Homework 3

The due date has been changed to May 25, 2022. Also, problem 5(a) was clarified to say that “light load” means no more than 1 request in the queue at a time.

Due: May 23, 2022
Points: 100

When you do the homework, please put the answers to questions 1–3 on one page, and the answers to each of the others on separate pages. You can save this file and put your answers on it. This will make using Gradescope to grade the assignment much easier than if you submitted everything without regard to pages.

Remember, you must justify all your answers.


  1. (6 points) What is “device independence”?

  2. (7 points) What is thrashing?

  3. (10 points) What is the difference between paging and segmentation?

Long Answer Questions

  1. (25 points) You are the president of Cheapo Computronics, Inc., and your star hardware designer has suggested a brilliant idea: Implement segmentation, but let the least significant m bits of a virtual address be used to select the segment, and let the other bits determine the offset. What is the problem with this idea?

  2. (25 points) This question asks you to compare different disk scheduling policies.
    1. Under very light loads (specifically, when there is no more than 1 request in the queue), all the disk scheduling policies we have discussed degenerate into which policy? Why?
    2. Consider a system on which a seek takes 0.5 + 0.4T ms, where T is the number of cylinders moved. Then assume the arm is initially at cylinder 100, the disk has 200 cylinders, and the arm is moving inward. Will requests scheduled by a FCFS disk scheduling policy ever have a lower mean waiting time than those scheduled by a SCAN policy? Than those scheduled by a SSTF policy? Justify your answers.

  3. (27 points) Consider a file currently consisting of 100 blocks of 512 words each. Ignoring the access to update the device directory, and assuming a disk block number fits into a single word, how many disk I/O operations are involved with contiguous, linked, and indexed allocation strategies, if one block:
    1. is removed from the beginning?
    2. is added in the middle?
    3. is added at the end?

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Matt Bishop
Office: 2209 Watershed Sciences
Phone: +1 (530) 752-8060
Email: mabishop@ucdavis.edu
ECS 150, Operating Systems
Version of May 19, 2022 at 10:45AM

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