RealTime Computing

1
Real-Time Computing

• Sunggu Lee
• EE Dept., POSTECH

2
Overview

• Introduction
• Characteristics and Challenges of Real-Time
  Computing Systems
• Definitions, Issues and Comparisons
• Tasks and Scheduling
• Worst-Case Execution Time Analysis
• Real-Time Software
• Real-Time Operating Systems
• Middleware

3
Introduction Crnkovic 2002

• Embedded computers
• Examples Medical control equipment, mobile
  phones, and vehicle control systems.
• Most such embedded systems can also be
  characterized as real-time systems.
• They must usually meet stringent specifications
  for safety, reliability, limited hardware
  capacity etc

4
Characteristics and Challenges of RTS Crnkovic
2002

• Real-time systems are computing systems in which
  the meeting of timing constraints is essential to
  correctness.
• If the system delivers the correct answer, but
  after a certain deadline, it could be regarded as
  having failed.

5
Types of Real-Time Systems

• Hard real-time system
• A system where something very bad happens if
  the deadline is not met
• Examples control systems for aircraft, nucluear
  reactors, chemical power plants, jet engines,
  etc.
• Soft real-time system
• A system where the performance is degraded below
  what is generally considered acceptable if the
  deadline is missed
• Example multimedia system

6
Utility Function (Task Value Function) Kim 2005
Task with a hard deadline
Utility
Task with a soft deadline
1
0
deadline
Time
7
Issues in Real-Time Computing Liu 2000

• Real-time computing deals with all problems in
  computer architecture, fault-tolerant computing
  and operating systems are also problems in
  real-time computing, with the added complexity of
  having to meet real-time constraints
• Real-time computer systems differ from
  general-purpose systems
• They are more specific in their applications
• The consequences of their failure are more
  drastic
• Emphasis is placed on meeting task deadlines

8
Example Problems in Real-Time Computing

• Example 1 Task Scheduling
• General-purpose system can use round-robin
  scheduling
• This is NOT suitable for real-time systems
  because high-priority tasks may miss their
  deadlines with round-robin scheduling
• A priority mechanism is necessary
• Example 2 Cache Usage Scheduling
• A general-purpose system typically allows the
  process that is currently executing the right to
  use the entire cache area
• This keeps the cache miss rate low
• Side effect task run times are less predictable
• Thus, not so desirable for real-time systems

9
Comparison of Typical Systems Liu 2000

• Jet Engine Control System
• Designer knows precise workload to be executed
• System must be designed to meet task deadlines
• If a deadline is not met, the jet engine may
  explode

• General-Purpose Computer Workstation
• Workload is not known in advance
• System should be designed to be fast on the
  average
• Task execution time variance is less important

10
Tasks Crnkovic 2002

• Real-time systems can be constructed out of
  sequential programs, but typically they are built
  out of concurrent programs, called tasks.
• Tasks are usually divided into
• Periodic tasks consist of an infinite sequence
  of identical activities, called instances, which
  are invoked within regular time periods.
• Non-periodic or aperiodic are invoked by the
  occurrence of an event.
• Sporadic aperiodic tasks with a bounded
  interarrival time

11
Scheduling Crnkovic 2002

• Offline scheduling
• The scheduler has complete knowledge of the task
  set and its constraints.
• Online scheduling
• Make their scheduling decisions during run-time.
• Deadline
• Is the maximum time within which the task must
  complete its execution with respect to an event.
• Real-time systems are divided into two classes,
  hard and soft real-time systems

12
Schedulability Analysis Crnkovic 2002

• At this point we must check that the temporal
  requirements of the system can be satisfied,
  assuming time budgets assigned in the detailed
  design stage.
• In other words, we need to make a schedulability
  analysis of the system based on the temporal
  requirements of each component

13
WCET Verification Crnkovic 2002

• Performing a worst-case analysis can either be
  based on measurements or on a static analysis of
  the source code.
• What is more interesting in the test cases is the
  execution time behavior shown as a function of
  input parameters as shown in the following slide.

14
An Execution Time Graph Crnkovic 2002

• The execution time shows different values for the
  different
• input sub-domains.

15
Maximum execution time per sub-domain Crnkovic
2002
16
Composition of Components Crnkovic 2002
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End-To-End Deadlines Crnkovic 2002

• End-to-end deadlines
• Are set such that the system requirements are
  fulfilled in the same way as the time budgets are
  set
• Should be specified for the input to and output
  from the component since the WCET cannot be
  computed since its parts may be executing with
  different periods.

18
Real-Time Operating System

• Multi-tasking OS designed to permit tasks
  (processes) to complete within precisely stated
  deadlines
• If deadline constraints cannot be met for a new
  task, it may be rejected
• If a new task would result in deadline violations
  for other tasks, it may be rejected
• Example commercial operating systems
• Vrtx Mentor Graphics Systems
• VxWorks and pSOS Wind River Systems
• RTLinux FSMLabs, later acquired by Wind River
  Systems

19
Real-Time Middleware

• ObjectWeb defines middleware as "The software
  layer that lies between the operating system and
  the applications on each side of a distributed
  computing system in a network." Wikipedia
• Collection of Tools for Real-Time Programming
• Real-Time Java
• Real-Time Middleware Systems
• Real-Time CORBA
• CORBA Component Object Request Broker
  Architecture
• Time-triggered Message-triggered Object (TMO)
• System developed at the University of California
  at Irvine

20
Essence of RT Programming Time-Triggered Action
Kim 2005

• At time T do S
• Start S during T - ?, T ?
• A fundamental distinguishing part of real-time
  programming
• If S is a function, a control signal for
  activation of the function in a node is derived
  from the progression of real-time
• Whenever the real-time clock within a node
  reaches a preset value T specified in a
  scheduling table, a control signal is generated
• In principle, S may be a single assignment
  statement, a compound statement, or a function.

21
Essence of RT Programming Time-Triggered Action
Kim 2005

• Factors impacting response times
• - Application, Middleware, OS, Hardware, Comm
  Network

Real-timeobjects
Middleware for real-time support
Middleware for real-time support
Middleware for real-time support
OS
OS
OS
H/W
H/W
H/W
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Essence of RT Programming Time-Triggered Action
(cont) Kim 2005

• If there are many factors impacting response
  times, analysis of response times becomes very
  complicated and often impossible unless every
  contributing factor has been very carefully
  designed.
• gt Conventional response
• Avoid most of the software layers
• Implement application software in assembly or C
  programs
• ?
• Very low design productivity !!!
• This practice CAN NOT continue in the 2nd half
  of 21st Century !!! (except in unusual
  circumstances)

23
Facilities in TMO for Time-Triggered Actions
Kim 2005
TMO Time-Triggered Message-Triggered
Object
24
New-Generation Real-Time (RT) Distributed
Computing (DC) Component Kim 2005
High-Level RT ObjectTime-triggered
Message-triggered Object (TMO)

• Initiated in early 1990's
• Meant to be a natural easy-to-use extension of
  the C/Java object

• Can support Hard-RT DC Software Engineering (SE)
  as well as Non-RT DC SE
• Contains only high-level intuitive and yet
  precise expressions of timing requirements

25
New-Generation RT DC Component Kim 2005
High-Level RT ObjectTime-triggered
Message-triggered Object (TMO)

• Meant to be a natural easy-to-use extension of
  the C/Java object

• Can support Hard-RT DC SE as well as Non-RT DC SE
• Contains only high-level intuitive and yet
  precise expressions of timing requirements
• Formulated from the beginning with the objective
  of enabling design-time guaranteeing of timely
  actions

26
TMO Network Structuring Kim 2005
All conceivable RT DC applications and Non-RT
applications can be Structured as TMO networks .
Kernel ( e.g. NT kernel )
Kernel ( e.g. NT kernel )
Kernel ( e.g. NT kernel )
H/W
H/W
H/W
27
Some New-Generation Applications Kim 2005

• Distributed multi-media processing
• Next-generation RT VR (Virtual Reality)
• Multi-party multi-media conferencing and
  collaboration
• Distributed orchestra (?)
• Drive-by-wire Tele-driving Telematics

F
Precisely timed actions, please !!!

• Military command-control
• Missile defense (ground targets, airborne
  targets, ship-borne targets) and
  sub-division level command-control
• Time-sensitive health care
• Monitoring of in- out-patients
• Remotely controlled surgery
• Non-stop web servers

Non-stop service, please !!!
28
An example of a TMO network
Ex 2
Telematics in a Car Fleet Kim 2005
29
References

• Jane Liu, Real-Time Systems, Prentice-Hall, Upper
  Saddle River, 2000.
• Ivica Crnkovic and Magnus Larsson, Building
  Reliable Component-Based Software Systems, Artech
  House Publishers, July 2002. PowerPoint slides
  from http//www.idt.mdh.se/cbse-book/presentations
  /13-chapterWC.ppt
• Kane (K. H.) Kim, PowerPoint slides used for
  undergraduate class ECES 123 Introduction to
  Real-Time Distributed Programming taught at the
  University of California at Irvine,
  http//dream.eng.uci.edu