Information Technology Reference
In-Depth Information
CHAPTER 3
Using Voltage and Frequency
Adjustments to Manage
Dynamic Power
Issues addressing dynamic power have predominated the power-aware architecture landscape.
Amongst these dynamic power techniques, methods for addressing voltage and frequency have
dominated in turn. Most of these methods have focused on dynamic adjustments to supply
voltage, clock frequency, of both, and they go under the broad title of
Dynamic Voltage and
Frequency Scaling
,or
DVFS
. This chapter discusses the motivation for these techniques overall,
and gives examples drawn from different categories of techniques.
Chapter Structure
: Decisions to engage voltage and frequency scaling are made at various
levels. The decision level, the level of the control policy, defines the structure of this chapter.
Starting from the top, the system (or operating system) level, the chapter unfolds to progressively
more focused levels: program (or program phase) level and the hardware (flip-flop) level. The
following section (Section 3.1) gives an overview of voltage/frequency scaling and discusses a
number of issues pertaining to the corresponding techniques.
3.1 DYNAMIC VOLTAGE AND FREQUENCY SCALING:
MOTIVATION AND OVERVIEW
The basic dynamic power equation:
P
CV
2
Af
clearly shows the significant leverage possible
by adjusting voltage and frequency [
47
,
101
]. If we can reduce voltage by some small factor, we
can reduce power by the square of that factor. Reducing supply voltage, however, might possibly
reduce the performance of systems as well. In particular, reducing supply voltage often slows
transistors such that reducing the clock frequency is also required. The benefit of this is that
within a given system, scaling supply voltage down now offers the potential of a cubic reduction
in power dissipation. The downside of this is that it may also linearly degrade performance. If
the program runs at lower power dissipation levels, but for longer durations, then the benefit
in terms of total energy will not be cubic. It is interesting to note that while voltage/frequency
=
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