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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 456
SEC. 5.8
POWER MANAGEMENT
425
more. Most mobile devices have programs that can be run to query and display all
these parameters. Smart batteries can also be instructed to change various opera-
tional parameters under control of the operating system.
Some notebooks have multiple batteries. When the operating system detects
that one battery is about to go, it has to arrange for a graceful cutover to the next
one, without causing any glitches during the transition. When the final battery is on
its last legs, it is up to the operating system to warn the user and then cause an
orderly shutdown, for example, making sure that the file system is not corrupted.
Driver Interface
Several operating systems have an elaborate mechanism for doing power man-
agement called
ACPI
(
Advanced Configuration and Power Interface
). The op-
erating system can send any conformant driver commands asking it to report on the
capabilities of its devices and their current states. This feature is especially impor-
tant when combined with plug and play because just after it is booted, the operat-
ing system does not even know what devices are present, let alone their properties
with respect to energy consumption or power manageability.
It can also send commands to drivers instructing them to cut their power levels
(based on the capabilities that it learned earlier, of course). There is also some traf-
fic the other way.
In particular, when a device such as a keyboard or a mouse
detects activity after a period of idleness, this is a signal to the system to go back to
(near) normal operation.
5.8.3 Application Program Issues
So far we have looked at ways the operating system can reduce energy usage
by various kinds of devices. But there is another approach as well: tell the pro-
grams to use less energy, even if this means providing a poorer user experience
(better a poorer experience than no experience when the battery dies and the lights
go out).
Typically, this information is passed on when the battery charge is below
some threshold.
It is then up to the programs to decide between degrading perfor-
mance to lengthen battery life or to maintain performance and risk running out of
energy.
One question that comes up here asks how a program can degrade its perfor-
mance to save energy.
This question has been studied by Flinn and Satya-
narayanan (2004).
They provided four examples of how degraded performance
can save energy.
We will now look at these.
In this study, information is presented to the user in various forms. When no
degradation is present, the best possible information is presented.
When degrada-
tion is present, the fidelity (accuracy) of the information presented to the user is
worse than what it could have been. We will see examples of this shortly.
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