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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 449
418
INPUT/OUTPUT
CHAP. 5
an industry used to a doubling of performance every 18 months (Moore’s law),
having no progress at all seems like a violation of the laws of physics, but that is
the current situation.
As a consequence, making computers use less energy so
existing batteries last longer is high on everyone’s agenda. The operating system
plays a major role here, as we will see below.
At the lowest level, hardware vendors are trying to make their electronics more
energy efficient. Techniques used include reducing transistor size, employing dy-
namic voltage scaling, using low-swing and adiabatic buses, and similar techni-
ques. These are outside the scope of this book, but interested readers can find a
good survey in a paper by Venkatachalam and Franz (2005).
There are two general approaches to reducing energy consumption. The first
one is for the operating system to turn off parts of the computer (mostly I/O de-
vices) when they are not in use because a device that is off uses little or no energy.
The second one is for the application program to use less energy, possibly degrad-
ing the quality of the user experience, in order to stretch out battery time.
We will
look at each of these approaches in turn, but first we will say a little bit about hard-
ware design with respect to power usage.
5.8.1 Hardware Issues
Batteries come in two general types: disposable and rechargeable. Disposable
batteries (most commonly AAA, AA, and D cells) can be used to run handheld de-
vices, but do not have enough energy to power notebook computers with large
bright screens.
A rechargeable battery, in contrast, can store enough energy to
power a notebook for a few hours. Nickel cadmium batteries used to dominate
here, but they gave way to nickel metal hydride batteries, which last longer and do
not pollute the environment quite as badly when they are eventually discarded.
Lithium ion batteries are even better, and may be recharged without first being
fully drained, but their capacities are also severely limited.
The general approach most computer vendors take to battery conservation is to
design the CPU, memory, and I/O devices to have multiple states: on, sleeping,
hibernating, and off. To use the device, it must be on.
When the device will not be
needed for a short time, it can be put to sleep, which reduces energy consumption.
When it is not expected to be needed for a longer interval, it can be made to hiber-
nate, which reduces energy consumption even more. The trade-off here is that get-
ting a device out of hibernation often takes more time and energy than getting it
out of sleep state. Finally, when a device is off, it does nothing and consumes no
power.
Not all devices have all these states, but when they do, it is up to the oper-
ating system to manage the state transitions at the right moments.
Some computers have two or even three power buttons. One of these may put
the whole computer in sleep state, from which it can be awakened quickly by typ-
ing a character or moving the mouse. Another may put the computer into hiberna-
tion, from which wakeup takes far longer.
In both cases, these buttons typically do
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