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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 450
SEC. 5.8
POWER MANAGEMENT
419
nothing except send a signal to the operating system, which does the rest in soft-
ware. In some countries, electrical devices must, by law, have a mechanical power
switch that breaks a circuit and removes power from the device, for safety reasons.
To comply with this law, another switch may be needed.
Power management brings up a number of questions that the operating system
has to deal with. Many of them relate to resource hibernation—selectively and
temporarily turning off devices, or at least reducing their power consumption when
they are idle. Questions that must be answered include these: Which devices can be
controlled? Are they on/off, or are there intermediate states?
How much power is
saved in the low-power states?
Is energy expended to restart the device? Must
some context be saved when going to a low-power state?
How long does it take to
go back to full power? Of course, the answers to these questions vary from device
to device, so the operating system must be able to deal with a range of possibilities.
Various researchers have examined notebook computers to see where the pow-
er goes.
Li et al. (1994) measured various workloads and came to the conclusions
shown in Fig. 5-40. Lorch and Smith (1998) made measurements on other ma-
chines and came to the conclusions shown in Fig. 5-40. Weiser et al. (1994) also
made measurements but did not publish the numerical values. They simply stated
that the top three energy sinks were the display, hard disk, and CPU, in that order.
While these numbers do not agree closely, possibly because the different brands of
computers measured indeed have different energy requirements, it seems clear that
the display, hard disk, and CPU are obvious targets for saving energy. On devices
like smartphones, there may be other power drains, like the radio and GPS. Al-
though we focus on displays, disks, CPUs and memory in this section, the princ-
ples are the same for other peripherals.
Device
Li et al. (1994)
Lorch and Smith (1998)
Display
68%
39%
CPU
12%
18%
Hard disk
20%
12%
Modem
6%
Sound
2%
Memory
0.5%
1%
Other
22%
Figure 5-40.
Power consumption of various parts of a notebook computer.
5.8.2 Operating System Issues
The operating system plays a key role in energy management.
It controls all
the devices, so it must decide what to shut down and when to shut it down. If it
shuts down a device and that device is needed again quickly, there may be an
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