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MIT 6 033 - Quiz I

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Department of Electrical Engineering and Computer ScienceMASSACHUSETTS INSTITUTE OF TECHNOLOGY6.033 Computer Systems Engineering: Spring 2009Quiz IThere are 14 questions and 15 pages in this quiz booklet. Answer each question according to theinstructions given. You have 50 minutes to answer the questions.Most questions are multiple-choice questions. Next to each choice, circle the word True or False,as appropriate. A correct choice will earn positive points, a wrong choice may earn negativepoints, and not picking a choice will score 0. The exact number of positive and negative pointsfor each choice in a question depends on the question and choice. The maximum score for eachquestion is given near each question; the minimum for each question is 0. Some questions areharder than others and some questions earn more points than others—you may want to skim allquestions before starting.If you find a question ambiguous, be sure to write down any assumptions you make. Be neat andlegible. If we can’t understand your answer, we can’t give you credit!Write your name in the space below AND at the bottom of each page of this booklet.THIS IS AN OPEN BOOK, OPEN NOTES QUIZ.NO PHONES, NO COMPUTERS, NO LAPTOPS, NO PDAS, ETC.CIRCLE your recitation section number:10:00 1. Girod/Badirkhanli11:00 2. Girod/Badirkhanli 3. Zeldovich/Reid12:00 4. Zeldovich/Reid1:00 5. Jackson/Benjamin 6. Newton/Dowgun2:00 7. Jackson/Benjamin 7. Newton/DowgunDo not write in the boxes below1-3 (xx/10) 4-6 (xx/20) 7-10 (xx/36) 11-14 (xx/34) Total (xx/100)Name:6.033 Spring 2009, Quiz 1 Page 2 of 15I Reading QuestionsThe following questions refer to Herbert Simon’s paper, “The Architecture of Complexity” (reading #2).1. [2 points]: Simon’s notion of hierarchy organizes a collection of components by their(Circle the BEST answer)A. physical containment relationshipsB. names and access patternsC. strengths of interaction2. [2 points]: Simon argues that systems naturally evolve in hierachical form because(Circle the BEST answer)A. hierarchies are inherently stableB. hierarchies are easily describedC. a component in a structure of size N can be accessed in logN steps3. [6 points]: Based on the description of the X Window System in the 1986 paper by Scheifler andGettys (reading #5), which of the following statements are true?(Circle True or False for each choice.)A. True / False X’s client/server architecture ensures that one misbehaving client cannot interfere withother clients running on the same display.B. True / False X’s asynchronous protocol ensures that clients never have to wait for a network round-trip time for the server to respond to a request.C. True / False The X protocol always requires the server to send an exposure event to the client toredraw its window when an obscured window becomes visible.Name:6.033 Spring 2009, Quiz 1 Page 3 of 154. [6 points]: Based on the description of the UNIX file system in the 1974 paper by Ritchie andThompson (reading #6), which of the following statements are true?(Circle True or False for each choice.)A. True / False The kernel does not allow users to create hard links to existing directories.B. True / False An application can ask the kernel to read any file by specifying its i-node number, aslong as that i-node represents a file that the application has permissions to read.C. True / False File names and i-node structures are stored within the data blocks of their containingdirectory.5. [6 points]: Based on the description of the MapReduce system in the 2004 paper by Dean andGhemawat (reading #8), which of the following statements are true?(Circle True or False for each choice.)A. True / False If there are m map tasks, using more than m workers in the map phase may stillimprove performance beyond that achieved with m workers.B. True / False To achieve locality, map workers always execute on the same machine as the input datathat they consume.C. True / False Intermediate data passed between the map workers and reduce workers is stored in theGoogle File System (GFS).Name:6.033 Spring 2009, Quiz 1 Page 4 of 156. [8 points]: The following question refers to the Eraser system, by Savage et al. (reading #7).Consider the following snippet of code, as part of a larger system:Lock L1;Lock L2;int x;function foo() {acquire(L1);print(x);release(L1);}function bar(int v) {acquire(L2);if (v == 0) {print(x);}release(L2);}The functions foo() and bar() are executed from separate threads, but Eraser never flags an error.Which of the following reasons might explain this?(Circle ALL that apply)A. foo() and bar() both execute, but never at the same time, so no race condition actually occursB. foo() and bar() run concurrently, but bar() is always called with 1 as an argumentC. Every time foo() or bar() is called, an additional lock L3 is also heldD. The value of x is changed for the last time before either foo() or bar() are called for the first time.Name:6.033 Spring 2009, Quiz 1 Page 5 of 15II FaceFeederInspired by Design Project 1, Ben BitDiddle decides to build a dataflow processing system for Facebookfeeds. A Facebook feed is a stream of notifications, informing Facebook users of changes to a given friend’sprofile page.Users upload programs, or operators, that are run by Ben’s dataflow server. Operators can read from oneor more feeds, and produce outputs which are themselves feeds. Users may subscribe to different feeds toreceive alerts. Multiple users may subscribe to the same feed, and feed alerts are delivered asynchronously(e.g., via email.)As an example of a FaceFeed application, one user might create an operator that combines their friends’“25 things you didn’t know about me” lists into “a whole lot of things you didn’t know about a whole lotof people” list. Another user might write an operator that takes in a stream of text and produces a graphshowing the most common words in that stream. A third user might combine these together to produce agraph of the most common words used in his or her friends’ “25 things you didn’t know about me” list.7. [8 points]: As a first approach, Ben decides to run all operators in the same address space, in asingle process, with each operator running in a thread. His thread scheduler is pre-emptive, meaningthat it can interrupt one thread and switch to another. Alyssa P. Hacker warns him that a single processis a bad idea, because running operators in the same process only provides soft modularity betweenthem. Which of the


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MIT 6 033 - Quiz I

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