MCS-378 Homework 4, Fall 1999
Due: November 10, 1999
The operating systems for general purpose workstation computers
normally read input packets from their network interfaces on an
interrupt-driven basis. In contrast, computers that are specially
designed as dedicated inter-network routers normally poll for packets
from their network interfaces. Explain why each strategy makes sense
Do exercise 13.2 from page 458.
Do exercise 13.3 from pages 458-459.
Part a says that seek time is
proportional to the square root of distance, but part b gives a
formula with a constant term as well as the term proportional to the
square root of distance. To understand this, you need to understand
that our authors have chosen to use "seek time" in two different senses
in one problem. The full seek time of a disk is combined out of two
parts: time to move the head arm, and "settling" time for the head to
settle into its new position once it is done being moved. (Ask me if
you care why settling is needed or how it is actually done.) It is
the motion time that part a is referring to as proportional to the
square root of distance. The settling time is independent of
distance. Therefore, part b's formula is the realistic one for the
total seek time, including settling as well as motion. (Note that
this formula only applies to seeks of one or more cylinders. A "seek"
of zero cylinders is actually no seek at all, and takes zero time, because
settling is not needed if there is no motion.) In part b, you are
being asked to determine specific numeric values for the parameters
x and y.
Instructor: Max Hailperin