practice
Doi: 10.1145/1378727.1378740
The processor is, perhaps, the most
obvious target of power management.
On a modern system the CPU is likely
to be the single component consuming the most power. Switching the state
BY matthe W GaRRett of hundreds of millions of transistors
takes power. Modern processors can
generate well over 100 watts of heat.
Powering How can this be reduced?
At the most basic level the answer
is simple: reduce the power taken to
switch those transistors. Making them
smaller is one mechanism, as is reducing the amount of power lost via leakage. There are limits, however, to how
Down much power can be saved via fundamental improvements in semiconductor technology, and vendors are being
forced to adopt increasingly high-tech
solutions to maintain the rate of progress in this respect. We cannot rely on
technological breakthroughs to be the
silver bullet. We need to be smarter in
how we use what we already have available.
The most reasonable approach to
reducing the power consumption of
processors is to realize that under normal usage patterns processors will not
run at 100% utilization all the time.
The first attempt to take advantage of
this was in the Pentium era with the
addition of idle power savings. The
HLT (halt) instruction allowed operating systems to indicate that they had
nothing to execute, letting the processor put itself into some form of power
saving until the next interrupt arrived.
Runtime processor power management had arrived.
PHO TOGRAPH BY PAUL TICHONCZUK
The APM (Advanced Power Management) specification extended this
functionality. Rather than simply halting the processor, the APM CPU idle
call allowed the processor to be put
into a lower power state where it continued to execute instructions. If system load reached a threshold level, the
operating system could then send a
CPU busy call to restore the processor
to full operating speed.
This concept of runtime power management has been further enhanced
in recent years. CPU clock and volt-
Smart power management is all about doing
more with the resources we have.
POWER MANAGEMENT—FROM laptops to rooms full
of servers—is a topic of interest to everyone. in the
beginning there was the desktop computer. it ran
at a fixed speed and consumed less power than the
monitor it was plugged into. Where computers were
portable, their sheer size and weight meant that you
were more likely to be limited by physical strength
than battery life. it was not a great time for power
management.
Now consider the present. Laptops have increased
in speed by more than 5,000 times. Battery capacity,
sadly, has not. With hardware becoming increasingly
mobile, however, users are demanding that battery life
start matching the way they work. People want to work
from cafés. Long-haul flights are now perceived as the
ideal opportunity to finish a presentation. Two hours
of battery life just isn’t going to cut it; users are looking
for upward of eight hours. What’s drawing that power,
and more importantly, how can we reduce it?
