TDDB47 Real Time Systems
Lecture 1: Introduction & cyclic scheduling
Real-time systems
Mikael Asplund
Real-Time Systems Laboratory
Department of Computer and Information Science
Linköping University, Sweden
The lecture notes are partly based on lecture notes by Calin Curescu, Simin NadjmTehrani, Jörgen Hansson, Anders Törne. They also loosely follow Burns’ and Welling
book “Real-Time Systems and Programming Languages”. These lecture notes should
only be used for internal teaching purposes at Linköping University.
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
42 pages
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
What is a real-time system?
2 of 42
Computer systems
• Correctness depends on time!
• Predictability
• Usually a component in a larger engineering
system - embedded
• Usually a critical system
– Safety-critical
– Dependable
– Fault-tolerant
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
3 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
4 of 42
Really good example
Why should you care?
• For the fun...
• For the money
• To save the world
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
5 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
6 of 42
Course Information
• Website: www.ida.liu.se/~TDDB47
• Examiner
Course
– Mikael Asplund, mikas@ida.liu.se
• Course assistant
– Jonas Elmqvist, jonel@ida.liu.se
• Course secretary
– Gunilla Mellheden, gunme@ida.liu.se
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
7 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
8 of 42
Aim and Prerequisites
• Aims
• Course Book
– To understand the methods and structuring mechanisms
involved in the design and implementation of real-time
systems.
– A. Burns and A. Wellings: Real-Time Systems and
Programming Languages, Addison-Wesley,
– NOT covered/followed chapter by chapter
• Prerequisites
• Additional material posted on the web
– Process Programming and Operating Systems
(TDDB68/72)
– Some Java knowledge for the labs
• Introductory lesson
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
Reading
– Get copies from assistant on material that is not
available on the course Literature page
9 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
Laboratory
10 of 42
Lectures and Lessons
• Apply your knowledge!
• Lectures vs. reading
• Real-time system
• Slides
• Please make sure that you
• Lessons:
– Read the compendium
– Introduction to labs
– Observe the instructions
– Possibility to ask questions before exam
– Keep the deadline (6th December)
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
11 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
12 of 42
Examination
• Written exam
– December 13th 14.00-18.00
– Old exams available on the web
Real-time systems
• Labs
– Practical part
– Theoretical part
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
13 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
Definitions of an RT System
14 of 42
What is an embedded system?
• Oxford Computing Dictionary
– ”Any system in which the time at which output is
produced is significant. This is usually because the input
corresponds to some movement in the physical world,
and the output has to relate to that same movement. The
lag from input time to output time must be sufficiently
small for acceptable timeliness.”
• Timeliness: milliseconds, seconds, hours
• 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
• Real-time vs. embedded
• Randell et. al 1995
– Usually an rt-system is embedded
– RT and not embedded?
– Embedded and not RT?
– ”A real-time system is a system that is required to react
to stimuli from the environment (including the passage
of physical time) within time intervals dictated by the
environment.”
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
15 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
16 of 42
Application areas
Hard real-time
Utility
• Manufacturing and process control
• Command and control and communication
• Generalised embedded system
r
Fly-by-wire, brake-by wire
Atomic power plant control
Garden sprinkle system
Computer games
Robot vision processing
Communication equipment
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
time
−∞
• Examples
–
–
–
–
–
–
d
• Missed deadline = failure
• Predictability crucial
• Critical applications
17 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
18 of 42
Soft real-time
Utility
Firm real-time
Utility
r
d
r
time
d
time
• A late response is worth nothing
• Deadlines should be met...
• Deadlines can be missed occasionally
• but a miss is just a miss.
• Performance!
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
19 of 42
Characteristics of a RTS
• Large and complex
20 of 42
Misconceptions about RT systems
• Real-time computing is equivalent to fast
computing.
• Concurrent control of separate system
components
• Advances in hardware will solve real-time
problem.
• Facilities to interact with special purpose
hardware
• There is no science in real-time system design.
• Extreme reliability and safety
• Real-time systems research is about performance
tuning.
• Guaranteed response times
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
21 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
Part II
22 of 42
Scheduling
• Allocating resources, especially the CPU time,
among all computational processes such that the
timeliness requirements are met.
Scheduling
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
• The scheduler decides which process is to be
executed next.
23 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
24 of 42
Process as an abstraction
Non Real-Time Scheduling
1. Interactive performance
• For different computational processes in the OS
2. Fairness
• For parallel developments within the physical
environment and in the control system
• Typical metrics and goals:
– Minimize response time
– Maximize throughput
• For nodes in a network, with interaction and
communication
• Examples
– FCFS (First-Come-First-Served)
– RR (Round-Robin)
– CFS (Completely Fair Scheduler)
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
25 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
Real-Time Scheduling
• Scheduling, resource management
2. The rest
• Dependability
Typical metrics and goals:
– Guarantee ratio (hard real-time)
– Maximize completion ratio / minimize miss ratio (hard / firm)
– Minimize overall lateness (soft real-time)
•
Content
• Introduction to Real-time Systems
1. Predictability
•
26 of 42
• Distributed real-time systems
• Real-time operating systems
• Real-time applications
Examples
– Cyclic Scheduling
– EDF (Earliest Deadline First)
– LS (Least Slack)
– RM (Rate-Monotonic)
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
27 of 42
Some parameters
R
r
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
28 of 42
Scheduling: Problem Space
• Uniprocessor / multiprocessor / distributed
system
d
• Periodic / sporadic / aperiodic
• Independent / interdependent
C
•
•
•
•
•
•
•
• Preemptive / non-preemptive
Release time: r
f
Computation time: C
Absolute deadline: d
Finish time: f
Response time: R = f-r
Relative deadline D = d-r
Task period: T (Usually D=T)
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
• Handle transient overloads
• Support fault tolerance
29 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
30 of 42
Simple Process Model
• Fixed set of processes, uniprocessor system
Performance Metrics
• Completion ratio / miss ratio
• Periodic
• Maximize total value (weighted sum)
• Independent
• Context switch is for free
• Maximize value of task
• D=T
• Minimize lateness
• Worst-case execution time is known
• Minimize error (imprecise tasks)
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
31 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
32 of 42
Static vs dynamic scheduling
Variants
• Static (off-line)
• Preemptive vs. non-preemptive
– complete a priori knowledge of the task set and its
constraints is available
– hard/safety-critical system
• Off-line vs. on-line scheduling analysis
• off-line analysis can be used with dynamic scheduling
• Dynamic (on-line)
– partial taskset knowledge, runtime predictions
– firm/soft/best-effort systems, hybrid systems
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
• off-line analysis very important for hard-rt systems
33 of 42
Scheduling Approaches
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
34 of 42
Cyclic Executive
• Cyclic executives
• Fixed priority scheduling
– RM - Rate Monotonic
– DM - Deadline Monotonic Scheduling
• Dynamic priority scheduling
– EDF - Earliest Deadline First
– LSF - Least Slack First
• OBS!
– Don’t confuse dynamic scheduling with dynamic priority
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
35 of 42
1: Introduction
& cyclic scheduling
•Lecture
What
happens
if the periods are not harmonic?
Mikael Asplund
36 of 42
General Procedure
Calculate major cycle =
lcm  T 1 ,T 2 , ,T n 
• Calculate minor cycle =
gcd  T 1 ,T 2 , ,T n 

• Schedulability analysis?
– See if a taskset is schedulable.
• Scheduling analysis?
• Populate all the minor cycles of a first major cycle with tasks
– If a task cannot fit (its WCET) in a minor cycle it needs to
be split
• Over several minor cycles
• If the programming technique allows it
– Every task should run to completion once per period
• Period might span several minor cycles
• A new task instance cannot be scheduled before its natural
release
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
Schedulability analysis
– Analyse the particular scheduling.
• Is a taskset schedulable using cyclic scheduling?
– Construct the schedule and find out.
• ∑ Ci /T i
≤ 1
?
– YES: If we assume a simple task model, and that tasks can
be split in arbitrarily small pieces, then the taskset is
schedulable using cyclic scheduling.
– NO: The taskset is not schedulable using cyclic scheduling.
37 of 42
Task dependencies & cyclic
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
38 of 42
Cyclic executives observations
• No actual processes exist at run-time
• Dependences due to
• Sharing resources
• Computation precedence requirements
• The procedures share a common address
space
• Make sure the running order of the tasks is
respected
• Data does not need to be protected
• A taskset that is schedulable if tasks are
independent might not be schedulable if
dependencies are introduced
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
39 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
Problems with Cyclic
40 of 42
Reading
• Sporadic processes
• Burns & Wellings, Ch 1. and Ch. 13
• Processes with long periods
• Tasks with dependencies
• Adding new tasks
• Processes with sizeable computation times.
– How do you split them?
– Can you split them?
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
41 of 42
Lecture 1: Introduction & cyclic scheduling
Mikael Asplund
42 of 42