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1 UNIVERSITY OF TORONTO Faculty of Arts and Science DECEMBER 2008 EXAMINATIONS CSC369H1F Duration: Aids Allowed: 3 hours One 8.5 x 11 sheet of paper (two-sided). No electronic aids. Student Number: i i i Last Name: First Name: Instructor: Scannell MARKING GUIDE Do not turn this page until you have received the signal to start. (In the meantime, please fill out the identification section above, and read the instructions below carefully.) This final exam consists of 11 questions on 18 pages (including this one). When you receive the signal to start, please make sure that your copy of the test is complete. Please indicate clearly the part of your work that should be marked. Vague answers will not receive full marks. In questions where you are asked to write code, unless indicated otherwise C-like pseudo-code is acceptable. #1: #2: #3 #4 #5 #6 #7: #8 #9 # 10 # ii io 7io TOTAL: 798

2 CSC 369H1F Final Examination Fall 2008 This page left blank intentionally. Page 2 of 18

3 CSC 369H1F Final Examination Fall 2008 Question 1. [10 MARKS] TRUE FALSE Some operating systems map the whole kernel in the virtual address space of every process. TRUE FALSE In OS/161, every new thread starts executing within the kernel in system mode. TRUE FALSE Following a context switch in OS/161, a thread immediately starts executing in user mode if it was pre-empted there. TRUE FALSE A scheduling algorithm named shortest-burst-next would likely discriminate against CPU-bound processes. TRUE FALSE Scheduling in OS/161 is pre-emptive. TRUE FALSE Two transactions don't conflict unless they must both contain at least one write on the same data. TRUE FALSE Our virtual memory subsystem in OS/161 used vmobjects which were essentially lists of lpage structures. The definition of the lpage structure is determined by the hardware architecture. TRUE FALSE The Fast File System achieves good performance by grouping related data on the same disk sector. TRUE FALSE By treating the disk as a circular log, log-structured file systems tend to wear out flash-based storage devices very quickly. TRUE FALSE Synchronization between remote systems is inherently more difficult than synchronization on a multiprocessor system. Page 3 of 18

4 Question 2. [8 MARKS] Define the following terms. Part (a) Priority Inversion Part (b) Unsafe State Part (c) Memory-mapped I/O Part (d) Copy-on-write Page 4 of 18

5 Question 3. [ H MARKS] Short answer questions. Part (a) [1 MARK] What is multiprogramming? Part (b) In early computers, I/O devices did not access memory directly - the CPU handled every byte of read or written. Describe how this affects multiprogramming. data Part (c) [3 MARKS] Briefly explain the difference between Access Control Lists and Capabilities. Part (d) Briefly describe two challenges that face operating systems for mobile devices. Part (e) [3 MARKS] Describe the difference between a Type 1 Hypervisor and a Type 2 Hypervisor. Page 5 of 18

6 Question 4. [5 MARKS] We are writing a new system call, myname. It copies the current thread's name to user space. Recall that each thread structure has a t_name field. The C-library prototype for our new system call is: /* * myname: Copies the current thread name to buffer. * Returns the number of characters copied. */ int myname(char* buffer); Consider the following version of mips.syscall with the case for SYS_myname unimplemented: void mips_syscall(struct trapframe C int32_t retval; int err; int callno = tf->tf_vo; *tf) switch { (callno) case SYS_reboot: err = sys_reboot(tf->tf_a0); break; case SYS_myname: } default: err = ENOSYS; break; > if { } else { } (err) tf->tf_vo tf->tf_a3 err; 1; /* signal an error */ tf->tf_vo = : retval; tf->tf_a3 = i 0; /* signal no error */ tf->tf_epc + : = 4; /* instruction completed successfully */ Page 6 of 18.

7 Part (a) There is a serious problem with our mips.syscall function, unrelated to myname. What is it and would it manifest in the running system? how Part (b) [3 MARKS] Implement the myname system call on the previous page (you may use C-like pseudo-code). Clearly state any assumptions that you make. If you wish, you may define a sys_myname helper function in the space below. Page 7 of 18

8 Question 5. Part (a) [4 MARKS] [6 MARKS] Suppose we are given a python implementation of binary semaphores, and we want to implement counting semaphores. A programmer has already implemented P, and you are given the task of correctly writing V given the existing implementation of P. You may use python-like pseudo code. Recall that self. field is used to access the class member named field. class CountingSemaphore: # init is the constructor function for the CountingSemaphore class def init (self, count): # L is an object field self.l = BinarySemaphore(l) # N is also an object field if count > 0: self.n = BinarySemaphore(l) else: self.n = BinarySemaphore(O) # C is also an object field self.c = count def def P(self): self.n.po self.l.po self.c -= 1 if self.c > 0: self.n.v() self.l.vo V(self): Part (b) What is the mechanism that OS/161 uses to enforce atomicity for the implementation of semaphores? When is this mechanism appropriate? Page 8 of 18

9 Question 6. [13 MARKS] Consider a system with 32 bit virtual and physical addresses and a page size of 4K. A page table entry in this system is same length as a physical address. We construct a multi-level page table such that all page tables and page table directories fit in a single page. Part (a) [4 MARKS] Draw a diagram, showing how the bits of this virtual address map to a physical address using page tables. Be sure to illustrate how the bits of the virtual address are used to index into different levels of the page table as well as how the page tables are found in memory. Bit Address Ol 00 Page 9 of 18

10 Part (b) [3 MARKS] Imagine our physical and virtual addresses increase to 48 bits (with page table entries the same length as a physical address). If our page size is still 4K, how many levels would our page tables require? Show your work for partial marks. Part (c) [3 MARKS] The following question refers to OS/161. Check all the boxes that apply to each scenario. You may check multiple boxes in any row. Do not check any boxes in the row if the scenario is impossible. Scenario TLB miss, Page in physical memory TLB miss, Page not physical memory TLB miss, Invalid address TLB hit, Page in physical memory TLB hit, Page not in physical memory TLB hit, Invalid address Process Continues Executing lpage_fault Executed swap_pagein Executed Part (d) [3 MARKS] Assume that a memory access costs 10 nanoseconds, a TLB miss costs 1 microsecond, and a page fault costs 1 millisecond. If our TLB hit rate is 99%, what percentage of TLB misses can result in page faults before the average memory access time is greater than 100 nanoseconds? You may express your answer as a fraction, or as an approximate percentage. Recall that 1 millisecond = 1000 microseconds = nanoseconds. Page 10 of 18

11 Question 7. [8 MARKS] The following question refers to cache replacement algorithms. Recall that page replacement is a specific instance of this problem. Based on the following reference string, fill in the caches according to the appropriate algorithm. Mark an X above columns where the request was not satisfied by the cache. Reference string: 3, 2, 1, 0, 3, 2, 4, 3, 2, 1, 0, 4, 2, 3, 2, 1, 0, 4 Part (a) Optimal Part (b) LRU Part (c) [1 MARK] Describe a specific situation where LRU performs very poorly as a page replacement algorithm. Part (d) [3 MARKS] All of these algorithms work by taking advantage of a specific form of locality. Name at least two different types of locality that programs typically exhibit. If we just have physical address access information in a system with paging, how might this affect observable locality? Page 11 of 18

12 Question 8. [12 MARKS] Synchronization and multiprocessors. Part (a) Some modern processors use out-of-order execution in order to improve performance. What is out-oforder execution and why might it be used to improve performance? Part (b) Explain why instructions cotild appear to execute out-of-order on multiprocessors, even if a processor does not use out-of-order execution? The following function uses Peterson's algorithm to solve the critical section problem. Recall that it may be called concurrently by two threads with ids 0 and 1 respectively. int flag [2] = {0, 0}; int turn = 0; void work(int id) { 1: flag[id] = 1; 2: turn = (1-id); 3: while (flag[1-id] && turn == (1-id)); 4: critical_section(); 5: flag[id] = 0; } Page 12 of 18

13 Part (c) [3 MARKS] Imagine that lines 1 and 2 in the work function are occasionally executed out-of-order. Give a specific schedule that would cause the above algorithm to violate mutual exclusion. Part (d) [3 MARKS] Is deadlock possible with out-of-order execution in this case? Give a specific schedule where deadlock occurs or argue why it cannot. Part (e) Suppose that we are given a function mb() which implements a memory barrier. Recall that a memory barrier flushes the cache and instruction pipelines, guaranteeing relative ordering of instructions on either side of the barrier. How could we use memory barriers to fix potential problems caused by out-of-order execution? Page 13 of 18

14 Question 9. [5 MARKS] What are the contents of design.txt at the end of the main function (below)? You should assume that this program uses the same file access semantics that you implemented in OS/161. Show intermediate steps for partial marks. For your convenience, an exert from the dup2 manual page is reproduced here: int dup2(int oldfd, int newfd); dup2 clones the file handle oldfd onto the file handle newfd. If newfd names an open file, that file is closed. The two handles refer to the same "open" of the file. This is different from opening the same file twice. char* stringl = "AAAA char* string2 = "BBBB char* string3a = "CCCC char* string3b = "DDDD char* string4a = "EEEE char* string4b = "FFFF void maino c int fl = open("design.txt", "w"); int f2 = open("design.txt", "w"); int f3 = open("design.txt", "r"); write(fl, stringl, strlen(stringl)); write(f2, string2, strlen(string2)); } int pid = fork(); ifc pid ) i int status = 0; waitpid(pid, festatus, 0); write(fl, string3a, strlen(stringl)); dup2(f2, fl); write(f2, string4a, strlen(string2)); } else { write(fl, string3b, strlen(stringl)); dup2(fl, f2); write(f2, string4b, strlen(string2)); } Page 14 of 18

15 Question 10. [10 MARKS] Imagine that we want to support multiple threads per process in OS/161. Describe how you would go about implementing this functionality, including any changes to data structures, system interfaces and steps that you would take to create and destroy threads. Be sure to mention how your implementation would handle fork, exec and exit system calls. You may use C-like pseudo-code if you wish. Page 15 of 18

16 Question 11. [10 MARKS] The following questions concern security. Part (a) [4 MARKS] Explain the following terms: Security through obscurity Exploit This relates to part (b). There are severe problems with the following code: char* readline(int socket) { char buffer[buffer_size] ; int index = 0; do i if( read(socket, febuffer[index], 1) < 0 ) return NULL; index += 1; } while( buffer[index-1]!= >\0> && buffer[index-1]!= '\n' ); > buffer[index-1] = '\0'; return buffer; Page 16 of 18

17 CSC369H1F Final Examination Fall 2008 Part (b) [4 MARKS] Identify two serious errors and how are they likely to affect program correctness and security? Be sure to specify in detail exactly how each of the identified errors could cause problems or be exploited. Part (c) [1 MARK] The code has one less serious, performance-related problem. What is it? Briefly justify your answer. Part (d) [1 MARK] If this code were used by the OS/161 menu thread to read user input, what specific consequences could that have? Page 17 of 18

18 (Extra page for work that did not fit. Do not tear this page off.) Page 18 of 18 END OF TEST

Operating Systems Comprehensive Exam. Spring Student ID # 2/17/2011

Operating Systems Comprehensive Exam Spring 2011 Student ID # 2/17/2011 You must complete all of Section I You must complete two of the problems in Section II If you need more space to answer a question,