OS_Ch05

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CPU SchedulingCPU Scheduling

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5.2Operating System Conce pts

CPU SchedulingCPU Scheduling

Basic ConceptsScheduling Criteria

Scheduling Algorithms

Multiple-Processor Scheduling

Real-Time Scheduling

Thread Scheduling

Operating Systems Examples

Java Thread Scheduling

Algorithm Evaluation

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Basic Conce ptsBasic Conce pts

Maximum CPU utilization obtained with multiprogrammingCPU±I/O Burst Cycle ± Process execution consists of acycle of CPU executionand I/O wait

CPU burst distribution

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Alternating Sequence of CPU And I/O BurstsAlternating Sequence of CPU And I/O Bursts

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CPU Scheduler CPU Scheduler

Selects from among the processes in memory that are ready to execute, andallocates the CPU to one of them

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

Scheduling under 1 and 4 isnonpr eem ptiv e

All other scheduling is pr eem ptiv e

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Dispatcher Dispatcher

Dispatcher module 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 programDispat ch l at ency ± time it takes for the dispatcher to stop one process and startanother running

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Scheduling CriteriaScheduling Criteria

CPU utilization ± keep the CPU as busy as possibleThroughput ± # of processes that complete their execution per time unit

Turnaround time ± amount of time to execute a particular process

Waiting time ± amount of time a process has been waiting in the readyqueue

Response time ± amount of time it takes from when a request wassubmitted until the first response is produced,not output (for time-sharing environment)

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Optimization CriteriaOptimization Criteria

Max CPU utilization

Max throughput

Min turnaround time

Min waiting time

Min response time

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FirstFirst--Come, FirstCome, First--Served (FCFS) SchedulingServed (FCFS) Scheduling

Process Burst TimeP 1 24P 2 3P 3 3

Suppose that the processes arrive in the order: P 1 , P 2 , P 3 The Gantt Chart for the schedule is:

Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: (0 + 24 + 27)/3= 17

P1 P2 P3

24 27 300

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FCFS Scheduling (Cont .)FCFS Scheduling (Cont .)

Suppose that the processes arrive in the order P 2 , P 3 , P 1

The Gantt chart for the schedule is:

Waiting time for P 1 = 6 ; P 2 = 0; P 3 = 3

Average waiting time: (6 + 0 + 3)/3= 3

Much better than previous caseC onvoy effec t short process behind long process

P1P3P2

63 300

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ShortestShortest--JobJob--First (SJR) SchedulingFirst (SJR) Scheduling

Associate with each process the length of its next CPU burst. Use these lengthsto schedule the process with the shortest time

Two schemes:

nonpreemptive ± once CPU given to the process it cannot be preempted untilcompletes its CPU burst

preemptive ± if a new process arrives with CPU burst length less thanremaining time of current executing process, preempt. This scheme is knowas theShortest-Remaining-Time-First (SRTF)

SJF is optimal ± gives minimum average waiting time for a given set of processes

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Process Arrival Time Burst TimeP 1 0.0 7P 2 2.0 4P 3 4.0 1P 4 5.0 4

SJF (non-preemptive)

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

Ex ample of NonEx ample of Non--Preemptive SJFPreemptive SJF

P1 P3 P2

73 160

P4

8 12

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Ex ample of Preemptive SJFEx ample of Preemptive SJF

Process Arrival Time Burst TimeP 1 0.0 7P 2 2.0 4P 3 4.0 1P 4 5.0 4

SJF (preemptive)

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

P1 P3P2

42 110

P4

5 7

P2 P1

16

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Determining Length of Nex t CPU BurstDetermining Length of Nex t CPU Burst

Can only estimate the lengthCan be done by using the length of previous CPU bursts, using exponentialaveraging

:Define 4.10, 3.

burst CPUnextthefor valuepredicted 2.

burst CPUof lenghtactual 1.

1

ee

!

!

EE

X n

th

nnt

.11 nnn

t XEEX !!

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Prediction of the Length of the Ne x t CPU BurstPrediction of the Length of the Ne x t CPU Burst

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Ex amples of Ex ponential AveragingEx amples of Ex ponential Averaging

E =0Xn+1 = Xn

Recent history does not countE =1

Xn+1 = E t nOnly the actual last CPU burst counts

If we expand the formula, we get:Xn+1 = E tn+(1 - E )E t n -1 + «

+( 1 - E ) j E t n - j + «+( 1 - E )n +1 X0

Since both E and (1 - E ) are less than or equal to 1, each successive term hasless weight than its predecessor

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Priority SchedulingPriority Scheduling

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

highest priority)

Preemptive

nonpreemptive

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

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Round Robin (RR)Round Robin (RR)

Each process gets a small unit of CPU time (ti me quant um), usually 10-100milliseconds. After this time has elapsed, the process is preempted andadded to the end of the ready queue.

If there aren processes in the ready queue and the time quantum is q, theneach process gets 1/ n of the CPU time in chunks of at mostq time units at

once. No process waits more than (n-1)q time units.Performance

q large FIFO

q small q must be large with respect to context switch, otherwiseoverhead is too high

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Ex ample of RR with Time Quantum = 20Ex ample of RR with Time Quantum = 20

Process Burst TimeP 1 53P 2 17P 3 68P

4 24The Gantt chart is:

Typically, higher average turnaround than SJF, but better r esponse

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

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Time Quantum and Conte x t Switch TimeTime Quantum and Conte x t Switch Time

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Turnaround Time Varies With The Time QuantumTurnaround Time Varies With The Time Quantum

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Multilevel QueueMultilevel Queue

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

Each queue has its own scheduling algorithm

foreground ± RR

background ± FCFSScheduling must be done between the queues

Fixed priority scheduling; (i.e., serve all from foreground then frombackground). 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 RR20% to background in FCFS

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Multilevel Queue SchedulingMultilevel Queue Scheduling

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Multilevel Feedback QueueMultilevel Feedback Queue

A process can move between the various queues; aging can be implemented thisway

Multilevel-feedback-queue scheduler defined by the following parameters:

number of queues

scheduling algorithms for each queuemethod used to determine when to upgrade a process

method used to determine when to demote a process

method used to determine which queue a process will enter when thatprocess needs service

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Ex ample of Multilevel Feedback QueueEx ample of Multilevel Feedback Queue

Three queues:Q0 ± RR with time quantum 8 millisecondsQ1 ± RR time quantum 16 millisecondsQ2 ± FCFS

Scheduling

A new job enters queue Q0 which is served FCFS. When it gains CPU, jobreceives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved toqueue Q1.

AtQ1 job is again served FCFS and receives 16 additional milliseconds. If itstill does not complete, it is preempted and moved to queueQ2.

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Multilevel Feedback QueuesMultilevel Feedback Queues

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MultipleMultiple--Processor SchedulingProcessor Scheduling

CPU scheduling more complex when multiple CPUs are availableH omog eneous processors within a multiprocessor Load sharing

Asymmetri c mul tiprocessing ± only one processor accesses thesystem data structures, alleviating the need for data sharing

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RealReal--Time SchedulingTime Scheduling

H ard r eal -ti me systems ± required to complete a critical taskwithin a guaranteed amount of timeS of t r eal -ti me computing ± requires that critical processesreceive priority over less fortunate ones

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Thread SchedulingThread Scheduling

Local Scheduling ± How the threads library decides which thread to putonto an availableLWP

Global Scheduling ± How the kernel decides which kernel thread to run next

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Operating System Ex amplesOperating System Ex amples

Solaris scheduling

Windows XP schedulingLinux scheduling

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Linux SchedulingLinux Scheduling

Two algorithms: time-sharing and real-timeTime-sharing

Prioritized credit-based ± process with most credits is scheduled nextCredit subtracted when timer interrupt occursWhen credit= 0, another process chosenWhen all processes have credit = 0, recrediting occurs

Based on factors including priority and historyReal-time

Soft real-timePosix.1b compliant ± two classes

FCFS and RRHighest priority process always runs first

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Algorithm E valuationAlgorithm E valuation

Deterministic modeling ± takes a particular predetermined workloadand defines the performance of each algorithm for that workload

Queueing models

Simulations

Implementation

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5.155.15

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Questions ?Questions ?

OS_Ch05

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