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# CS439: Principles of Computer Systems

## Homework 8

(Source: Alison Norman.)

### Due: 11:59 PM Monday, April 1, 2013

Homeworks will be submitted electronically. Please refer to the homework turnin instructions.

1. Sam P. Hacker is a Head Guru in a software company known for operating systems with very sorry quality. Hacker suggested a trick to reduce the pressure on the swap space. Instead of swapping out pages that belong to code texts into the swap area, the operating system could just take away the memory frames. If the code page is needed later, it could be paged directly from its binary file. Hacker's argument is that the operating system will save time by not storing the code page into the swap space, and will save space in the swap area. The code text exists on disk anyway, and could be fetched from there. Would you approve of this design? Why or why not?
2. Disk requests come into the disk driver for tracks 10, 22, 20, 2, 40, 6, and 38, in that order. A seek takes 6 msec per track moved. How much seek time is needed for the following scheduling algorithms?
a) FIFO
b) SSTF
c) LOOK (SCAN, but doesn't move to the end)
In all cases, the arm is initially at track 20. (Note that SCAN is a synonym for the elevator algorithm.)

Adapted from Tannenbaum Chapter 5 Number 24.

3. Suppose I have a disk such as the 320GB SATA drive described here and I have a workload consisting of 10,000 reads to sectors randomly scattered across the disk. How long in expectation will these 10,000 request take (total) assuming the disk services requests in FIFO order?
4. The MegaGiga hard disk rotates at 5400 rpm with an arm seek time given by = 4 + 0.05t msec, where t is the number of tracks the arm seeks. Assume a sector size of 1024 bytes, and 2048 tracks with 512 sectors/track. The disk controller and DMA read or write data from/to disk at a rate of 4MB/sec. What is the storage capacity of the disk?