Process as an abstraction
TDDB47 – Real-time Systems
Lecture 1: Introduction
Simin Nadjm-Tehrani
Real-time Systems Laboratory
Department of Computer and Information Science
Linköping university
Undergraduate course on Real-time Systems
Linköping
40 pages
Autumn 2005
To model:
• Management of different
computational processes by operating
systems
• Parallel developments within the
physical environment as well as the
control system
• Nodes in a network, with interaction
and communication
Undergraduate course on Real-time Systems
Linköping
An example
Performance criteria
• For operating systems it is enough to
show that processes will not end up in
deadlock or livelock
• With real-time systems it is not enough
that the system eventually responds
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The film…
The time that the result of the computation is
delivered is as important as the result itself
• Predictability!
Undergraduate course on Real-time Systems
Linköping
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Undergraduate course on Real-time Systems
Linköping
What is meant by predictable?
Release time
Computation
time
Deadline
time
Event
Reaction
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Hard real-time
• If the sensor detects a crash, the air
bag in the car should be activated within
a given time
• If the signal is red, the train must be
stopped within a given time
• If the liquid level has reached the max.
value, the valve must be opened within
a given time
Hard real-time: All processes meet their deadline!
Undergraduate course on Real-time Systems
Linköping
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Linköping
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Deadlines
• Hard: Not meeting any deadline is a
failure of the system
How often?
• Soft: It is desirable that deadlines are
met, but OK if they are missed now and
again
• Firm: It is OK that they are missed now
and again, but after the deadline the
result is of no use
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Linköping
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Real-time systems research
• Traditionally on hard real-time problems
• Most systems consist of subsystems
with hard, firm, and soft deadlines
• Many exciting applications of today have
problems that no longer fit in the
framework of traditional hard real-time!
Undergraduate course on Real-time Systems
Linköping
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Real-time vs. embedded
• Many Real-time computations are
embedded in application systems:
Embedded systems
A computer system that
– is part of a larger system
– is tailor-made for a specific
application
– is characterised by restricted
programmability by the user
– has tight interaction with a physical
environment
• But there are embedded systems that
are not real-time
• And Real-time systems that are not
embedded
Undergraduate course on Real-time Systems
Linköping
Embedded systems
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Undergraduate course on Real-time Systems
Linköping
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Typical application areas
•
•
•
•
Less obvious ...
• Avoiding overload in CPU that runs
traffic functions in RNC despite
fluctuations in user requests
Command and control
Transportation
Medical devices
Multimedia and voice transfer
RNC
RBS
Core
Network
RBS
Load control
Cells
Undergraduate course on Real-time Systems
Linköping
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Undergraduate course on Real-time Systems
Linköping
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Lecture topics
• One lecture with two novel applications!
– Embedded databases in engine
control
– Bandwidth allocation to differentiated
users in a 3G telecom network
Sharing the brain
Labs based on football playing robots
• Computational processes given,
parameters given, how to allocate CPU
so that a team plays better football?
• You will implement three schedulers and
compare them
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Linköping
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Linköping
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Information
• Posted on: www.ida.liu.se/~TDDB47
• Your other contacts:
– Course assistant: Aleksandra
Tesanovic
– Lab assistant: Calin Curesu
– Course secretary: Anne Moe
Now a little on course organisation!
Undergraduate course on Real-time Systems
Linköping
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Undergraduate course on Real-time Systems
Linköping
Lab organisation
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Extra lesson
• Follow the instructions on the web for
registration and forming the lab groups
• For DI students, or others with no Java
background
• Get the compendium, read it, and
attend the lab introductory lesson, for a
complete preparation before the labs
• This year: two sessions that can be
taken at different times, covering
introductory parts to Java!
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Linköping
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Undergraduate course on Real-time Systems
Linköping
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Important notice
• Lab environment is under development
• Please make sure that you observe the
instructions
– for finishing the labs before deadline
– and followup examination sessions
• By next year the labs may be different!
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Linköping
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Readings
• The course book is not covered/followed
chapter by chapter!
• There is additional reading material
posted to the web:
– Book chapters: get the reference
copies
– Articles: will be posted
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Linköping
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Scheduling
... is about allocating resources,
specially the CPU time, among all
computational processes such that
the timeliness requirements are met.
Now back to business!
The scheduler decides which
process is to be executed next.
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Linköping
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Scheduling
• Performed off-line or on-line
• With information available statically or
dynamically
• Preemptive or non-preemptive
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Linköping
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Linköping
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Schedulability Analysis
• To confirm that the deadlines for a set
of processes can be guaranteed using a
given policy
• The process set is then considered
”schedulable”
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Linköping
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Order matters!
Computation
time
Release time
Consider following tasks:
Deadline
time
Computation time (Ci)
Deadline (Di)
Event
Reaction
t0
How should we take care of several
parallel activities?
Undergraduate course on Real-time Systems
Linköping
t0
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a1
a2
5 ms
20 ms
a2
•
•
•
•
Worst case execution time (WCET)
Deadline
Release time
...
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Linköping
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a1
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Linköping
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10 ms
12 ms
time
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Linköping
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Parameters
• How to find the maximum computation
time for each process?
• How to determine deadlines?
• When (how often) is a process released?
Undergraduate course on Real-time Systems
Linköping
Release times
• Periodicity: when reading and reacting
to continuous signals
• Minimum inter-arrival time: when
recognising/reacting to some aperiodic
events (sporadic processes)
a2
time
Which parameters?
Scheduling policy induces an order on
executions using an algorithm and a set
of parameters:
a1
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Off-line scheduling
... With a cyclic executive
• A static schedule is fixed based on pre-runtime
analysis
• The analysis might have to be repeated
[Iterative check: Will it work with the natural
periods and computation times? Otherwise,
change the parameters!]
• When executing: Run the processes in predetermined order using a table look-up
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Linköping
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A cyclic executive
every_major_cycle do{
read all in_signals;
run_minor_cycle_1_processes;
wait_for_interrupt;
write all out_signals;
...
read all in_signals;
run_minor_cycle_n_processes;
wait_for_interrupt;
write all out_signals;
}
Example (1)
Consider following processes:
P1
P2
Period/Deadline(Ti)
Worst case execution time (Ci)
50
10
100
30
Note: repetition!
...
0
Undergraduate course on Real-time Systems
Linköping
50
100
150
200
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Linköping
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time
250
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Harmonic processes
Minor cycle: greatest common devisor
(sv. sgd)
What if periods are not harmonic?
Major cycle: least common multiplier
(sv. mgn)
Example:
process
period
A
20
B
40
C
60
...
0
20
120
Undergraduate course on Real-time Systems
Linköping
Undergraduate course on Real-time Systems
Linköping
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How to construct?
• Each process Pi is assumed to be
strictly periodic (i.e. should be run once
every Ti)
• Place the processes in minor cycle and
major cycle until repetition appears
• If the periods are not harmonically
related
– change the periods
– all processes should be run at least as
often as every (original) Ti
Undergraduate course on Real-time Systems
Linköping
Example (2.1)
process
period
A
75
B
100
Alternative 1:
Run process B more often than necessary,
e.g. once every 75 time units.
minor cycle
0
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75
Undergraduate course on Real-time Systems
Linköping
Drawbacks?
...
time
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Example (2.2)
process
period
A
75
B
100
Alternative 2:
Choose minor cycle as greatest
common divisor, and move processes
backwards in time when they clash.
100
time
150
Undergraduate course on Real-time Systems
Linköping
A
75
B
100
Drawbacks?
...
25
process
period
Alternative 3:
A mix of the last two
Drawbacks?
0
Example (2.3)
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Schedulability test
• Sum of processes’ execution times
(WCET) in each minor cycle is less than
the cycle’s length, and processes run at
the ”right” frequency.
minor cycle
0
...
time
50
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Linköping
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Reading for this lecture
• Burn & Wellings, Ch 1. and Ch. 13
If succeeded:
• the schedule, whose length corresponds
to the major cycle, is repeated for all
executions.
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Linköping
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Linköping
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