Final Exam April 14, 2007 COSC 3407: Operating Systems

Transcription

1 Laurentian University Department of Mathematics & Computer Science Winter 2007 Name: Student ID: Final Exam April 14, 2007 COSC 3407: Operating Systems Kalpdrum Passi Instructions: 1. This is a closed book and notes exam. 2. You have three hours to answer as many questions as possible. 3. Write all of your answers directly on this paper. 4. You should read all of the questions before starting the exam, as some of the questions are substantially more time consuming. 5. Make your answers as concise as possible. Good Luck!! Problem Possible Score 1. (4 parts) (6 parts) (4 parts) (4 parts) (4 parts) (4 parts) 18 Total 120 Page 1 of 13

2 1. (13 points total) Short answers (a) (4 points) For the following error messages, state if the message would be generated during the first or second pass of the linker. Be sure to justify your answer. (i) External Reference FOO not found. (ii) Duplicate symbol definition FOO. (b) (4 points) A system you admin is thrashing, and the users are mad. What are some ways to get into this mess? Page 2 of 13

3 (c) (3 points) Assume your Nachos file system needed to update a multiple sector long data structure on disk. Describe how you might arrange to detect if your system failed in the middle of writing the data structure. (d) (2 points) In a paging system where page table is stored in memory and if a memory reference takes 175 nanoseconds, how long does a paged memory reference take? 2. (22 points total) Demand Paging: (a) (3 points) Consider a demand paging system that uses 4 KByte pages. In such a system, does a Least Recently Used (LRU) page replacement policy exploit temporal locality? If so, then how? If not, why not? Page 3 of 13

4 (b) (3 points) Consider a demand paging system that uses 4 KByte pages. In such a system, does a Least Recently Used (LRU) page replacement policy exploit spatial locality? If so, then how? If not, why not? (c) (4 points) Consider a demand paging system with four physical memory frames and the following reference string over seven pages: 1, 2, 3, 4, 2, 1, 5, 6, 2, 1, 2, 3, 7, 6 Assuming that memory starts empty, how many page faults will occur and what will be the final contents of memory under the FIFO page replacement policy? Page 4 of 13

5 (d) (4 points) Answer the problem of part c for the LRU policy. Recall that the reference string is: 1, 2, 3, 4, 2, 1, 5, 6, 2, 1, 2, 3, 7, 6. (e) (4 points) Answer the problem of part c for the MIN (i.e., optimal) policy. Recall that the reference string is: 1, 2, 3, 4, 2, 1, 5, 6, 2, 1, 2, 3, 7, 6 (f) (4 points) What architectural constraint is there in demand paging when one instruction may modify several memory locations? What is the solution to this constraint? Page 5 of 13

6 3. (19 points total) Virtual Memory (a) (3 points) Consider a paging system with the page table stored in memory. If we add associative registers, and 90% of all page-table references are found in the associative registers, what is the effective memory reference time? Assume that finding a page-table entry in the TLBs takes zero time, if the entry is there. (b) (6 points) Explain the address translation for a system with 64-bit address space with page size 4K and a four level paging scheme where the three inner levels are 1K each. What will be the size of the outermost page table (in bytes)? Further, assume we use TLBs with a hit ratio of 98%. It takes 20 nanoseconds to search the TLB and 100 nanoseconds to access the memory. What will be the effective access time for the system? Page 6 of 13

7 (c) (6 points) Consider a demand-paging system with a paging disk that has an average access and transfer time of 20milliseconds. Addresses are translated through a page table in main memory, with an access time of 1 microsecond per memory access. Thus, each memory reference through the page table takes two accesses. To improve his time, we have added an associative memory that reduces access time to one memory reference, if the page-table entry is in the associative memory. Assume that 80 percent of the accesses are in the associative memory, and that, of the remaining, 10 percent (or 2 percent of the total) cause page faults. What is the effective memory access time? Page 7 of 13

8 (d) (4 points) Given a virtual memory system that always gives equal portions of the physical memory to each process, would the system necessitate a global or local replacement policy? Justify your answer. 4. (28 points) Disk Drives and File Systems (a) (6 points) Itsy Bitsy Machines Corporation develops a new disk drive that has a separate read/write head for each track and thus its disk heads don t move. Which of the following file-system features are no longer important? State whether the feature is useful or not useful and in 1 2 sentences explain why or why not. i) Cylinder groups (e.g., Unix 4.2 BSD): ii) A bitmap free list (e.g., Unix 4.2 BSD) Page 8 of 13

9 (b) (10 points) List the set of disk blocks that must be read into memory in order to read the file /home/cosc3407/test.doc in its entirety from a UNIX BSD 4.2 filesystem. Assume the file is 15,234 bytes long and that disk blocks are 1,024 bytes long. Assume that the directories in question all fit into a single disk block each. Note that this is not always true in reality. (c) (9 Points) Suppose a file system can have three disk allocation strategies, contiguous, linked, and indexed. We have just read the information for a file from its parent directory. For contiguous and linked allocation, this gives the address of the first block, and for indexed allocation this gives the address of the index block. Now we want to read the 10th data block into the memory. How many disk blocks (R) do we have to read for each of the allocation strategies? For partial credit, explicitly list which block(s) you have to read. Contiguous allocation: R = Linked allocation: R = Indexed allocation: R = Page 9 of 13

10 (d) (3 points) Assume your system uses SCAN or elevator algorithm as a diskscheduling algorithm. How would you modify this algorithm if rather than having a single disk your storage device was a RAID (20 points total) Networking and Distributed Systems (a) (8 points) The following are names given to several fields in the Internet protocol (IP) packet header. For each field describe its likely purpose: (i) Header checksum (ii) Time to live (TTL) (iii) IP fragment offset (iv) Protocol Page 10 of 13

11 (b) (4 points) Why can an Ethernet get away with a flat address of 48bits while the Internet with a smaller 32bit address needed to go to a hierarchical organization? (c) (4 points) Assume you have two machines (A and B) sending datagram packets between one another using Internet Protocol (IP) over an internetwork. You notice from statistics being kept on the machines that machine A sent X packets to B and B received X packets from A. Machine B reports to have sent Y packets to A yet A reports to have received 4*Y packets from B. Explain how this is possible. (d) (4 points) Explain how an Ethernet decides which machine goes next after a collision. Page 11 of 13

12 6. (18 points) Distributed File Systems (a) (6 points) What are the three naming schemes for Distributed File Systems (DFS)? Explain. (b) (4 points) What is the cache consistency problem with Distributed File System? How is it solved? Give one method. (c) (4 points) What cache update policy is used in Network File System (NFS)? Explain. Page 12 of 13

13 (d) (4 points) What is the cache policy in the Andrew File System (AFS)? Explain. Page 13 of 13