptiveversions:

" Preemptive:

! schedule a new process even when the current process does notintend to give up the CPU

! Currently running process may be interrupted and moved to the

Ready state by the OS! Prevents one process from monopolizing the processor

" Non-preemptive:

! Once a process is in the running state, it will continue until itterminates or blocks for an I/O

! only schedule a new process when the current one does not want

CPU any more.


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Dispatcher

! The dispatcher is the module that gives control of the CPU to theprocess selected by the short-term scheduler

! Dispatcher module gives control of the CPU to the processselected by the short-term scheduler; this involves:

" switching context

" switching to user mode

" jumping to the proper location in the user program to restartthat program

! Dispatch latency time it takes for the dispatcher to stop oneprocess and start another running


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Scheduling Criteria

! CPU utilization percentage of the time that CPU is busy

! Throughput number of completed processes per time unit

! Turnaround time the interval from the time of submission of aprocess to the time of completion

! Waiting time the sum of the periods spent waiting in the readyqueue

! Response time amount of time it takes from when a request wassubmitted until the first response is produced (for time-sharing

environment)

! Fairness give everyone an equal amount of CPU time


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Scheduling goals

! All Systems:

" Fairness: giving each process a fair share of the CPU

" Policy enforcement: seeing that stated policy is carried out

" Balance: keeping all parts of the system busy

! Batch Systems:

" Maximize jobs per hour" Minimize time between submission and termination of a task

" CPU utilization: keep the CPU busy at all times

! Interactive systems:

" Respondto requests quickly

" Meet usersexpectations (a task that is supposed to take a short time shouldfinish quickly, and not surprise the user in terms of his/her expectations)

! Real-time systems:

" Meeting deadlines: avoid losing data

" Predictability: avoid quality degradation in multimedia systems


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Scheduling Algorithm Optimization Criteria

! Maximize:

" CPU utilization

" throughput

" fairness

! Minimize:

" turnaround time

" waiting time

" response time.


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Methods for evaluating CPU scheduling algorithms

! Deterministic Modeling" Take a predetermined workload

" Run the scheduling algorithm manually

" Find out the value of the performance metric that you care about.


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Deterministic modeling example:

! Suppose we have processes A, B, and C, submitted at time 0

! We want to know the response time, waiting time, and turnaround time ofprocess A

A B C A B C A C A C Time

response time = 0+ +wait time

turnaround time

Gantt chart: visualize how processes execute.


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Deterministic modeling example


! We want to know the response time, waiting time, and turnaround time ofprocess B


response time+wait time

turnaround time


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Deterministic modeling example


! We want to know the response time, waiting time, and turnaround time ofprocess C


response time+ ++wait time

turnaround time


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Scheduling Policies

! FCFS (first come, first served)! SJF (shortest job first)! Priority Scheduling! Round robin! Multilevel feedback queues! Lottery scheduling! This is obviously an incomplete list....


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FCFS (First Come First Served)

! FCFS: assigns the CPU based on the order of requests

" Nonpreemptive: A process keeps running on a CPU until it is blockedor terminated

" when a process arrives, all in ready list will be processed before this job

" and none of the following processes will be serviced before this process

" The process that currently has the CPU must finish.

+ Simple

- Short jobs can get stuck behind long jobs

- Turnaround time is not ideal


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FCFS Scheduling (Cont)

Suppose that the processes arrive in the order

P2, 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.

P1P3P2

63 300


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Shortest-Job-First (SJF) Scheduling

! SJFruns whatever job puts the least demand on the CPU, also known as

STCF (shortest time to completion first)

! Associate with each process the length of its next CPU burst. Use theselengths to schedule the process with the shortest time+ Probably optimal in terms of turn-around time

+ Gives minimum average waiting time for a given set of processes

+ Great for short jobs+ Small degradation for long jobs

The difficulty is knowing the length of the next CPU request


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Example of SJF

Process Burst Time

P1 6

P2 8

P3 7

P4 3

! SJF scheduling chart

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

P4 P3P1

3 160 9

P2

24


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5.42 Silberschatz, Galvin and Gagne 2009Operating System Concepts 8thEdition 42

Shortest Job First Scheduling

ProcessArrival Time Burst Time

P1 0 7

P2 2 4

P3 4 1P4 5 4


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Shortest Job First Scheduling

ProcessArrival Time Burst Time

P1 0 7

P2 2 4

P3 4 1P4 5 4

P1 P3 P2

7

P1(7)

160

P4

8 12

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

2 4 5

P2(4)

P3(1)

P4(4)

P1s wating time = 0

P2s wating time = 6

P3s wating time = 3P4s wating time = 7


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Shortest Remaining Time First (SRTF)

! SRTF: a preemptiveversion of SJF

" If a job arrives with a shorter time to completion, SRTF preempts theCPU for the new job

" Also known as SRTCF (shortest remaining time to completion first)


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Shortest Remaining Time First

Process Arrival Time Burst Time

P1 0 7

P2 2 4

P3 4 1P4 5 4

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)

P1s wating time = 9

P2s wating time = 1

P3s wating time = 0P4s wating time = 2

P1(5)

P2(2)


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Turn-around time for SJF

! Let assume that all the processes are available at the same time

! Processes = {T1,T2, T3, ..., Tn } in order of duration

! Total Turn-around time =

= T1+ (T1 +T2 ) + (T1 +T2 +T3 ) + ....+ (T1 +T2 +. . .+Tn ) =

= n * T1 + (n-1) * T2 + (n-2) * T3 + ... + 2* Tn-1 + Tn


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Determining Length of Next CPU Burst

! Optimal scheduling

! Preemptive SJF is superior to non preemptive SJF.

! However, there are no accurate estimations to know the length of the nextCPU burst

! Can only estimate the length

! Can be done by using the length of previous CPU bursts, using exponential

averaging

:Define4.

10,3.

burstCPUnexttheforvaluepredicted2.

burstCPUoflengthactual1.

!!

=

=

+

""

# 1n

th

n nt

( ) .11 nnn

t !""! #+=+


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Examples of Exponential Averaging

! !=0" "n+1= "n" Recent history does not count

! !=1

" "n+1= tn" Only the actual last CPU burst counts.

! Consider t4, we get:

"4= !t3+(1 - !) "3+

= !t3+(1 - !)(!t2+(1 - !)"2)

= !t3+(1 - !)!t2+ (1 - !)2"2

= !t3+(1 - !)!t2+ (1 - !)2(!t1+ (1 -!) "1)

= !t3+(1 - !)!t2+ (1 - !)2!t1+ (1 -!)

3"1

= !t3+(1 - !)!t2+ (1 - !)2!t1+ (1 -!)

3(!t0+ (1 -!) "0)

= !t3+(1 - !)!t2+ (1 - !)2!t1+ (1 -!)

3!t0+ (1 -!)4"0

! Since both !and (1 - !) are less than or equal to 1, each successive term has less weight

than its predecessor.


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Examples of Exponential Averaging

0

5

10

15

Time

CPU burst 10 6 4 6 4 13 13 13

guess 10 8 6 6 5 9 11 12

0 1 2 3 4 5 6 7


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Drawbacks of Shortest Job First

- Starvation: constant arrivals of short jobs can keep long ones from running

- There is no way to know the completion time of jobs (most of the time)

" Some solutions

! Ask the user, who may not know any better

! If a user cheats, the job is killed


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Priority Scheduling

! Priority scheduling: A priority number (integer) is associated with each

process

! The CPU is allocated to the process with the highest priority (smallestinteger #highest priority)

" Priority 0:

" Priority 1:

" Priority 2:

A

B

C

A B TimeC

Priority Scheduling


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Priority Scheduling

+ Generalization of SJF: SJF is a priority scheduling where priority is the

predicted next CPU burst time

" With SJF, priority = 1/requested_CPU_time

- Problem #Starvation low priority processes may never execute

+ Solution #Aging as time progresses increase the priority of the process


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Real-time Systems

! On-line transaction systems! Real-time monitoring systems

! Signal processing systems

" multimedia

! Embedded control systems:

" automotives

" Robots

" Aircrafts

" Medical devices


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Real-Time Workload

! Job (unit of work)

" a computation, a file read, a message transmission, etc

! Attributes

" Resources required to make progress

" Timing parameters

Released

Absolute

deadline

Relative deadline

Execution time


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Real-Time Task

! Task : a sequence of similar jobs

" Periodic task (p,e)

!Its jobs repeat regularly

!Period p= inter-release time (0 < p)

!Execution time e= maximum execution time (0 < e < p)

!Utilization U = e/p

5 10 150


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Deadlines: Hard vs. Soft

! Harddeadline

" Disastrous or very serious consequences may occur if the deadline ismissed

" Validation is essential : can allthe deadlines be met, even under worst-case scenario?

" Deterministic guarantees

! Softdeadline

" Ideally, the deadline should be met for maximum performance. Theperformance degrades in case of deadline misses.

" Best effort approaches / statistical guarantees


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Schedulability

! Property indicating whether a real-time system (a set of real-time tasks) can

meet their deadlines

(4,1)

(5,2)

(7,2)


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Real-Time Scheduling

! Determines the order of real-time task executions

! Static-priority scheduling

! Dynamic-priority scheduling


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RM (Rate Monotonic)

! Optimal static-priority scheduling

! It assigns priority according to period

! A task with a shorter period has a higher priority

! Executes a job with the shortest period

(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3


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RM (Rate Monotonic)

! Executes a job with the shortest period

(4,1)

(5,2)

(7,2)

Deadline Miss !

5

5

10

10 15

15

T1

T2

T3


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RM Utilization Bound

! Real-time system is schedulable under RM if

$Ui%n (21/n-1)

Liu & Layland,

Scheduling algorithms for multi-programming in a hard-real-timeenvironment, Journal of ACM, 1973.


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$Ui%n (21/n-1)

! Example: T1(4,1), T2(5,1), T3(10,1),

$Ui= 1/4 + 1/5 + 1/10

= 0.55

3 (21/3-1) &0.78

Thus, {T1, T2, T3} is schedulable under RM.


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RM Utilization Bounds

0.5

0.6

0.7

0.8

0.9

1

1.1

1 4 16 64 256 1024 4096

The Number of Tasks

Utilization



$Ui%n (21/n-1)


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EDF (Earliest Deadline First)

! Optimal dynamic priority scheduling

! A task with a shorter deadline has a higher priority

! Executes a job with the earliest deadline

(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3


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EDF (Earliest Deadline First)

! Executes a job with the earliest deadline

! Optimal scheduling algorithm

" if there is a schedule for a set of real-time tasks,

EDF can schedule it.

(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3


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EDF Utilization Bound

! Real-time system is schedulable under EDF if and only if

$Ui%1

Liu & Layland,

Scheduling algorithms for multi-programming in a hard-real-timeenvironment, Journal of ACM, 1973.


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! Domino effect during overload conditions

" Example: T1(4,3), T2(5,3), T3(6,3), T4(7,3)

EDF Overload Conditions

T1

50 7

T2 T3 T4

3 6

Deadline Miss !

T1

50 7

T3

3 6

Better schedules :

T1

50 7

T4

3 6


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Round Robin

! Goal is an equal distribution of processing time

! If there are nprocesses, each gets 1/nof the CPU time.

! Round Robin (RR) periodically releases the CPU from long-running jobs

" Based on timer interrupts

" Preemptive: a process can be forced to leave its running state andreplaced by another running process

" Time slice (or time quantum): interval between timer interrupts

!usually 10-100 milliseconds

" Most widely used scheduling algorithm


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Round Robin (RR)

! After the time slice has elapsed, the process is preempted andadded to the end of the ready queue.

" the ready queue is treated as a circular queue

! If there are nprocesses in the ready queue and the timequantum is q, then each process gets 1/nof the CPU time in

chunks of at most qtime units at once. No process waits morethan (n-1)q time units.


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More on Round Robin

! If time slice is too long

" Scheduling degrades to FCFS

! If time slice is too short

" Context switching cost dominates

! Timeslice frequently set to ~100 milliseconds

! Context switches typically cost < 1 millisecond

" Context switch is usually negligible (< 1% per timeslice) otherwise youcontext switch too frequently and lose all productivity.


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! Time Quantum (Q) = 20

Process Burst Time Wait Time

P1 53 57 +24 = 81P2 17 20P3 68 37 + 40 + 17= 94P4 24 57 + 40 = 97

Round Robin Scheduling

P1 P2 P3 P4 P1 P3 P4 P1 P3 P30 20 37 57 77 97 117 121134 154162

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)


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Time Quantum and Context Switch Time


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FCFS vs. RR (varying Q)

Quantum

CompletionTime

Wait

Time

AverageP4P3P2P1

P2[8]

P4[24]

P1[53]

P3[68]

0 8 32 85 153

Best FCFS:

6257852284Q = 1

104!11215328125Q = 20

100!8115330137Q = 1

66"88852072Q = 20

31"885032Best FCFS

121#14568153121Worst FCFS

69!32153885Best FCFS

83!121014568Worst FCFS

95!8015316133Q = 8

57"5685880Q = 8

99!9215318135Q = 10

99!8215328135Q = 5

61"68851082Q = 10

61"58852082Q = 5


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Turnaround Time Varies With The Time Quantum

Average turnaround time does not

necessarily decrease with larger timequanta


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Round Robin (RR)

!Performance

" Average turnaround time decreases if most processes finish theirnext CPU burst in a singletime quantum.

" Rule of thumb: 80% of the CPU bursts should be shorter thanthe time quantum.


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FCFS vs. Round Robin

! Suppose we have three jobs of equal length (3 process of 10 time units

each)

" time quantum = 1 then turnaround time is 29

" time quantum = 10, average turnaround time is 20

A B C A B C TimeA B C

turnaround time of A

turnaround time of B

turnaround time of C

Round Robin

A B TimeC

turnaround time of A

turnaround time of B

turnaround time of C

FCFS


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FCFS vs. Round Robin

! Round Robin

+ Shorter response time

+ Fair sharing of CPU

- Not all jobs are preemptable

- Not good for jobs of the same length

- Not good in terms of the turnaround time.


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Multi-Level Queues

! All processes of the same priority are placed into the same queue

! Scheduler uses one strategy to allocate processor time to each pool

! Uses a different strategy for all the processes in the same pool


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Multi-Level Queues

! Example:

" Use priority scheduling among pools

!Then all processes in pool i are run before any process in pool j if i

Documents

Ch05 - Cpu Scheduling.ppt