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

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
Masked
event
Unmasked
event
interrupt
Supervisor
Interrupt
analysis
Interrupt
end
Process
calling
end of
handling
Interrupt routines
1 st type
interruption
2 nd type
interruption
Interrupts
enabled
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
Interrupts
disabled
3 rd type
interruption
D. Bertrand 2

Implementation
Registers :
• General interrupt mask (MM = 1 : enabled; MM = 0 disabled) MM
• Interrupt requests : array of bits IR
• Interrupt masks : array of bits MS
• Supervisor entry point for interrupt handling ES: 32 bits
• PC saving memory zone
PS: 32 bits
Interrupts
enabled ?
MM=1 ?
Interrupts
masked ?
yes
n
∑IRi.MS
i
≠ 0?
i=1
yes
1 0 1 0 0 0 0 0 0 1 1 1 0 1 0 0
0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1
MM 0; PS PC
PC ES
1
no
I [PC]
Execute I
no
• ES register can be replaced by an array
• Each entry pointer to a specific service routine
• Working registers saving (stack) !
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
D. Bertrand 3

Multiprogramming
Variation in speed of various modules of a computer dead times
Task A
input
channel 1
Processing
output
channel 2
Task B
Task B : timing ≡ timing task A; independent I/O channels
input
channel 3
1
Processing 1
output
channel 4 1
1
1 2 3 4 5 6 7 8 9 10
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
2
3
1 2
3
4
5
2
1
2
2
ideal configuration !!!
3
2
3
4
3
4
3
4
5
4
5
D. Bertrand 4
4
5
time

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
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
D. Bertrand 5

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)
Task A
switching time = supervision cost As small as possible !
from task A
1 2 3 4 5 6 7 8 9 10
time
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
waiting
Task B ready
Task C ready ready
Chan. 1 ready
interrupt
from chan. 1
ready
ready
waiting
D. Bertrand 6

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
Task A
Task B
ready
ready
waiting
ready
ready
waiting
ready
ready
well-balanced
system
(use of clock inter.)
Task A I/O
Task A
Task B
ready
ready
waiting
ready
ready
1 2 3 4 5 6 7
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
Unbalanced
system
(optimised for proc.
not for periph.)
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
• 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
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
I/O device spooling
D. Bertrand 8

Input
device
Input
queue
Input
symbiont
Job
scheduler
working
Request
Job evolution
Process
Process
scheduler
waiting
Resources
scheduler
ready
Output
scheduler
Output
symbiont
Termination
Output
device
Output
queue
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)
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
D. Bertrand 9

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
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
D. Bertrand 10

1 st case :
Task A
Task B
ready
0 11
• Looks like mono-programmation !
60 61 time
• But control unit is quite efficiently used
• User A happy !
• User B must wait 50 minutes for the execution of one minute job !
2 nd 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)
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
D. Bertrand 11

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
Task A
Task B
Task C
ready
ready
Time-sharing :
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
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
ready
ready
ready
D. Bertrand 12

Multi-programmation
Time-sharing
Comparison
Simultaneous management
of several processes
Multi-management systems
Main aim Method Consequence
Multi-programmation
optimisation of
control unit usage
Management of
several processes
quasi-parallelism
Time-sharing
Response time
optimisation (users)
Management of
several processes
better use of
active resources
Some operating systems based on a compromise
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
D. Bertrand 13

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
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
D. Bertrand 14

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, …
First Year University Studies in Science. ULB . Computer Principles. Chapter 12
D. Bertrand 15