CS 610

Homework 5

Due Date: 12/10/98

Note: You are required to show all work to receive full credit.

1. (10 points)

What is the effect of allowing two entries in a page table to point to the same page frame in memory? Explain how this effect could be used to decrease the amount of time needed to copy a large amount of memory from one place to another. What would the effect of updating some byte in the one page be on the other page?

2. (10 points)

Consider a demand-paging system with the following time-measured utilizations:

CPU utilization20%
Paging disk97.7%
Other I/O devices 5%

Which (if any) of the following will (probably) improve CPU utilization? Explain your answer.

(a)
Install a faster CPU.
(b)
Install a bigger paging disk.
(c)
Increase the level of multiprogramming.
(d)
Decrease the level of multiprogramming.
(e)
Install more main memory.
(f)
Install a faster hard disk, or multiple controllers with multiple hard disks.
(g)
Add prepaging to the page fetch algorithms.
(h)
Increase the page size.

3. (10 points)

Explain the purpose of the file open and close operations. How can having these operations improve the performance of a system?

4. (10 points)

Consider a file system where free space is kept in a free-space list.

(a)
Suppose that the pointer to the free-space list is lost. Can the system reconstruct the free-space list? Explain your answer.
(b)
Suggest a scheme to ensure that the pointer is never lost as a result of memory failure.

5. (10 points)

Suppose that a disk drive has 5000 cylinders, numbered 0 to 4999. The drive is currently serving a request at cylinder 143, and the previous request was at cylinder 125. The queue of pending requests, in FIFO order, is:

86, 1470, 913, 1774, 948, 1509, 1022, 1750, 130

Starting from the current head position, what is the total distance (in cylinders) that the disk arm moves to satisfy all the pending requests, for each of the following disk-scheduling algorithms?

(a)
FCFS
(b)
SSTF
(c)
SCAN
(d)
LOOK
(e)
C-SCAN

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