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CPSC 457
OPERATING SYSTEMS
FINAL EXAM SOLUTION
Department of Computer Science
University of Calgary
Professor: Carey Williamson
December 10, 2008
This is a CLOSED BOOK exam. Textbooks, notes, laptops, calculators, personal digital assistants, cell phones, and Internet access are NOT allowed. It is a 120-minute exam, with a total of 100 marks. There are 18 questions, and 11 pages (including this cover page). Please read each question carefully, and write your answers legibly in the space provided. You may do the questions in any order you wish, but please USE YOUR TIME WISELY. When you are finished, please hand in your exam paper and sign out. Good luck!
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Multiple Choice
Choose the best answer for each of the following 12 questions, for a total of 12 marks.
1 1. Three file descriptors associated with every Linux process are:
(a) standard input, standard output, and standard pipe
(b) standard input, standard output, and standard error
(c) standard input, standard output, and standard deviation
(d) standard input, standard output, and standard terminal
(e) standard input, standard output, and standard transmission
1 2. User Mode Linux (UML) is an example of a virtual machine environment in which:
(a) Linux runs on top of Windows
(b) Linux runs on top of Linux
(c) Windows runs on top of Linux
(d) Windows runs on top of Windows
(e) none of the above
1 3. During the boot process, a computer obtains its initial bootstrapping information from:
(a) a special “boot block” on disk
(b) the superblock in the root file system
(c) a pre-configured file vmunix within the file system
(d) the /tmp file system
(e) none of the above
1 4. The copy-on-write mechanism provides:
(a) an efficient way to create new processes
(b) a clever way to share virtual memory pages (at least temporarily)
(c) a way to avoid unnecessary page copying
(d) all of the above
(e) none of the above
1 9. The Banker’s Algorithm is an example of a technique for:
(a) deadlock prevention
(b) deadlock avoidance
(c) deadlock detection
(d) deadlock recovery
(e) stabilizing turbulent financial markets
1 10. With asynchronous I/O, file system changes will be committed to disk when:
(a) the in-memory inode is updated
(b) the sync daemon runs
(c) the system administrator feels like doing it
(d) nightly file system backups are run
(e) the system is rebooted
1 11. The operation of defragmenting a hard disk:
(a) uses compaction to combat internal fragmentation
(b) uses compaction to combat external fragmentation
(c) uses compression to combat internal fragmentation
(d) uses compression to combat external fragmentation
(e) all of the above
1 12. Which of the following is an idempotent request?
(a) read the next byte from file foople
(b) read block 3 from file foople
(c) write this block to the end of file foople
(d) append file foople to file boople
(e) link file foople to file boople
OS Concepts and Definitions
15 13. For each of the following pairs of terms, identify the context(s) in which they occur. Then define each term and clarify the key difference(s) between the two terms.
(a) (3 marks) “host OS” and “guest OS” context: virtual machines host OS: underlying OS layer, with access to physical hardware guest OS: runs on top of host OS, provides services to user, using the resources provided by the host OS Virtual machines provide flexible execution environments for users.
(b) (3 marks) “page” and “frame” context: (virtual) memory management frame: fixed-size basic unit of physical memory allocation page: fixed-size logical unit of process address space; a page fits in a frame; any page can be put in any frame
(c) (3 marks) “reference bit” and “dirty bit” context: paging-based memory management reference bit: indicates if a page has been accessed (recently) dirty bit: indicates if a page has been modified (relative to disk) These bits influence decision-making in page replacement policies
(d) (3 marks) “file” and “directory” context: file systems file: a named logical collection of related info, as defined by user directory: a logical collection of related files, as defined by user The directory has metadata about files (name, size, location, etc)
(e) (3 marks) “disk partition” and “file system volume” context: file and storage systems disk partition: a logical piece of a disk (or set of disks) file system volume: a partition that contains a file system (as opposed to being empty or used as raw disk or swap space) A partition can hold a file system
Memory Management
15 15. Answer the following questions about OS memory management.
(a) (4 marks) One of the design decisions in OS memory management is the choice between swapping and paging. Define each of these terms, and clarify their respective roles in OS memory management. swapping: copies entire process image between memory and disk. Assumes contiguous allocation, entire process needed for execution. Used to limit multiprogramming level and avoid thrashing. paging: divides logical address space of process into fixed-size pieces; process can execute with only a subset of these pages being resident in memory at a time. Provides flexible and efficient memory management, with lots of processes active at a time.
(b) (5 marks) Another key design decision in OS memory management is the choice be- tween paging and segmentation. Compare and contrast these two approaches to memory management, making sure to identify the strengths and weaknesses of each. paging: separates physical organization from logical address space Uses fixed-size units called pages, which are stored in page frames. Requires page table to keep track of (possibly many) pages. segmentation: preserves user’s structural view of logical address space Uses variable-size units called segments. Can go anywhere in memory. Requires segment table to keep track of the (few) segments. Need base and limit register for each segment. (These two techniques can also be combined!)
(c) (6 marks) In pure on-demand paging, a page replacement policy is used to manage system resources. Suppose that a newly-created process has 3 page frames allocated to it, and then generates the page references indicated below. (i) How many page faults would occur with FIFO page replacement? 12
A B C B A D A B C D A B A C B D The bold font indicates the references that cause a page fault. (ii) How many page faults would occur with LRU page replacement? 10
A B C B A D A B C D A B A C B D The bold font indicates the references that cause a page fault. (iii) How many page faults would occur with OPT page replacement? 7
A B C B A D A B C D A B A C B D The bold font indicates the references that cause a page fault.
File and Storage Systems
15 16. Answer the following questions about file systems in general.
(a) (3 marks) In Unix, Linux, and Windows file systems, there are multiple timestamps (usually 3) associated with each file. What do each of these timestamps represent? creation: time when file was first created modification: time when file was most recently modified (e.g., written) access: time when file was most recently accessed (e.g., read)
(b) (6 marks) In class, we discussed three different techniques for organizing the data blocks for each file in a file system, namely contiguous allocation, linked allocation, and indexed allocation. Briefly describe each approach, identifying the strengths and weaknesses of each. contiguous: allocate file blocks consecutively on disk. Easy to do sequential or direct (random) access to file. Simple to implement, but prone to (external) fragmentation, and very difficult to ‘‘grow’’ file dynamically. linked: linked list of available blocks from anywhere on disk. Each block points to the next. Ameliorates fragmentation problem, and easy to grow a file, but random access is difficult and slow because of many seek times required to traverse chain of pointers. indexed: best of both worlds; table of (direct and/or indirect) pointers to data blocks, which can be anywhere on disk. Solves fragmentation problem. Supports sequential and random access. Can optimize data block layout using cylinder groups, as in Unix.
(c) (6 marks) In a storage system with conventional magnetic-media disks, several different delays occur when servicing a request. Identify at least three of these delays, and comment on their relative contribution to the total delay for servicing a request. seek time: time to move read/write head from its current position to the desired track. May take 5-10 milliseconds (often dominant). rotational latency: time for desired sector on target track to spin under the read/write head. May take 1-2 milliseconds (medium) transfer time: time to read the target sector and transfer bytes to host computer. A millisecond or less (low). wait time: time spent in I/O queue waiting for service. Could be 0 of more milliseconds, depending on number of requests queued and the disk request scheduling policy used.
[carey@csl]$ df Filesystem 1K-blocks Used Available Use% Mounted on /dev/sda2 20315844 7513824 11753380 39% / /dev/sda5 10581704 5181092 4854404 52% /tmp /dev/sda1 1019208 50356 916244 6% /boot tmpfs 1684784 77164 1607620 5% /dev/shm nsi:/export/research 247709760 203879488 31247360 87% /home/research nse:/export/scratch 3361364788 528637368 2661979928 17% /home/scratch nsj:/export/proj/dsl 485097928 184513976 275942416 41% /home/dsl nsg:/export/ug 381885660 79136396 283350608 22% /home/ugc nsf:/export/ug 381885660 88768580 273718424 25% /home/ugb nsh:/export/grads 789574392 507946744 241519616 68% /home/grads nsb:/export/ug 381885660 76958700 285528304 22% /home/uga
[carey@csl]$ cat /etc/fstab LABEL=/ / ext3 defaults 1 1 LABEL=/tmp /tmp ext3 defaults 1 2 LABEL=/boot /boot ext3 defaults 1 2 tmpfs /dev/shm tmpfs defaults 0 0 devpts /dev/pts devpts gid=5,mode=620 0 0 sysfs /sys sysfs defaults 0 0 proc /proc proc defaults 0 0 LABEL=SWAP-sda3 swap swap defaults 0 0
[carey@csl]$ mount /dev/sda2 on / type ext3 (rw) proc on /proc type proc (rw) sysfs on /sys type sysfs (rw) devpts on /dev/pts type devpts (rw,gid=5,mode=620) /dev/sda5 on /tmp type ext3 (rw) /dev/sda1 on /boot type ext3 (rw) tmpfs on /dev/shm type tmpfs (rw) none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw) sunrpc on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw) nsi:/export/research on /home/research type nfs (rw,intr,tcp,rsize=32768,wsize=4096) nse:/export/scratch on /home/scratch type nfs (rw,intr,tcp,rsize=32768,wsize=4096) nsj:/export/proj/dsl on /home/dsl type nfs (rw,intr,tcp,rsize=32768,wsize=4096) nsg:/export/ug on /home/ugc type nfs (rw,intr,tcp,rsize=32768,wsize=4096) nsf:/export/ug on /home/ugb type nfs (rw,intr,tcp,rsize=32768,wsize=4096) nsh:/export/grads on /home/grads type nfs (rw,intr,tcp,rsize=32768,wsize=4096) nsb:/export/ug on /home/uga type nfs (rw,intr,tcp,rsize=32768,wsize=4096)
General Operating Systems Knowledge
15 18. Throughout CPSC 457 this year, there were several recurring themes (i.e., ideas that applied quite broadly across several topics).
(a) (5 marks) One of these themes was virtualization. Identify three contexts in which virtualization was used as a solution technique. Briefly discuss the technical issues involved, and the benefits of the virtualization approach to the problem. virtual machines: can run multiple OS on same machine at same time virtual memory: separates physical memory management from logical view virtual file system: common API for all file systems (local or remote) Virtualization abstracts away the physical hardware and its constraints, providing simplicity and flexibility for the users.
(b) (5 marks) A second theme was hardware support. Identify three contexts in which hardware support was used as a solution technique. Briefly discuss the technical issues involved, and the benefits of a hardware-based approach to the problem. multi-processors: hardware support for parallel computing multi-core: hardware support for fine-grain thread concurrency MMU/TLB: hardware support for address translation, page table lookup test-and-set, SWAP: atomic hardware instructions for mutual exclusion protection: distinguish user-mode priviliges from kernel-mode Hardware solution provides fast, specialized, high-performance solution for key OS features and requirements.
(c) (5 marks) A third theme was caching. Identify three contexts in which caching was used as a solution technique. Briefly discuss the technical issues involved, and the benefits of caching as a solution. CPU: on-chip instruction/data caches to avoid memory latency buffer cache: in-memory caching to avoid disk latency disk cache: remember recent data blocks to avoid disk access latency write cache: buffer outgoing writes to improve system performance storage: sequential read-ahead and prefetch for I/O controller file system: in-memory caching of superblock, inodes, directory info NFS: client-side caching of info to avoid network latency Caching optimizes for the common case, and improves performance.
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