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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 454
SEC. 5.8
POWER MANAGEMENT
423
more data to transmit. Finally, it will give up and go to sleep, because continuous
polling is very bad for power consumption. Shortly after, the producer provides
more data, but now the network stack is fast sleep. Waking up the stack takes time
and slows down the throughput. One possible solution is never to sleep, but this is
not attractive either because doing so would increase the power consumption—ex-
actly the opposite of what we are trying to achieve. A much more attractive solu-
tion is to run the network stack on a slower core, so that it is constantly busy (and
thus never sleeps), while still reducing the power consumption. If the network core
is slowed down carefully, its performance will be better than a configuration where
all cores are blazingly fast.
The Memory
Two possible options exist for saving energy with the memory. First, the cache
can be flushed and then switched off. It can always be reloaded from main memo-
ry with no loss of information. The reload can be done dynamically and quickly, so
turning off the cache is entering a sleep state.
A more drastic option is to write the contents of main memory to the disk, then
switch off the main memory itself. This approach is hibernation, since virtually all
power can be cut to memory at the expense of a substantial reload time, especially
if the disk is off, too. When the memory is cut off, the CPU either has to be shut off
as well or has to execute out of ROM. If the CPU is off, the interrupt that wakes it
up has to cause it to jump to code in ROM so the memory can be reloaded before
being used. Despite all the overhead, switching off the memory for long periods of
time (e.g., hours) may be worth it if restarting in a few seconds is considered much
more desirable than rebooting the operating system from disk, which often takes a
minute or more.
Wireless Communication
Increasingly many portable computers have a wireless connection to the out-
side world (e.g., the Internet).
The radio transmitter and receiver required are often
first-class power hogs.
In particular, if the radio receiver is always on in order to
listen for incoming email, the battery may drain fairly quickly.
On the other hand,
if the radio is switched off after, say, 1 minute of being idle, incoming messages
may be missed, which is clearly undesirable.
One efficient solution to this problem has been proposed by Kravets and Krish-
nan (1998).
The heart of their solution exploits the fact that mobile computers
communicate with fixed base stations that have large memories and disks and no
power constraints. What they propose is to have the mobile computer send a mes-
sage to the base station when it is about to turn off the radio. From that time on, the
base station buffers incoming messages on its disk.
The mobile computer may in-
dicate explicitly how long it is planning to sleep, or simply inform the base station
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