Student ID: ____________CS-736 Final: To Err is Human(Fall 2003)An Exam offered to you by Professor Remzi Arpaci-DusseauPlease Read (And Answer) All Questions Carefully!There are twelve (12) total numbered pages; there are 20 equally-weighted questions.Please put your NAME on this page, and your STUDENT ID on this and all other pagesName:1Student ID: ____________IntroductionWelcome to an alternate universe!All semester long, we’ve talked about systems that have been built throughout the history ofsystems research. All of these systems have both flaws as well as numerous qualities that makethem worth studying. Of course, not every aspect of each system is the best or only way to buildsaid system; indeed, it is conceivable to think of many other variants of each.In this exam, we present slight variants of each of the systems we have studied; assumingthe system we studied is called SuperSystem, we will call the variant introduced in this examSuperSystemBizarro. Your job will be to answer questions about these alternate systems, as de-scribed in more detail below in each question. Good luck!2Student ID: ____________1. The designers of DiscoBizarrowanted it to run on a new machine from SGI, called the OriginBizarro. Thismachine was the same as the machine described in the Disco paper, except that it only had two processor modes:user mode (restricted) and kernel mode (full privileges). Do you think the designers of DiscoBizarrowill beable to implement their system efficiently? Discuss.2. In F F SBizarro, there is a feature that tries to take advantage of temporal locality. Specifically, when twodirectories are created at nearly the same time, FF SBizarrotries to place them within the same cylinder group.Compare and contrast F F S and F F SBizarroallocation. What are the strengths and weaknesses of eachapproach?3Student ID: ____________3. AF SBizarrois quite similar to AF S, with this key difference. Instead of moving whole files to the client uponopen, AF SBizarroonly moves the first 64KB. Other 64KB chunks are moved to the client lazily, on demand.Compare and contrast AF S and AF SBizarro, both in terms of performance and consistency.4. In Gray − boxBizarro, a different method for determining the contents of the OS file cache is used. Specifically,a library is used to monitor read and write requests, and the OS cache contents are derived via a simulation.For example, if LRU is the underlying policy, an LRU list is maintained in the library; a read of a page wouldmove it to the front of the simulated LRU list, as it does in the kernel file cache. Applications can then lookat this user-level LRU list to determine what is in the cache. Compare this approach to the probe-basedapproach taken in the original gray-box system. What are the strengths of each approach, and what arethe weaknesses?4Student ID: ____________5. The design of MulticsBizarroincluded all of the same support for segmentation as in M ultics; however,MulticsBizarrodoes not include any support for paging. Discuss the strengths and weaknesses of theMulticsBizarrodesign.6. In the paper we read for class, Baker observes all deletions that have occured within the trace sample (of say 24hours), finds the creation time of the file, and thereby can compute average file lifetime for all files that have beendeleted. In a new measurement paper, a different method of measuring file lifetime is introduced. Specifically,BakerBizarrodecides to observe all file creations that have occured within a trace sample, and then computethe average lifetime for those files; specifically, if a file is deleted during the sample, the exact lifetime goes intothe average, and if not, the lifetime is assumed to be the length of the sample. Compare and contrast Bakerand BakerBizarro; which is the better technique for computing average file lifetime?5Student ID: ____________7. When issuing a long duration call in the Xerox RP C system, the client repeatedly probes the remote server,getting acknowledgments from it, until the result of the call finally is returned. RP CBizarrodoes not do this;instead, the server will send a single ack that it has received the call and is working on it, and of course an ackin the form of the final result. What is the problem with the RP CBizarroapproach?8. LF SBizarrois a variant of LF S that streamlines the contents of a segment. In fact, LF SBizarrodoes not eveninclude a segment summary block within each segment, just inodes, pieces of the imap, and data blocks. Whataspect of LFS cleaning is made more difficult without a segment summary block? (Just focus on cleaning,and not other aspects of LF S that may also need to change.)6Student ID: ____________9. In ExokernelBizarro, there is an additional feature that lets user-level “OS libraries” download code into theOS, specifically for the scheduler. A library can download scheduler code which changes the default multiplex-ing employed by exokernel. Why would such a feature be useful? What are the inherent difficulties inproviding such a feature?10. Mesa is a concurrent programming environment, with monitors and condition variables used to build parallelprograms and even the Pilot operating system. One feature included in Mesa is called the “naked notify”. Thistype of notify, as you may recall, can signal a condition but does so without grabbing the monitor lock, andrequired changes in other parts of the M esa code to support it. Our alternate version of M esa, M esaBizarro,includes a naked notify, but does not include those changes. What problems can a naked notify lead to? Howare these problems avoided in the original Mesa system?7Student ID: ____________11. Resource Containers (RC) are used to charge resource consumption within a system on a per-activity basis.The slight variant, RCBizarro, also provides a flexible mechanism for accounting, but only accounts for kernelresources that are consumed. In an event-based web server, is the functionality provided by RCBizarro“good enough” to deliver differentiated services to clients?12. Scheduler Activations (SA) are used to inform the user level of kernel activity that is relevant to user-levelthread management. Specifically, four upcalls are provided: Add this processor, Processor has been preempted,Scheduler activation has blocked, Scheduler activation has unblocked. The designers of SABizarrothoughtthis was one too many upcalls, and decided to remove one. Specifically, they removed Scheduler activation