Comp111 - Operating Systems
Final Exam Review #2, Dec 17, 2009
Open Books and Notes

Please write all answers on the exam. Please feel free to use the back of each sheet if appropriate. Please show all of your work for full credit.

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  1. There are 200 people typing on a time-sharing system, where each one types at an average rate λ=1 character/second. The observed average response time is 1 second between typing a character and seeing the result. There is no assumption that the inter-arrival time distribution or service distribution is exponential.
    1. (15 points) What is the average number of characters "in system", either queued or being processed?














    2. (15 points) Suppose that the clock rate of the machine is C, and that we replace the machine with another one with clock rate 2C. Suppose that the time in system for the current machine is T1 and the time in system for the new machine is T2. Is T2 equal to T1/2, greater than T1/2, or less than T1/2? Why? List all reasons that apply.

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  2. A real-time scheduler attempts to wake up processes on demand in response to time-critical I/O events. It runs at the same time as a round-robin scheduler. When a selected I/O event occurs, a real-time scheduler disables any scheduler that might in use otherwise, processes the event, and then returns control to the regular scheduler.
    1. (10 points) Exhibit the result of a real-time scheduling event as a process schedule diagram, with processes on the y-axis and time on the x-axis. Show context switches.














    2. (10 points) It is typical for the real-time scheduler to invoke only a small producer for a non-real-time consumer. Why is this design desirable?









    3. (10 points) What limits must be imposed upon the "real-time" process for the process to respond as fast as possible. Why?

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  3. A translucent filesystem makes a read-only device act as if it is read-write, by associating two filesystems to one another: a read-only filesystem (e.g., a DVD) and a writeable filesystem (e.g., a disk). If a file is written into the translucent filesystem, it is actually written onto the writeable drive, but other read-only files surround it. To open a file for reading, the OS first queries the writeable device, and then the read-only device, for a match. The first match is opened.
    1. (20 points) As a matter of filesystem design, should the translucent filesystem driver interact directly with the raw disk devices? Why or why not?














    2. (20 points) Based upon just the description above, can one delete a file from the read-only part of a translucent filesystem? Why or why not?