Student ID CS 736 Final To Err is Human Fall 2003 An Exam offered to you by Professor Remzi Arpaci Dusseau Please 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 pages Name 1 Student ID Introduction Welcome to an alternate universe All semester long we ve talked about systems that have been built throughout the history of systems research All of these systems have both flaws as well as numerous qualities that make them worth studying Of course not every aspect of each system is the best or only way to build said 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 assuming the system we studied is called SuperSystem we will call the variant introduced in this exam SuperSystemBizarro Your job will be to answer questions about these alternate systems as described in more detail below in each question Good luck 2 Student ID 1 The designers of DiscoBizarro wanted it to run on a new machine from SGI called the OriginBizarro This machine 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 DiscoBizarro will be able 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 two directories are created at nearly the same time F F SBizarro tries to place them within the same cylinder group Compare and contrast F F S and F F SBizarro allocation What are the strengths and weaknesses of each approach 3 Student ID 3 AF SBizarro is quite similar to AF S with this key difference Instead of moving whole files to the client upon open AF SBizarro only 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 would move it to the front of the simulated LRU list as it does in the kernel file cache Applications can then look at this user level LRU list to determine what is in the cache Compare this approach to the probe based approach taken in the original gray box system What are the strengths of each approach and what are the weaknesses 4 Student ID 5 The design of M ulticsBizarro included all of the same support for segmentation as in M ultics however M ulticsBizarro does not include any support for paging Discuss the strengths and weaknesses of the M ulticsBizarro design 6 In the paper we read for class Baker observes all deletions that have occured within the trace sample of say 24 hours finds the creation time of the file and thereby can compute average file lifetime for all files that have been deleted In a new measurement paper a different method of measuring file lifetime is introduced Specifically BakerBizarro decides to observe all file creations that have occured within a trace sample and then compute the average lifetime for those files specifically if a file is deleted during the sample the exact lifetime goes into the average and if not the lifetime is assumed to be the length of the sample Compare and contrast Baker and BakerBizarro which is the better technique for computing average file lifetime 5 Student 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 CBizarro does 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 ack in the form of the final result What is the problem with the RP CBizarro approach 8 LF SBizarro is a variant of LF S that streamlines the contents of a segment In fact LF SBizarro does not even include a segment summary block within each segment just inodes pieces of the imap and data blocks What aspect of LF S 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 6 Student ID 9 In ExokernelBizarro there is an additional feature that lets user level OS libraries download code into the OS specifically for the scheduler A library can download scheduler code which changes the default multiplexing employed by exokernel Why would such a feature be useful What are the inherent difficulties in providing such a feature 10 M esa is a concurrent programming environment with monitors and condition variables used to build parallel programs and even the Pilot operating system One feature included in M esa is called the naked notify This type of notify as you may recall can signal a condition but does so without grabbing the monitor lock and required 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 How are these problems avoided in the original M esa system 7 Student 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 kernel resources 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 level thread management Specifically four upcalls are provided Add this processor Processor has been preempted Scheduler activation has blocked Scheduler activation has unblocked The designers of SABizarro thought this was one too many upcalls and decided to remove one Specifically they removed Scheduler activation has unblocked Is this a good decision Why or why not 8 Student ID 13 The V ax V M S paging system uses first in first out FIFO queues to manage pages in memory V ax V M SBizarro instead uses last in