Survey #3. Final Exam. Today is totally optional! University of British Columbia CPSC 111, Intro to Computation Alan J. Hu. Readings.

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1 University of British Columbia CPSC 111, Intro to Computation Alan J. Hu Readings This Week: No new readings. Consolidate! (Reminder: Readings are absolutely vital for learning this stuff!) Multithreading Lecture 37 Final Exam Wednesday, April 15, 7pm in SRC A This wasn t a good room last year, but we re stuck with the date, time, and room UBC assigns. Note: You are allowed to do scratch work on the exam paper! (But write your answer where indicated if the problem tells you to.) Survey #3 Ben Yu s 3 rd survey is up on WebCT now. Due April 14. Purely optional, but tiny amount of extra credit, etc Today is totally optional! Learning Goals By the end of class today you will be able to Explain what terms like dual core, quad core, and multicore mean. Write simple multi-threaded programs in Java that can take advantage of the multiple cores in modern computers. Give examples to illustrate some basic pitfalls and problems that occur in concurrent programs. Explain why you shouldn t write concurrent programs for anything that really matters until you ve learned more CS! 1

2 Multicore Is Coming! All the top companies are going completely nuts about this. People will cite many technological reasons (beyond the scope of what we can talk about today) for why every computer you buy in the future will be multicore: Limits to uniprocessor performance Power consumption Design complexity etc. etc. etc. Multicore Is Here Already! You see ads Intel Core 2 Duo AMD Athlon X2 Dual-Core Intel Core 2 Quad AMD Quad-Core Opteron etc. Many of you probably have a multicore computer already So, what does multicore mean? At the beginning of the course, we learned about the fetch-decode-execute cycle: The CPU (central processing unit) of your computer fetches, decodes, and executes one instruction at a time. In a multicore processor, you have several CPUs (actually, CPU cores), each of which independently does its own fetch-decodeexecute cycle. Terminology Multitasking one CPU does multiple tasks How can it do that? Multiprocessing multiple CPUs in a computer system work together. Multicore the multiple CPUs are on a single chip, and they usually share some resources, so it s only the core of the CPUs that you have multiple of. From programmer s perspective, the above are basically the same. Although multitasking doesn t give any speedup. Pentium Die Photo Intel Core 2 Duo Die Photo 2

3 IBM Cell Processor (Used in PS3) More Terminology Thread/Process One thread/process is basically one chunk of code that s executing. The difference between these two you will learn in 213 or 313. Not important for now. Multi-threading Writing a program that uses multiple threads to achieve a task. Concurrent/Parallel Programming the general term for any sort of programming (for multitasking, multiprocessing, multithreading) that has different processors/threads/processes interacting. Concurrent Programming in Java Java has libraries that make it easy to experiment with multi-threaded programming I m going to show you the very simplest way to get going. It s not the best way to do thing. Chapter 20 (23 in old book) in your book gives better ways to do things. Java Thread Class Java has a class called Thread You make your own class extend Thread You supply a run() method. When you call the start() method on an object of your class, the code in the run() method starts running in its own thread! Class foo extends Thread { public void run() { // Your thread code goes here } } Example: doblur from Assignment 2 Note to self: On Columbia, use neighborhood 5, up to 8 processors. java picapp/pictureprocessorapp Promises and Pitfalls Multithreading is like getting a team of people to work on a task together. If you do it right, you get your work done much faster. If you do it kind of wrong, you get your work done slower, and everyone wastes effort. If you do it really wrong, you get wrong answers or other bad effects! 3

4 Common Pitfall: Deadlocks A deadlock occurs when everyone (or every thread/process) is stuck waiting for someone else to finish Real Life Example: Two lovers in a quarrel waiting for each other to apologize first. Real Computer Example: Mars Pathfinder bug almost caused total mission failure. Common Pitfall: Race Conditions Wrong outcome because people/threads compute based on out-of-date picture of another person/thread s state. Human Examples: O. Henry, The Gf Gift of the Magi Lane changing on a highway. Computer Examples: The textbook illustrates with bank accounts Therac-25 Bank Account Example Bank Account Example You withdraw some money from ABM: Your parents give you $1000 for your birthday You withdraw some money from ABM: Your parents give you $1000 for your birthday bal = acc.getbalance(); bal -= withdrawal; bal=acc.getbalance(); bal += 1000; bal = acc.getbalance(); bal -= withdrawal; bal=acc acc.getbalance(); bal += 1000; Therac-25 This is not the only disaster, and probably not the worst one, but it was unusually wellstudied, so we know a lot about what happened. Caution/Moral: Don t write code for important applications until you know what you re doing! (Especially not concurrent code.) Therac-25 Outline Therac-25 was a medical radiation treatment machine made by AECL in the late 1980s. Delivers precise radiation beam to destroy tumors: Electron beam for some treatments X-ray beam for other treatments Computer-controlled, easy-to-use interface Massive radiation overdose to at least 6 patients 4

5 Therac-25 One Patient s Case East Texas Cancer Center, March 1986 Patient to receive 180 rads of radiation Very experienced machine operator Machine repeatedly indicates minor malfunction, with a does of 6 rads. Operator repeats treatment several times. Operator only has audio and video monitor of patient. Video monitor was unplugged; audio was broken. Therac-25 One Patient s Case After first treatment attempt, patient said that someone had poured hot coffee on his back. He knew something was wrong and tried to get up. He got hit by second treatment while trying to get up, felt like his arm was being shocked by electricity. Pounded on door to alert operator. Therac-25 One Patient s Case After-accident investigation suggests he received 16,500 to 25,000 rads of radiation. He lost the function of his left arm and had periodic bouts of nausea and vomiting. He was eventually hospitalized for radiation-induced myelitis of the cervical cord, causing paralysis of his left arm and both legs, left vocal cord paralysis (which left him unable to speak), Died from the radiation overdose 5 months later. Therac-25 What Happened This is the simplified basics. The machine had two modes. A single accelerator provides electron beams at low (treatment) levels, or a much higher level. The high-power electron beam is supposed to hit a target that generates X-rays at a treatment level of power. Both the target and the beam power are under computer control. Therac-25 What Happened The software for the machine was poorly and unsafely written. For no particular reason, it was written as multithreaded code. There was a race condition: The user-interface task updated variables based on what kind of treatment mode was needed. Under certain conditions, the beam power would be set based on one value of these variables, but the position of the X-ray target would be set based on a different value. Result: electron beam at high power without X-ray target in place. Moral of the Story By learning how to program, you re starting to gain great power in controlling computers, which in turn can control many other things. With this power, you have a responsibility to use your power wisely and carefully. Learn more CS: to develop your powers, to learn to control your powers, to use your powers for good! 5