fault-tolerant computing jenn-wei lin department of computer science and information engineering fu...

FAULT-TOLERANT COMPUTING

Jenn-Wei LinDepartment of Computer Science and Information Engineering

Fu Jen Catholic University

Motivation and IntroductionLecture Set 1

ECE 753 Fault Tolerant Computing 2

General Information• Textbook

– Marin L. Shooman: Reliability of Computer Systems and Networks: Fault Tolerance, Analysis, and Design, John Wiley and Sons, 2002.

– D.P. Siewiorek and R.S. Swarz: Reliable Computer Systems: Design and Evaluation, 3rd ed. A. K. Peters, 1999.

– D. K. Pradhan, editor, Fault-Tolerant Computer System Design, Prentice-Hall, 1996. The book is out of print

• Paper– Dependable Computing Conference

• Grading Policy– Exam. 20%– Presentation 40% (four)– Term report & Project 40%


Overview• Motivation

• Introduction

• Terminology

• Fundamental Principles

• Fault-Error-Failure concept


Motivation

• Informal Definition

• Key Attributes

• Who, What and Why Study

• Examples


Motivation

• What is Fault-Tolerance?

A “fault-tolerant system” is one that continues to perform at desired level of service in spite of failurs in some componetns that constitute the system.


Motivation (contd.)• Who is concerned about fault-tolerance?

– System Users• Who is concerned at design stages?

– Universities• R, d, and a (Research, development,

applications)– Industry

• r, D, and A (research, Development, Applications)


Motivation (contd.)

Examples

• General Purpose Systems– PCs: RAMs with parity checks– Workstations: error detection (HW), occasional

corrective action (SW), ECC (HW), keeping log (SW)


Motivation (contd.)

Examples

• Reliable Systems– Telephone systems– Banking systems e.g. ATM– Stock market– Football games display/ticketing


Motivation (contd.)

Examples

• Critical and Life Critical Systems– Manned and unmanned space borne systems– Aircraft control systems– Nuclear reactor control systems– Life support systems


Motivation (contd.)

Examples

• Reliable -> Critical Systems– 911 telephone switching system– Traffic light control system– Automobile control system (ABS, Fuel

injection system)


Introduction

– Historical perspective and major push

– Goals of fault-tolerance

– Applications of fault-tolerance


Introduction (contd.)

• Historical Perspective– not a new concept

– first use by J. van Neumann 1956

• Major push– Space program

– HW Fault tolerance - then

– SW Fault tolerance later

– Merge the two


Introduction (contd.)

• Applications– Space borne system

• long life system

– Airplane control system• critical system

– Transaction processing system• high availability system

– Switching system• high availability over certain level of performance


Terminology

• Reliability and concept of probability– R(t): conditional probability that a system provides continuous

proper service in the interval [0,t] given that it provided desired service at time 0.

• Availability– The probability that an item is up at any point in time– Uptime/(Uptime+Downtime)

• Dependability– Property of computer system that allows

reliance to be placed justifiably on service it delivers


Fundamental Principles

• Dependability

• Impairments– Faults, errors, failures

• Means– Fault Avoidance, Fault Tolerance, Fault Removal, Fault

Forecasting

• Measures– Reliability, Availability, Maintainability


Fundamental Principles (contd.)

• A set of methods, tools and solutions that enable development of dependable systems.

- Fault Prevention: how to prevent fault occurrence or

introduction,- Fault Tolerance: how to ensure a service up to fulfilling the system’s function in the presence of faults,- Fault Removal: how to reduce the presence (number seriousness) of faults,- Fault Forecasting: how to estimate the present number, the future incidence, and the consequences of faults



• Fault Avoidance: To prevent by construction fault occurrence. E.g., nearly fault-free components, shielding against electromagnetic fields– Drawbacks:

- Cost of near-perfect components high- Cost of maintenance personnel

• Fault Tolerance: To provide, by redundancy, service complying with specification in spite of faults occurring



• Fault Removal: To minimize, by verification, the presence of faults. E.g. Am I building the right system? Concepts of coverage,etc.

• Fault Forecasting: To estimate, by evaluation, the presence, occurrence and consequences of faults. E.g. For how long will the system be right ?



• Reliability: A measure of continuous delivery of proper service (or equivalently, of the time to failure) from a reference initial time

• Availability: A measure of the delivery of the proper service with respect to the alternation of delivery of proper and improper service

• Maintainability: A measure of continuous delivery of improper service (time to restoration or repair)



• Hardware redundancy• Low level

• High level

• Software Redundancy• Time Redundancy• Information Redundancy



• Hardware Redundancy - Low level– logic level

• Example 1 - Self checking circuits

• Example 2 - Arithmetic code A modular adder using the mathematical principle

(A+B+|) mod k = ((A mod k) + (B mod k)) mod k

• Hardware Redundancy - High level– Triplicate or 5-copies as in space shuttle



• Software Redundancy – Use two different programs/algorithms

• Time Redundancy– Re-compute or redo the task and compare the results

– May or may not use the same hardware/software

• Information Redundancy– backup information

– Use of ECC

• Question - What kind of FT is achieved?


Fault-Error-Failure concept

• Intuitive definitions• Origins of faults• Methods to break FEF chain• Attribute of faults


Fault-Error-Failure concept (contd.)

Intuitive definitions

• Fault -– An anomalous physical condition caused by a

manufacturing problem, fatigue, external disturbance (intentional or un-intentional), desgin flaw, …

– Causes

• Error - Effect of activation of a fault

• Failure - over-all system effect of an error

Fault -> Error -> Failure



• Failure occurs when the delivered service deviates from the specified service; failures are caused by errors

• Error is the manifestation of a fault within a program or data structure

• Fault is an incorrect state of hardware or software resulting from failures of components, physical interferences from the environment, operator error or incorrect design


Fault-Error-Failure concept (contd.) Causes of faults

• Specification mistakes– Incorrect algorithms, architectures, or hardware and software design

specification• Implementation mistakes

– Process of transforming hardware and software specifications into the physical hardware and the actual software

– Poor design, poor component selection, poor construction, software coding mistakes

• Component defects– Manufacturing imperfections, random device defects, and component

wear-out• External disturbance

– Radiation, electromagnetic interference, operator mistakes, battle damage, and environmental extremes



Causes of faultsSpecification

Mistakes

Implementaion Mistakes

External Disturbances

Component Defects

SoftwareFaults

HaredwareFaults

ErrorsSystemFailures


Fault-Error-Failure concept (contd.) Characteristics of faults

• Fault nature– Specify the type of fault

• Is the fault a hardware or a software fault?• Fault duration

– Specify the length of time that a fault is active• Permanent fault• Transient fault

– Appear and disappear within a very short period of time• Intermittent fault

– Appear, disappear, and reappear repeatedly• Fault extent

– Fault is localized to a given hardware or software module or globally affects the hardware, the software, or both.

• Fault value– Determinate or indeterminate

• Fault sensitive to either the data or time

fault-tolerant computing jenn-wei lin department of computer science and information engineering fu...

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