Realtime

Real-Time Computing
Sunggu Lee EE Dept., POSTECH

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

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 2

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

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 3

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.

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 4

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

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 5

Utility Function (Task Value Function) [Kim 2005]
Utility 1
Task with a hard deadline Task with a soft deadline

0

deadline
Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Time
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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 generalpurpose systems

 

They are more specific in their applications
The consequences of their failure are more drastic Emphasis is placed on meeting task deadlines

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 7

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 roundrobin 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 Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 8

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

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 9

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]

 

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 10

Scheduling [Crnkovic 2002]
 Offline scheduling:


The scheduler has complete knowledge of the task set and its constraints. Make their scheduling decisions during run-time. 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

 Online scheduling:


 Deadline:
 

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 11

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

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 12

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.

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 13

An Execution Time Graph [Crnkovic 2002]
The execution time shows different values for the different input sub-domains.

Execution time

Input domain 1 domain 2 domain 3
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Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Maximum execution time per sub-domain [Crnkovic 2002]

Execution time

Input domain 1

domain 2

domain 3

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 15

Composition of Components [Crnkovic 2002]

New Component (Cnew) Component 1 (C1)

in1_Cnew

in_C1

out_C1 out1_Cn out1_Cnew out2_Cnew

in2_Cnew

in2_Cn

Component n (C2)

out2_Cn

in3_Cnew in4_Cnew

in1_C2 in2_C2

Component 2 (C3)

out_C2

out3_Cnew

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 16

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.

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 17

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 Vrtx – Mentor Graphics Systems VxWorks and pSOS – Wind River Systems RTLinux – FSMLabs, later acquired by Wind River Systems
Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

 Example commercial operating systems
  

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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 CORBA
CORBA: Component Object Request Broker Architecture

 Real-Time Middleware Systems
 

Time-triggered Message-triggered Object (TMO)
System developed at the University of California at Irvine

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 19

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.
Building Reliable Component-based Systems
EECE 426 - Embeddede Systems
Page 20

Essence of RT Programming: Time-Triggered Action [Kim 2005]
• Factors impacting response times
- Application, Middleware, OS, Hardware, Comm Network Real-time objects

Middleware for real-time support

Middleware for real-time support

Middleware for real-time support

OS
H/W

OS
H/W

OS
H/W

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 21

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. => 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)
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Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Facilities in TMO for Time-Triggered Actions [Kim 2005]
TMO := Time-Triggered Message-Triggered Object

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 23

New-Generation Real-Time (RT) Distributed Computing (DC) Component [Kim 2005] High-Level RT Object:

Time-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
C++ object
var AAC AAC


SpM 1 SpM 2

 

DC SE
 Contains only high-level intuitive and yet precise expressions of timing

SvM 1 SvM 2

requirements
Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

 

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New-Generation RT DC Component [Kim 2005]

High-Level RT Object:

Time-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

C++ object

var AAC AAC



SpM 1 SpM 2

 Formulated from the beginning with the objective of enabling design-

 

SvM 1 SvM 2

time guaranteeing of timely actions
Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

 

Page 25

TMO Network Structuring [Kim 2005]
All conceivable RT DC applications and Non-RT applications can be Structured as TMO networks .

Middleware
FT support NT service TMOSM

Middleware
FT support NT service TMOSM

Middleware
FT support NT service TMOSM

Kernel ( e.g. NT kernel )
H/W

Kernel ( e.g. NT kernel )
H/W

Kernel

( e.g. NT kernel ) H/W

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 26

Some New-Generation Applications [Kim 2005]
F

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 Military command-control  Missile defense (ground targets, airborne targets, shipNon-stop borne targets) and service, please sub-division level command-control !!! Time-sensitive health care  Monitoring of in- & out-patients  Remotely controlled surgery Non-stop web servers Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Precisely timed actions, please !!!

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Ex 2

Telematics in a Car Fleet [Kim 2005]
Base Intf TMO
ODSS1 SpM1

An example of a TMO network

Base Intf TMO
ODSS1 SpM1

SvM1

SvM1

Car Intf TMO
ODSS1 SpM1

SvM1

Car Intf TMO
ODSS1 SpM1

Car Intf TMO
ODSS1 SpM1

SvM1

SvM1

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

Page 28

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/13chapterWC.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

Building Reliable Component-based Systems
EECE 426 - Embeddede Systems

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