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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 451
420
INPUT/OUTPUT
CHAP. 5
annoying delay while it is restarted.
On the other hand, if it waits too long to shut
down a device, energy is wasted for nothing.
The trick is to find algorithms and heuristics that let the operating system make
good decisions about what to shut down and when. The trouble is that ‘‘good’’ is
highly subjective.
One user may find it acceptable that after 30 seconds of not
using the computer it takes 2 seconds for it to respond to a keystroke. Another user
may swear a blue streak under the same conditions.
In the absence of audio input,
the computer cannot tell these users apart.
The Display
Let us now look at the big spenders of the energy budget to see what can be
done about each one. One of the biggest items in everyone’s energy budget is the
display.
To get a bright sharp image, the screen must be backlit and that takes sub-
stantial energy. Many operating systems attempt to save energy here by shutting
down the display when there has been no activity for some number of minutes.
Often the user can decide what the shutdown interval is, thus pushing the trade-off
between frequent blanking of the screen and draining the battery quickly back to
the user (who probably really does not want it).
Turning off the display is a sleep
state because it can be regenerated (from the video RAM) almost instantaneously
when any key is struck or the pointing device is moved.
One possible improvement was proposed by Flinn and Satyanarayanan (2004).
They suggested having the display consist of some number of zones that can be in-
dependently powered up or down. In Fig. 5-41, we depict 16 zones, using dashed
lines to separate them. When the cursor is in window 2, as shown in Fig. 5-41(a),
only the four zones in the lower righthand corner have to be lit up. The other 12
can be dark, saving 3/4 of the screen power.
When the user moves the cursor to window 1, the zones for window 2 can be
darkened and the zones behind window 1 can be turned on. However, because win-
dow 1 straddles 9 zones, more power is needed.
If the window manager can sense
what is happening, it can automatically move window 1 to fit into four zones, with
a kind of snap-to-zone action, as shown in Fig. 5-41(b). To achieve this reduction
from 9/16 of full power to 4/16 of full power, the window manager has to under-
stand power management or be capable of accepting instructions from some other
piece of the system that does. Even more sophisticated would be the ability to par-
tially illuminate a window that was not completely full (e.g., a window containing
short lines of text could be kept dark on the right-hand side).
The Hard Disk
Another major villain is the hard disk.
It takes substantial energy to keep it
spinning at high speed, even if there are no accesses. Many computers, especially
notebooks, spin the disk down after a certain number of minutes of being idle.
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