Operating sistem

D. Bertrand 1First Year University Studies in Science. ULB . Computer Principles. Chapter 12

Multi-programming, Time-sharing& Real-time systems

• Under the supervisor control• Input/output organisation• Interrupt handling :

o External interrupts (generally disconnected from current process) I/O interrupt (channel interrupt) Operator/user interrupt Clock interrupt (alarm clock, …) Processor interrupt Dis-functioning interrupt

o Internal interrupts (directly dependent from the current process) Supervisor call Program errors Trapping or extra-code (micro-coding)


A simple mechanism• The interrupt handling task is chosen by the supervisor• No interrupt during the current interrupt handling• Interrupts can be masked (to avoid uncontrolled sequence breaking)

Maskedevent

Unmaskedevent

User process

InterruptanalysisInterrupt

end

Supervisor1st type

interruption

2nd typeinterruption

3rd typeinterruption

Interrupt routinesin

terr

upt

Processcalling

end ofhandling

Interruptsenabled

Interruptsdisabled


ImplementationRegisters :• General interrupt mask (MM = 1 : enabled; MM = 0 disabled)

ES: 32 bitsPS: 32 bits

• Interrupt requests : array of bits• Interrupt masks : array of bits• Supervisor entry point for interrupt handling• PC saving memory zone

1 0 1 0 0 0 0 0 0 1 1 1 0 1 0 0IR0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1MS

1MM

Execute I

MM 0; PS PCPC ES

yes

no

I [PC]

no

≠∑ 0?n

i ii=1

.MIR Syes

Interruptsmasked ?

MM=1 ?

Interruptsenabled ?

• ES register can be replaced by an array• Each entry pointer to a specific service routine• Working registers saving (stack) !


1 32 4 108765 9

MultiprogrammingVariation in speed of various modules of a computer dead times

time

432

2

1 5

5

4

4

3

3

2

Task A1input

channel 1

1outputchannel 2

Processing

43

5

5

4

4

3

3

1inputchannel 3

2

1Processing

Task B

Task B : timing ≡ timing task A; independent I/O channels

ideal configuration !!!

outputchannel 4 1

2

2


Processes states• Concurrent processing : 3 states recognised by the scheduler• Working state : task using the control unit (computational state)• Waiting state : task waiting for an external resource• Ready state : task waiting for the control unit (main resource)

1 32 4 108765 9 time

waiting

ready

waitingready ready ready

waiting ready

waiting

waiting readyready

ready

waiting

waiting

Task A

Task B

Task C

• Scheduler serving the first ”ready” task• Priority system can be implemented

system idle


Interrupt use• Task entering waiting state control given to supervisor :

o Special interrupt : Supervisor callo System records the transition state (working waiting)o System records the restart condition (resource availability)o System chooses a task ready to work (priorities !)

1 32 4 108765 9 time

ready

waiting ready

waiting

from task A

Task A

Task BTask C ready ready

ready ready

from chan. 1

Chan. 1interrupt

switching time = supervision cost As small as possible !

o Interrupt handlingo State transition handling (scheduling work : next task to run)


Job classes and prioritiesTwo main categories of processes :• CP-bound : computation in central memory; few I/O (number crunching)• IO-bound : little computation; many I/O operations• If only CP-bound processes : processor idle time very small

no gain (even losses !) with multi-programming• If only IO-bound processes : efficiency depends on synchronisation

time

waitingTask ATask A I/O

Task Bwaiting

ready

ready

ready

ready

ready

ready

1 32 4 765

Task ATask A I/O

Task Bwaiting

ready

ready

ready

ready

Mixture of CP-bound and I/O bound processes

well-balancedsystem

(use of clock inter.)

Unbalancedsystem

(optimised for proc.not for periph.)


Spooling• Sharing of ”sequential” peripherals (printers, magnetic tapes) difficult• Not a problem for random access devices (different areas)To solve the problem :

Deferral output system• Output sent to a secondary storage device (magnetic disk, …)• At the end of the task output put into an output queue• When the device is ready : get one of the files of the output queue (special task : the output symbiont)

Remarks• Symbiont : part of system software but handled as a user process

• Queues can be built for different classes of peripherals (staging …)

• A class of devices can be handled by the same or different symbionts

• Same system can be applied to input queues

• Priorities can be defined at the level of the queues

I/O device spooling


Job evolution

waiting

working ready

Inputdevice

Outputdevice

Inputqueue

Outputqueue

Processscheduler

Resourcesscheduler

Request

Inputsymbiont

Jobscheduler

Outputsymbiont

Outputscheduler

Termination

Process

The operating system manages this evolution : tables (status of the system and of its components)

o queue contento processes statuso memory/peripherals occupation

choices algorithms (scheduling) resource management (allocation)


Time sharingMulti-programmation resource usage optimisation

But …

Response time ?

Response time = reading time + waiting time in input queue + execution time + waiting time in output queue + output time

execution time = Σ working times + Σ waiting times + Σ ready times

Reduction of processor idle time may be inefficient for response time !

Example :• Two CP-bound tasks• Different execution times (60 minutes ↔ 1 minute)• Tasks are submitted at different times


time6011

ready

61

time11 12 61

0

Task ATask B

0

Task ATask B

• Looks like mono-programmation !• But control unit is quite efficiently used• User A happy !• User B must wait 50 minutes for the execution of one minute job !

1st case :

2nd case :

• Less efficient (more switching time)• User B happy (response time reduced by 50)• User A still happy (response time increased by 1.5 %)• Average response time : 31 minutes (55 minutes in the first case)


Remarks• Many short processes delay for medium and long processes• Execution time is not always a priori known (errors, convergences, …)

Other technique :• Put a working process in waiting state after a given time slice• Activate a ready process• Continue with another process …

Time-sharing :

• All processes systematically put to working state (quasi-parallelism)• Short processes quickly finished• Medium and long processes not indefinitely waiting

1 32 4 108765 9 time

ready

Task ATask BTask C

ready

ready

ready

ready

ready

clock interrupt


ComparisonMulti-programmation

Time-sharingSimultaneous managementof several processes

Multi-management systems

optimisation ofcontrol unit usage

Response timeoptimisation (users)

Management ofseveral processes

Management ofseveral processes

quasi-parallelism

better use ofactive resources

Main aim Method Consequence

Multi-programmation

Time-sharing

Some operating systems based on a compromise


Interactive systems• Task directives given interactively by the user• Each user has its own access units (terminal : keyboard + screen)• The user units do not need to be driven by a symbiont• Commands generate interrupts and tasks state transition

Average response time = n . tn : number of users; t : average execution time of a command

• In general system in waiting state• Each user get the impression to have his own computer


Real time systems

Time-sharing systems with strict timing constraints

Systems requiring minimum response time :• Process control in industry• Probe control in space• Electronic assistance systems in cars• Flying systems in planes• Data acquisition systems (physics, chemistry, …)

Systems requiring fast but not critical response time :• Reservation systems (plane, trains, …)• Automatic banking systems, …

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Operating sistem

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