Operating sistem

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

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)

User process Interrupt routines
Supervisor Process
Masked Interrupt calling 1st type
interrupt
event analysis interruption
Interrupt
end end of nd
Unmasked handling 2 type
event interruption

3rd type
Interrupts Interrupts interruption
enabled disabled

Implementation
Registers :
• General interrupt mask (MM = 1 : enabled; MM = 0 disabled) MM 1
• Interrupt requests : array of bits IR 1 0 1 0 0 0 0 0 0 1 1 1 0 1 0 0
• Interrupt masks : array of bits MS 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1
• Supervisor entry point for interrupt handling ES: 32 bits
• PC saving memory zone PS: 32 bits
Interrupts Interrupts
enabled ? masked ?
yes n yes MM
MM=1 ? ∑IRi .MSi ≠ 0?
i=1
0; PS PC
PC ES

no no

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

Multiprogramming
Variation in speed of various modules of a computer dead times
Task B : timing ≡ timing task A; independent I/O channels
Task A
input 1 2 3 4 5
channel 1
Processing 1 2 3 4 5
output
channel 2 1 2 3 4
Task B
input 1 2 3 4 5
channel 3
Processing 1 2 3 4 5
output
channel 4 1 2 3 4
1 2 3 4 5 6 7 8 9 10 time
ideal configuration !!!

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

Task A waiting ready waiting ready ready

Task B waiting ready waiting ready waiting ready

Task C ready waiting ready waiting

1 2 3 4 5 6 7 8 9 10 time

Interrupt use
• Task entering waiting state control given to supervisor :
o Special interrupt : Supervisor call
o System records the transition state (working waiting)
o System records the restart condition (resource availability)
o System chooses a task ready to work (priorities !)
o Interrupt handling
o State transition handling (scheduling work : next task to run)
switching time = supervision cost As small as possible !

Task A waiting ready
Task B ready waiting
Task C ready ready
Chan. 1 ready ready
interrupt from task A from chan. 1
1 2 3 4 5 6 7 8 9 10
First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 6time

Job classes and priorities
Two 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
Mixture of CP-bound and I/O bound processes
Task A I/O ready ready ready
well-balanced
Task A waiting waiting system
Task B ready ready ready (use of clock inter.)

Task A I/O ready ready
Unbalanced
Task A waiting ready system
Task B ready (optimised for proc.
1 2 3 4 5 6 7 not for periph.)
First Year University Studies in Science. ULB . Computer Principles. Chapter 12 time D. Bertrand 7

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 I/O device spooling
• 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

Input Output
device Job evolution device
Process
Output
Input scheduler
symbiont working Process Output
scheduler ready
symbiont
Job
scheduler Termination
Resources
Input scheduler Output
queue Request queue
waiting

The operating system manages this evolution :
tables (status of the system and of its components)
o queue content
o processes status
o memory/peripherals occupation
choices algorithms (scheduling)
resource management (allocation)

Time sharing
Multi-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

1st case :
Task A
Task B ready
0 11 60 61 time
• 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 !
2nd case :
Task A
Task B
0 11 12 61 time
• 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 …
clock interrupt
Time-sharing :
Task A ready ready
Task B ready ready
Task C ready ready
1 2 3 4 5 6 7 8 9 10
time
• All processes systematically put to working state (quasi-parallelism)
• Short processes quickly finished
• Medium and long processes not indefinitely waiting

Comparison
Multi-programmation Simultaneous management
Time-sharing of several processes

Multi-management systems
Main aim Method Consequence

optimisation of Management of
Multi-programmation quasi-parallelism
control unit usage several processes

Response time Management of better use of
Time-sharing
optimisation (users) several processes active resources

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 . t
n : 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, …


Operating sistem

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