CPSC 457: Principles of Operating Systems Assignment 2 due June 9, 2015

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1 CPSC 457: Principles of Operating Systems Assignment 2 due June 9, 2015 Assignment Objectives This is an individual assignment. You must submit your own work. This assignment has two objectives: (1) reviewing important concepts covered in lectures so far, and (2) practicing multi-process and multi-threaded programming. Start the assignment early and avoid procrastination. Deadline Written part: 9:00am on June 9, 2015 (delivered in class). Programming part: 10:00pm on June 9, 2015 (uploaded on D2L). Helps Available The second tutorial session in the week of June 1 st will be dedicated to providing help for this assignment. Forum for Assignment 2 on Piazza provides a platform for online discussion for this assignment. Robin Gonzalez (TA) will be in charge of this forum. Robin Gonzalez (TA) will offer two office hours, 3pm - 4pm on Thursday, June 4, and 2pm - 3pm on Friday, June 5. You may find him in ICT 616. Robin Gonzalez (TA) will be available at CT desk in MS computer lab from 3pm to 5pm on the assignment due date (June 9). The instructor is also available during office hours (TR, 12pm - 1pm) or by appointment. PART 1: Written Questions (40 marks) 1. (2 marks) What is context switching? What are the actions taken by the kernel to context switch among processes? 2. (3 marks) How a thread is created in terms of resource allocation? How does it differ from creating a process? 1

2 3. (5 marks) Two-level thread model is a combination of Many-to-Many and One-to-One models. Describe a scenario that such a model outperforms both Many-to-Many and One-to-One models. You can provide a general scenario or use an arbitrary application program to explain your answer. 4. (5 marks) In this problem you are to compare reading a file using a single-threaded file server and a multithreaded server. It takes 15 msec to process a task, if the data needed are in the cache. If a disk operation is needed (a cache miss), as is the case one- third of the time, an additional 15 msec is required. Assuming the overhead introduced by context switching is negligible, how many requests/sec can the server handle if it is single threaded? What about a multithreaded server, i.e., one thread for disk operation and one thread for regular processing? 5. (5 marks) In a system with threads, is there one stack per thread or one stack per process when user-level threads are used? What about when kernel-level threads are used? Explain. 6. (5 marks) Consider a banking system with two functions: deposit(amount) and withdraw(amount). These two functions accept one parameter, the amount that is to be deposited or with- drawn from a bank account. Assume that a shared bank account exists between a husband and a wife and the current balance is $ Concurrently, the husband calls the withdraw($400) function and the wife calls the deposit($200) function. Describe how a race condition is possible and what might be done to prevent the race condition from occurring. 7. (5 marks) Consider the following solution to the mutual-exclusion problem involving two processes P0 and P1. Assume that the variable turn is initialized to 0. Process P0 s code is presented below. /* Other code */ while (turn!= 0) { } /* Do nothing and wait. */ Critical Section /*... */ turn = 0; /* Other code */ For process P1, replace 0 by 1 in above code. provides mutual-exclusion. Determine if the solution correctly 8. (10 marks) We have learned that the operating system enforces a limit on the number of running processes. Follow the instructions below and describe what happens on your virtual machine. 2

3 (a) Use the command ulimit to find the maximum allowed user processes on your virtual machine. (b) Download the sample program maxprocesses.cpp for this assignment, try to change the number of processes to be created by this program. What is the maximum number of processes that you can run without seeing an error message? Is it the same number that you found in (a)? Why or why not? (c) Run the program again as root using sudo. According to the output of the program, how many processes can you successfully create as root before seeing an error message? Is it the same number that you found in (b)? Why or why not? PART 2: Programming Question (30 marks) In this part of assignment, you must write a program in C or C++ language and upload the source file (lcm.c or lcm.cpp) to D2L Dropbox. Threads are commonly used for parallelizing large computation tasks, e.g., matrix multiplications. In this question, you will write a multi-threaded solution for computing the least common multiple (also called the lowest common multiple or smallest common multiple) of a series of positive integers. Your program will take two arguments from the command line. The first argument is the name of the file that contains the positive integer numbers, and the second argument is the number of threads T to be created. You may assume that the number of threads is always smaller than the number of integers given in the input file. Your program will check the input parameters to make sure that two parameters are provided. If there was more or less parameters, an error message will be printed and the program terminates. If there are two input parameters, you may assume that the order and format is correct, i.e., the first parameter is a file name and the second parameter is an integer number. Your program will read the integer numbers from the input file. You can assume that each number is provided in a separate line, and the format of the input file is correct. Then the numbers are divided to T groups. All groups, except the last group, should contain the same number of numbers. After you divided the numbers to T groups, you must create T threads, each one receiving a group of numbers. Each thread will calculate the LCM for the given set of numbers. When all the threads are finished, you will have a set of T numbers. Now your program creates a new thread, and sends the T calculated LCMs to that thread to calculate the final value of the LCM for all the given numbers. Input Format As mentioned earlier, you can assume that the given input file contains a set of positive integer numbers, each one in a new line. A sample input file will be like this: 3

4 Input Parameters As mentioned before, your program accepts two parameters: the file name and the number of threads. An example command to run your program will be as follows: $./lcm numbers.txt 3 If the number of parameters are correct, you can assume that the first parameter is a valid file name and the file exists, and the second parameter is a positive integer. Output Format In the given example, N = 10 (the number of given positive integers) and T = 3. Therefore, the first group contains (15, 19, 21, 12), the second group contains (7, 6, 14, 33), and the third group contains (4, 15). You should send each group of numbers to the respective thread. After the thread finishes calculating LCM of the given group of numbers, it must print out these information: Thread thread ID the group of numbers sent to the thread the calculated LCM When the LCM results are passed to the final thread, it should print the same information after the phrase Final Thread. The expected output for the above example is as follows: $./lcm numbers.txt 3 Thread : (15, 19, 21, 12) : LCM = 7980 Thread : (7, 6, 14, 33) : LCM = 462 Thread : (4, 15) : LCM = 60 Final Thread : (7980, 462, 60) : Final Least Common Multiple:

5 The number after the word Thread in each line is the thread ID. You can use pthread self() to retrieve the ID of the running thread. Your program must be able to handle large numbers in order to calculate the LCM properly. Hence, you are suggested to use unsigned long instead of int where appropriate. It is important that your program accept command-line arguments to avoid hardcoding the number of threads or the name of the input file. During the marking session, your TAs are not allowed to modify your code for different test cases. Passing the group of numbers to the threads To pass each group of numbers to the target thread and also to let the thread know if it is the final thread, you are not allowed to use any global or shared variables. Instead, you should create a proper data structure and pass a variable from the type of the data structure to the threads. How to calculate LCM? You are free to use any correct algorithm to calculate the LCM. You can find more information here: Among different suggested methods, you may want to look at A method using a table. This method is easy to implement and allows you to calculate LCM using a simple tablebased algorithm. This method uses the prime numbers to calculate the LCM. You can find the first 1000 prime numbers in this address: Even though it is not a good programming practice, you are allowed to hardcode the prime numbers as an array in your code. Clearly the better method is to read them from a file. In this case you are allowed to hardcode the name of the file in your code and submit your file with your code. Coding style Please use the proper coding style in your code. Use the indentation and curly brackets properly, add proper amount of comments where needed, and use meaningful names for your variables and data structures. Testing your program Three sample inputs and the expected outputs are provided. You can use these sample file to test your program. The number of threads used to generate the output files are specified in the name of the files. 5

6 Using the skeleton code CPSC 457: Assignment 2 A skeleton code is provided. You are allowed to use the skeleton code as the base for your code. The skeleton code opens a file where the name of the file is given as a parameter, reads two line-separated numbers from the file, puts these numbers in a data structure, and passes the data structure to a thread. In the thread start routine, some calculations are done and the result is saved back to the data structure. When the thread is done, main function prints the final result on the screen. Deliverables Please upload your program (lcm.c or lcm.cpp) to D2L Dropbox before 10pm on assignment due date (June 9). Note that you must test your programs on your Fedora virtual machine on Rapid Access Cloud. The standard Fedora virtual machine on RAC will be used to compile and test your programs by your TAs. 6