CS 3500 Final Exam Sample Questions

Transcription

1 CS 3500 Final Exam Sample Questions (10) 1. The VAX used a paging system with 32-bit addresses, divided into three fields.. a 2 bit segment, 21 bits of page number, and 9 bits of offset. The 2-bit segment meant that each process could have up to four page tables associated to it (although segment 10 was used by the system and segment 11 was unused, leaving only segments 00 and 01 for the process). (a) How large are the pages? Solution: 2 9 = 512 bytes (b) How large is each page table? (Give the number of entries and the size in bytes) Solution: Each table has 2 21 = 2M entries, each of which needs 4 bytes, for a total of 8Mb. (10) 2. Suppose a memory manager (such as malloc) uses a linked freelist. An application makes successive memory requests of 10 and 30 bytes. (a) Is it possible that a first-fit method could fail to while a best-fit method is able to fill them both from the freelist? If so, what would the freelist need to look like? Solution: Yes. For example, if the list has a 30 byte and a 10 byte block on it in that order. (b) Is it possible that a best-fit method could fail to while a first-fit method is able to fill them both from the freelist? If so, what would the freelist need to look like? Solution: No. With only these two allocation requests, best-fit will always work if there is any possible allocation. (10) 3. Two threads simultaneously run the following code to insert an item onto a linked list. item->next = the_list; the_list = item; Assume the_list is the global linked list, and item is a local variable for each thread. (a) Explain how one thread s item could be left off the list. Solution: Thread 1 does item->next = the_list, thread 2 does the same. Then thread one does the_list=item, then thread 2 does it. Since the_list now points to thread 2 s item, and thread 2 s item->next field points to the start of the old list, thread 1 s item is not on the list.

2 (b) What are the implications for the malloc/free functions as implemented in this course? Solution: Our malloc/free is not thread safe. In particular, freed memory may be lost from the freelist. (c) What would you change in malloc() to fix the problem? Solution: Add a mutex to protect the freelist (10) 4. Consider two threads running at once: Thread 1 lock(a_lock); a++; if (a == 10) { lock(b_lock); b++; unlock(b_lock); a = 0; } unlock(a_lock); Thread 2 lock(b_lock); b--; if (b == 0) { lock(a_lock); a--; unlock(a_lock); } unlock(b_lock); Suppose a is initially 9, b is initially 1, and both mutex variables a_lock and b_lock are unlocked. If both threads execute concurrently, what are the possible outcomes? Solution: a = 0 and b = 1 (Thread 1 finishes then thread 2 starts) a =9 and b = 0 (Thread 2 finishes then thread 1 starts) both threads hang, deadlocked. (10) 5. A disk has the following free space bitmap:

3 The superblock is stored in block 0, and the root directory is stored in block 1. The root directory contains: Filename Start block Size (in blocks) afile 2 3 bigfile ccode.c 8 4 The file system is inconsistent. What is wrong? How would you suggest repairing it to minimize potential data loss? Solution: Bits at 12, 13, 17, 25 are inconsistent with files. Fix by changing the size of ccode.c to 6, marking bit 25 as 1, and marking bit 17 as 0. (10) 6. Consider a backup program that copies every file on a hard disk to a backup drive so that the disk can be restored in case of a crash. (a) How do hard links affect the program? What can it do to deal with hard links? Solution: Multiple directory entries for same file, not one of which is special. It can mark each file on disk when it backs it up so that it only makes one backup of each file, and then note when a hard link needs to be rebuilt on restoring. (b) How do symbolic links affect the program? What can it do to deal with symbolic links? Solution: Needs to detect symbolic links and save the link, not the file it links to. Also, for links to directories, the program shouldnt follow the symbolic link when walking the directory tree. (10) 7. Give an example of a single command line Unix command that will create three processes. Solution: ps sort more (10) 8. Name two services provided by the TCP layer. Solution: Process-to-process communication via ports. Reliable, ordered, byte stream, persistent connections.

4 (10) 9. Fetching a web page requires many layers of protocol. A common stack uses the four protocols: HTTP, TCP, IP, Ethernet. (a) Which protocols does the web browser need to understand? (b) Which protocols does the OS need to understand? (c) Which protocols does the network card (NIC) need to understand? Solution: The browser knows HTTP, the OS knows TCP and IP, the NIC handles the Ethernet. But this is somewhat oversimplified. (10) 10. Explain how a proxy server provides anonymous web browsing. Solution: Since all of the client s requests go through the proxy server, outside web servers do not see the client as the source of HTTP requests. They only see requests coming from the proxy server. (10) 11. True or false: (a) A thrashing process spends more time paging than doing useful work. (b) In a hardware TLB, the entries are kept sorted to make lookups faster. (c) Paged memory management is susceptible to internal fragmentation. (d) After a fork(), the two processes share frames which are marked copy-on-write. (e) Hierarchical page tables are too large to use on 64 bit machines. Solution: True, False, True, True, False. 12. (10) Suppose the virtual memory map for a process looks like the diagram below, with unmapped areas shown in white. Which of the following operations will result in a segmentation fault?

5 a) Read at $ b) Write at $1FFFFFF0 c) Read at $ d) Write at $ e) Read at $3FFFFFF0 f) Write at $ g) Read at $ h) Write at $B Solution: a, d, e, h will segfault. Sometimes, code can be execute only with no read permissions, and then c will segfault as well.