chapter09 new

7/30/2019 Chapter09 New

1/58

Click to edit Master subtitle style

1/12/13

Chapter 9Uniprocessor Scheduling

Operating Systems:Internals and Design Principles, 6/EWilliam Stallings

Dave BremerOtago Polytechnic, N.Z.2008 Prentice Hall


2/58

Roadmap

Types of Processor Scheduling Scheduling Algorithms Traditional UNIX Scheduling


3/58

Scheduling

An OS must allocate resources amongstcompeting processes.

The resource provided by a processor is

execution time The resource is allocated by means of a

schedule


4/58

Overall Aimof Scheduling

The aim of processor scheduling is toassign processes to be executed by theprocessor over time, in a way that meets system objectives, such

as response time, throughput, and processorefficiency.


5/58

Scheduling Objectives

The scheduling function should Share time fairlyamong processes Prevent starvation of a process Use the processor efficiently Have low overhead Prioritise processes when necessary (e.g. real

time deadlines)


6/58

Types of Scheduling


7/58

Two Suspend States

Remember this diagram from Chapter 3


8/58

Scheduling andProcess State Transitions


9/58

Nesting ofScheduling Functions


10/58

Queuing Diagram


11/58

Long-Term Scheduling

Determines which programs are admittedto the system for processing May be first-come-first-served Or according to criteria such as priority, I/O

requirements or expected execution time

Controls the degree of multiprogramming

More processes, smaller percentage oftime each process is executed


12/58

Medium-TermScheduling

Part of the swapping function Swapping-in decisions are based on the

need to manage the degree of

multiprogramming


13/58

Short-Term Scheduling

Known as the dispatcher Executes most frequently Invoked when an event occurs

Clock interrupts I/O interrupts Operating system calls

Signals


14/58

Roadmap



15/58

Aim of ShortTerm Scheduling

Main objective is to allocate processortime to optimize certain aspects of systembehaviour.

A set of criteria is needed to evaluate thescheduling policy.


16/58

Short-Term SchedulingCriteria: User vs System

We can differentiate between user andsystem criteria

User-oriented Response Time

Elapsed time between the submission of a requestuntil there is output.

System-oriented Effective and efficient utilization of the

processor


17/58

Short-Term SchedulingCriteria: Performance

We could differentiate betweenperformance related criteria, and thoseunrelated to performance

Performance-related Quantitative, easily measured E.g. response time and throughput

Non-performance related Qualitative Hard to measure


18/58

InterdependentScheduling Criteria


19/58

InterdependentScheduling Criteria cont.


20/58

Priorities

Scheduler will always choose a process ofhigher priority over one of lower priority

Have multiple ready queues to represent

each level of priority


21/58

Priority Queuing


22/58

Starvation

Problem: Lower-priority may suffer starvation if there is

a steady supply of high priority processes.

Solution Allow a process to change its priority based

on its age or execution history


23/58

Alternative SchedulingPolicies


24/58

Selection Function

Determines which process is selected forexecution

If based on execution characteristics then

important quantities are: w= time spent in system so far, waiting e = time spent in execution so far

s = total service time required by the process,including e;


25/58

Decision Mode

Specifies the instants in time at which theselection function is exercised.

Two categories: Nonpreemptive Preemptive


26/58

Nonpreemptive vsPremeptive

Non-preemptive Once a process is in the running state, it will

continue until it terminates or blocks itself for

I/O Preemptive

Currently running process may be interruptedand moved to ready state by the OS

Preemption may occur when new processarrives, on an interrupt, or periodically.


27/58

Process SchedulingExample

Example set ofprocesses,

consider each abatch job

Service time represents total execution time


28/58

First-Come-First-Served

Each process joins the Ready queue When the current process ceases to

execute, the longest process in the Ready

queue is selected


29/58

First-Come-First-Served

A short process may have to wait a verylong time before it can execute

Favors CPU-bound processes I/O processes have to wait until CPU-bound

process completes


30/58

Round Robin

Uses preemption based on a clock also known as time slicing, because each

process is given a slice of time before being

preempted.


31/58

Round Robin

Clock interrupt is generated at periodicintervals

When an interrupt occurs, the currently

running process is placed in the readyqueue Next ready job is selected


32/58

Effect of Size ofPreemption Time Quantum


33/58

Effect of Size ofPreemption Time Quantum


34/58

Virtual Round Robin


35/58

Shortest Process Next

Nonpreemptive policy Process with shortest expected

processing time is selected next Short process jumps ahead of longer

processes


36/58

Shortest Process Next

Predictability of longer processes isreduced

If estimated time for process not correct,

the operating system may abort it Possibility of starvation for longer

processes


37/58

CalculatingProgram Burst

Where: Ti = processor execution

time for the ith instance of

this process Si = predicted value for

the ith instance S1 = predicted value for

first instance; notcalculated


38/58

Exponential Averaging

A common technique for predicting afuture value on the basis of a time seriesof past values is exponential averaging


39/58

Exponential SmoothingCoefficients

Of


40/58

Use Of ExponentialAveraging

U Of


41/58

Use OfExponential Averaging

Sh t t R i i


42/58

Shortest RemainingTime

Preemptive version of shortest processnext policy

Must estimate processing time and

choose the shortest

Hi h t R


43/58

Highest ResponseRatio Next

Choose next process with the greatestratio


44/58

Feedback Scheduling

Penalize jobs thathave been runninglonger

Dont knowremaining timeprocess needs to

execute


45/58

Feedback Performance

Variations exist, simple version pre-emptsperiodically, similar to round robin But can lead to starvation

P f


46/58

PerformanceComparison

Any scheduling discipline that chooses thenext item to be served independent ofservice time obeys the relationship:


47/58

Formulas

O ll N li d


48/58

Overall NormalizedResponse Time

N li d R


49/58

Normalized ResponseTime for Shorter Process

N li d R


50/58

Normalized ResponseTime for Longer Processes

N li d


51/58

NormalizedTurnaround Time


52/58

Fair-Share Scheduling

Users application runs as a collection ofprocesses (threads)

User is concerned about the performance

of the application Need to make scheduling decisions based

on process sets


53/58

Fair-Share Scheduler


54/58

Roadmap


Traditional UNIX


55/58

Traditional UNIXScheduling

Multilevel feedback using round robinwithin each of the priority queues

If a running process does not block or

complete within 1 second, it is preempted Priority is based on process type and

execution history.


56/58

Scheduling Formula


57/58

Bands

Priorities are recomputed once per second Base priority divides all processes into

fixed bands of priority levels Swapper (highest) Block I/O device control File manipulation

Character I/O device control User processes (lowest)

Example of Traditional


58/58

Example of TraditionalUNIX Process Scheduling

chapter09 new

Documents