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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
Showing 996 out of 1137
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 996
SEC. 11.9
WINDOWS POWER MANAGEMENT
965
the current generation of multiprocessors, both hibernation and resume can be per-
formed in a few seconds even on systems with many gigabytes of RAM.
An alternative to hibernation is
standby mode
where the power manager re-
duces the entire system to the lowest power state possible, using just enough power
to the refresh the dynamic RAM.
Because memory does not need to be copied to
disk, this is somewhat faster than hibernation on some systems.
Despite the availability of hibernation and standby, many users are still in the
habit of shutting down their PC when they finish working. Windows uses hiberna-
tion to perform a pseudo shutdown and startup, called
HiberBoot
, that is much fast-
er than normal shutdown and startup.
When the user tells the system to shutdown,
HiberBoot logs the user off and then hibernates the system at the point they would
normally login again. Later, when the user turns the system on again, HiberBoot
will resume the system at the login point.
To the user it looks like shutdown was
very, very fast because most of the system initialization steps are skipped.
Of
course, sometimes the system needs to perform a real shutdown in order to fix a
problem or install an update to the kernel. If the system is told to reboot rather
than shutdown, the system undergoes a real shutdown and performs a normal boot.
On phones and tablets, as well as the newest generation of laptops, computing
devices are expected to be always on yet consume little power.
To provide this
experience Modern Windows implements a special version of power management
called
CS
(
connected standby
). CS is possible on systems with special network-
ing hardware which is able to listen for traffic on a small set of connections using
much less power than if the CPU were running.
A CS system always appears to be
on, coming out of CS as soon as the screen is turned on by the user.
Connected
standby is different than the regular standby mode because a CS system will also
come out of standby when it receives a packet on a monitored connection.
Once
the battery begins to run low, a CS system will go into the hibernation state to
avoid completely exhausting the battery and perhaps losing user data.
Achieving good battery life requires more than just turning off the processor as
often as possible.
It is also important to keep the processor off as long as possible.
The CS network hardware allows the processors to stay off until data have arrived,
but other events can also cause the processors to be turned back on.
In NT-based
Windows device drivers, system services, and the applications themselves fre-
quently run for no particular reason other than to
check on things
. Such
polling
activity is usually based on setting timers to periodically run code in the system or
application. Timer-based polling can produce a cacophony of events turning on the
processor.
To avoid this, Modern Windows requires that timers specify an impreci-
sion parameter which allows the operating system to coalesce timer events and re-
duce the number of separate occasions one of the processors will have to be turned
back on.
Windows also formalizes the conditions under which an application that
is not actively running can execute code in the background.
Operations like check-
ing for updates or freshening content cannot be performed solely by requesting to
run when a timer expires. An application must defer to the operating system about
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