Information Technology Reference
In-Depth Information
equally between different users or different applications, or should some get
preferential treatment? If so, how does the operating system decide what tasks
get priority?
Two related concepts are response time and throughput .
Response time,
Definition: response time
Definition: throughput
sometimes called delay, is how long it takes for a single specific task from when
it starts until it completes. For example, a highly visible response time for
desktop computers is the time from when the user moves the hardware mouse
until the pointer on the screen reects the user's action. An operating system
that provides poor response time can be unusable. Throughput is the rate at
which a group of tasks can be completed. Throughput is a measure of eciency
for a group of tasks rather than a single one. While it might seem that designs
that improve response time would also necessarily improve throughput, this is
not the case, as we will discuss later in this topic.
A related consideration is performance predictability , whether the system's
Definition: predictability
response time or other metric is consistent over time. Predictability can often
be more important than average performance. If a user operation sometimes
takes an instant, and sometimes much longer, the user may find it dicult to
adapt. Consider, for example, two systems. In one, the user's keystrokes are
almost always instantaneous, but 1% of the time, a keystroke takes 10 seconds
to take eect. In the other system, the user's keystrokes always take 0.1 seconds
to be reflected on the screen. Average response time may be the same in both
systems, but the second is more predictable. Which do you think would be more
user-friendly?
For a simple example illustrating the concepts of eciency, overhead, fair-
ness, response time, throughput, and predictability, consider a car driving to its
destination. If there were never any other cars or pedestrians on the road, the
car could go quite quickly, never needing to slow down for stop lights. Stop signs
and stop lights enable cars to share the road, at some cost in overhead and re-
sponse time for each individual driver. As the system becomes more congested,
predictability suffers. Throughput of the system is improved with carpooling.
In congested situations and especially with dedicated carpool lanes, carpooling
can also improve latency even though carpoolers need to coordinate their pick-
ups. Predictability, throughput, and arguably fairness can all be improved by
scrapping the car and installing mass transit.
1.2.5
Adoption
In addition to reliability, portability and performance, the success of an oper-
ating system depends on two factors outside its immediate control: the (wide)
availability of applications ported to that operating system, and the (wide) avail-
ability of hardware that the operating system can support. An iPhone runs iOS,
but without the preinstalled applications and the contents of the App Store, the
iPhone would be just a cellphone with (allegedly) bad phone reception.
