Debugging - The 9 Indispensable Rules for Finding Even the Most ....pdf

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Header

Table of Contents

DebuggingŠThe Nine Indispensable Rules for Finding Even the Most Elusive Software and Hardware Problems

Chapter 1: Introduction

Overview

How Can That Work?

Isn't It Obvious?

Anyone Can Use It

It'll Debug Anything

But It Won't Prevent, Certify, or Triage Anything

More Than Just Troubleshooting

A Word About War Stories

Stay Tuned

Chapter 2: The Rules-Suitable for Framing

Chapter 3: Understand the System

Overview

Read the Manual

Read Everything, Cover to Cover

Know What's Reasonable

Know the Road Map

Know Your Tools

Look It Up

Remember

Understand the System

Chapter 4: Make it Fail

Overview

Do It Again

Start at the Beginning

Stimulate the Failure

Don't Simulate the Failure

What If It's Intermittent?

What If I've Tried Everything and It's Still Intermittent?

A Hard Look at Bad Luck

Lies, Damn Lies, and Statistics

Did You Fix It, or Did You Get Lucky?

"But That Can't Happen"

Never Throw Away a Debugging Tool

Remember

Make It Fail

Chapter 5: Quit Thinking and Look

Overview

See the Failure

See the Details

Now You See It, Now You Don't

Instrument the System

Design Instrumentation In

Build Instrumentation In Later

Don't Be Afraid to Dive In

Add Instrumentation On

Instrumentation in Daily Life

The Heisenberg Uncertainty Principle

Guess Only to Focus the Search

Remember

Quit Thinking and Look

Chapter 6: Divide and Conquer

Overview

Narrow the Search

In the Ballpark

Which Side Are You On?

Inject Easy-to-Spot Patterns

Start with the Bad

Fix the Bugs You Know About

Fix the Noise First

Remember

Divide and Conquer

Chapter 7: Change One Thing at a Time

Overview

Use a Rifle, Not a Shotgun

Grab the Brass Bar with Both Hands

Change One Test at a Time

Compare with a Good One

What Did You Change Since the Last Time It Worked?

Remember

Change One Thing at a Time

Chapter 8: Keep an Audit Trail

Overview

Write Down What You Did, in What Order, and What Happened

The Devil Is in the Details

Correlate

Audit Trails for Design Are Also Good for Testing

The Shortest Pencil Is Longer Than the Longest Memory

Remember

Keep an Audit Trail

Chapter 9: Check the Plug

Overview

Question Your Assumptions

Don't Start at Square Three

Test the Tool

Remember

Check the Plug

Chapter 10: Get a Fresh View

Overview

Ask for Help

A Breath of Fresh Insight

Ask an Expert

The Voice of Experience

Where to Get Help

Don't Be Proud

Report Symptoms, Not Theories

You Don't Have to Be Sure

Remember

Get a Fresh View

Chapter 11: If You Didn't Fix it, It Ain't Fixed

Overview

Check That It's Really Fixed

Check That It's Really Your Fix That Fixed It

It Never Just Goes Away by Itself

Fix the Cause

Fix the Process

Remember

If You Didn't Fix It, It Ain't Fixed

Chapter 12: All the Rules in One Story

Chapter 13: Easy Exercises for the Reader

Overview

A Light Vacuuming Job

A Flock of Bugs

A Loose Restriction

The Jig Is Up

Chapter 14: The View From the Help Desk

Overview

Help Desk Constraints

The Rules, Help Desk Style

Understand the System

Make It Fail

Quit Thinking and Look

Divide and Conquer

Change One Thing at a Time

Keep an Audit Trail

Check the Plug

Get a Fresh View

If You Didn't Fix It, It Ain't Fixed

Remember

The View From the Help Desk Is Murky

Chapter 15: The Bottom Line

Overview

The Debugging Rules Web Site

If You're an Engineer

If You're a Manager

If You're a Teacher

Remember

List of Figures

List of Sidebars

Current

Debugging

Table of Contents Debugging—The Nine Indispensable Rules for Finding Even the Most Elusive Software and Hardware Problems...................................................................................................................1 Chapter 1: Introduction.....................................................................................................................4 Overview..................................................................................................................................4 How Can That Work?...............................................................................................................4 Isn't It Obvious?.......................................................................................................................4 Anyone Can Use It...................................................................................................................5 It'll Debug Anything..................................................................................................................5 But It Won't Prevent, Certify, or Triage Anything.....................................................................6 More Than Just Troubleshooting.............................................................................................6 A Word About War Stories.......................................................................................................7 Stay Tuned...............................................................................................................................7 Chapter 2: The Rules−Suitable for Framing....................................................................................9 Chapter 3: Understand the System................................................................................................10 Overview................................................................................................................................10 Read the Manual....................................................................................................................11 Read Everything, Cover to Cover..........................................................................................12 Know What's Reasonable......................................................................................................13 Know the Road Map..............................................................................................................14 Know Your Tools....................................................................................................................14 Look It Up...............................................................................................................................15 Remember.............................................................................................................................16 Understand the System....................................................................................................16 Chapter 4: Make it Fail.....................................................................................................................17 Overview................................................................................................................................17 Do It Again.............................................................................................................................19 Start at the Beginning............................................................................................................19 Stimulate the Failure..............................................................................................................19 Don't Simulate the Failure......................................................................................................20 What If It's Intermittent?.........................................................................................................21 What If I've Tried Everything and It's Still Intermittent?..........................................................22 A Hard Look at Bad Luck.................................................................................................22 Lies, Damn Lies, and Statistics........................................................................................23 Did You Fix It, or Did You Get Lucky?.............................................................................23 "But That Can't Happen"........................................................................................................24 Never Throw Away a Debugging Tool...................................................................................25 Remember.............................................................................................................................26 Make It Fail.......................................................................................................................26 Chapter 5: Quit Thinking and Look................................................................................................27 Overview................................................................................................................................27 See the Failure.......................................................................................................................29 See the Details.......................................................................................................................31 Now You See It, Now You Don't............................................................................................33 Instrument the System...........................................................................................................33 Design Instrumentation In................................................................................................33 i

Table of Contents Chapter 5: Quit Thinking and Look Build Instrumentation In Later..........................................................................................35 Don't Be Afraid to Dive In.................................................................................................36 Add Instrumentation On...................................................................................................36 Instrumentation in Daily Life.............................................................................................37 The Heisenberg Uncertainty Principle...................................................................................37 Guess Only to Focus the Search...........................................................................................38 Remember.............................................................................................................................38 Quit Thinking and Look....................................................................................................38 Chapter 6: Divide and Conquer......................................................................................................40 Overview................................................................................................................................40 Narrow the Search.................................................................................................................42 In the Ballpark..................................................................................................................43 Which Side Are You On?.................................................................................................44 Inject Easy−to−Spot Patterns................................................................................................44 Start with the Bad...................................................................................................................45 Fix the Bugs You Know About...............................................................................................46 Fix the Noise First..................................................................................................................46 Remember.............................................................................................................................47 Divide and Conquer.........................................................................................................47 Chapter 7: Change One Thing at a Time.......................................................................................48 Overview................................................................................................................................48 Use a Rifle, Not a Shotgun....................................................................................................49 Grab the Brass Bar with Both Hands.....................................................................................50 Change One Test at a Time...................................................................................................51 Compare with a Good One....................................................................................................51 What Did You Change Since the Last Time It Worked?........................................................52 Remember.............................................................................................................................54 Change One Thing at a Time...........................................................................................54 Chapter 8: Keep an Audit Trail.......................................................................................................55 Overview................................................................................................................................55 Write Down What You Did, in What Order, and What Happened..........................................56 The Devil Is in the Details......................................................................................................57 Correlate................................................................................................................................58 Audit Trails for Design Are Also Good for Testing.................................................................58 The Shortest Pencil Is Longer Than the Longest Memory.....................................................59 Remember.............................................................................................................................59 Keep an Audit Trail...........................................................................................................59 Chapter 9: Check the Plug..............................................................................................................61 Overview................................................................................................................................61 Question Your Assumptions..................................................................................................62 Don't Start at Square Three...................................................................................................63 Test the Tool..........................................................................................................................63 Remember.............................................................................................................................65 Check the Plug.................................................................................................................65 ii

Table of Contents Chapter 10: Get a Fresh View.........................................................................................................66 Overview................................................................................................................................66 Ask for Help...........................................................................................................................66 A Breath of Fresh Insight.................................................................................................66 Ask an Expert...................................................................................................................67 The Voice of Experience..................................................................................................67 Where to Get Help.................................................................................................................68 Don't Be Proud.......................................................................................................................69 Report Symptoms, Not Theories............................................................................................69 You Don't Have to Be Sure..............................................................................................70 Remember.............................................................................................................................70 Get a Fresh View.............................................................................................................70 Chapter 11: If You Didn't Fix it, It Ain't Fixed................................................................................71 Overview................................................................................................................................71 Check That It's Really Fixed..................................................................................................72 Check That It's Really Your Fix That Fixed It.........................................................................72 It Never Just Goes Away by Itself..........................................................................................73 Fix the Cause.........................................................................................................................73 Fix the Process......................................................................................................................74 Remember.............................................................................................................................75 If You Didn't Fix It, It Ain't Fixed.......................................................................................75 Chapter 12: All the Rules in One Story..........................................................................................76 Chapter 13: Easy Exercises for the Reader..................................................................................78 Overview................................................................................................................................78 A Light Vacuuming Job..........................................................................................................78 A Flock of Bugs......................................................................................................................79 A Loose Restriction................................................................................................................81 The Jig Is Up..........................................................................................................................85 Chapter 14: The View From the Help Desk....................................................................................88 Overview................................................................................................................................88 Help Desk Constraints...........................................................................................................89 The Rules, Help Desk Style...................................................................................................89 Understand the System....................................................................................................90 Make It Fail.......................................................................................................................91 Quit Thinking and Look....................................................................................................91 Divide and Conquer.........................................................................................................92 Change One Thing at a Time...........................................................................................92 Keep an Audit Trail...........................................................................................................92 Check the Plug.................................................................................................................93 Get a Fresh View.............................................................................................................93 If You Didn't Fix It, It Ain't Fixed.......................................................................................94 Remember.............................................................................................................................94 The View From the Help Desk Is Murky...........................................................................94 iii

Table of Contents Chapter 15: The Bottom Line..........................................................................................................95 Overview................................................................................................................................95 The Debugging Rules Web Site.............................................................................................95 If You're an Engineer.............................................................................................................95 If You're a Manager................................................................................................................95 If You're a Teacher.................................................................................................................96 Remember.............................................................................................................................96 List of Figures..................................................................................................................................98 List of Sidebars..............................................................................................................................100 iv

Debugging—The Nine Indispensable Rules for Finding Even the Most Elusive Software and Hardware Problems David J. Agans American Management Association New York • Atlanta • Brussels • Buenos Aires • Chicago • London • Mexico City San Francisco • Shanghai • Tokyo • Toronto • Washington , D.C. Special discounts on bulk quantities of AMACOM books are available to corporations, professional associations, and other organizations. For details, contact Special Sales Department, AMACOM, a division of American Management Association, 1601 Broadway, New York, NY 10019. Tel.: 212−903−8316. Fax: 212−903−8083. Web site: http://www.amacombooks.org/ This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold with the understanding that the publisher is not engaged in rendering legal, accounting, or other professional service. If legal advice or other expert assistance is required, the services of a competent professional person should be sought. Library of Congress Cataloging−in−Publication Data Agans, David J., 1954− Debugging: the 9 indispensable rules for finding even the most elusive software and hardware problems / David J. Agans. p. cm. Includes index. ISBN 0−8144−7168−4 1. Debugging in computer science. 2. Computer software—Quality control. I. Title. QA76.9.D43 A35 2002 005.1 4—dc21 2002007990 Copyright © 2002 David J. Agans. All rights reserved. Printed in the United States of America. This publication may not be reproduced, stored in a retrieval system, 1

or transmitted in whole or in part, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of AMACOM, a division of American Management Association, 1601 Broadway, New York, NY 10019. Printing number 10 9 8 7 6 5 4 3 2 1 To my mom, Ruth (Worsley) Agans, who debugged Fortran listings by hand at our dining room table, fueled by endless cups of strong coffee. And to my dad, John Agans, who taught me to think, to use my common sense, and to laugh. Your spirits are with me in all my endeavors. Acknowledgments This book was born in 1981 when a group of test technicians at Gould asked me if I could write a document on how to troubleshoot our hardware products. I was at a loss—the products were boards with hundreds of chips on them, several microprocessors, and numerous communications buses. I knew there was no magical recipe; they would just have to learn how to debug things. I discussed this with Mike Bromberg, a long time mentor of mine, and we decided the least we could do was write up some general rules of debugging. The Ten Debugging Commandments were the result, a single sheet of brief rules for debugging which quickly appeared on the wall above the test benches. Over the years, this list was compressed by one rule and generalized to software and systems, but it remains the core of this book. So to Mike, and to the floor techs who expressed the need, thanks. Over the years, I've had the pleasure of working for and with a number of inspirational people who helped me develop both my debugging skills and my sense of humor. I'd like to recognize Doug Currie, Scott Ross, Glen Dash, Dick Morley, Mike Greenberg, Cos Fricano, John Aylesworth (one of the original techs), Bob DeSimone, and Warren Bayek for making challenging work a lot of fun. I should also mention three teachers who expected excellence and made learning enjoyable: Nick Menutti (it ain't the Nobel Prize, but here's your good word), Ray Fields, and Professor Francis F. Lee. And while I never met them, their books have made a huge difference in my writing career: William Strunk Jr. and E. B. White (The Elements of Style), and Jeff Herman and Deborah Adams (Write the Perfect Book Proposal). To the Delt Dawgs, my summer softball team of 28 years and counting, thanks for the reviews and networking help. I'm indebted to Charlie Seddon, who gave me a detailed review with many helpful comments, and to Bob Siedensticker, who did that and also gave me war stories, topic suggestions, and advice on the publishing biz. Several people, most of whom I did not know personally at the time, reviewed the book and sent me nice letters of endorsement, which helped get it published. Warren Bayek and Charlie Seddon (mentioned above), Dick Riley, Bob Oakes, Dave Miller, and Professor Terry Simkin: thank you for your time and words of encouragement. I'm grateful to the Sesame Workshop, Tom and Ray Magliozzi (Click and Clack of Car Talk—or is it Clack and Click?), and Steve Martin for giving me permission to use their stories and jokes; to Sir Arthur Conan Doyle for creating Sherlock Holmes and having him make so many apropos comments; and to Seymour Friedel, Bob McIlvaine, and my brother Tom Agans for relating 2

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