U. Glaeser

the parameter to be maximized since the user and/or operating environment determines the average
throughput of the processor.
The run-time of a portable system is constrained by battery life. To maximize battery life, these systems
require minimum energy consumption. Even for wired desktop machines, the drive toward “green”
computers is making energy-efficient design a priority. Therefore, the computation per battery-life per
watt-hour should be maximized, or equivalently, the average energy consumed per operation should be
minimized.
This is in contrast to low-power design, which attempts to minimize power dissipation, typically to
meet thermal design limits. Power relates to energy consumption as follows:
© 2002 by CRC Press LLC
Power
(17.2)
Thus, while reducing throughput can minimize power dissipation, the energy/operation remains
constant.
Quantifying Energy Efficiency
An energy efficiency metric must balance the desire to maximize TMAX,
and minimize the average
energy/operation. A good metric to quantify processor energy efficiency is the energy-throughput ratio
(ETR) [6]:
(17.3)
A lower ETR indicates lower energy/operation for equal throughput or equivalently indicates greater
throughput for a fixed amount of energy/operation, satisfying the need to equally optimize TMAX
and
energy/operation. Thus, a lower ETR represents a more energy-efficient solution. The energy-delay
product [7] is a similar metric, but does not include the effects of architectural parallelism when the
delay is taken to be the critical path delay.
Common Design Approaches
=
Energy
---------------------- × Throughput
Operation
Energy/Operation Power
ETR ----------------------------------------
Throughput Throughput 2
= =
-----------------------------
With the ETR metric, three common design approaches for processor systems can be analyzed, and their
impact on energy efficiency quantified.
Compute ASAP
In this approach, the processor always performs the desired computation at maximum throughput. This
is the simplest approach, and the benchmark to compare others against. When an interrupt comes into
the processor, it wakes up from sleep, performs the requested computation, then goes back into sleep
mode, as shown in Fig. 17.2(a). In sleep mode, the processor’s clock can be halted to significantly reduce
idle energy consumption, and restarted upon the next interrupt. This approach is always high throughput,
but unfortunately, it is also always high energy/operation.
Clock Frequency Reduction
A common low-power design technique is to reduce the clock frequency, fCLK.
This in turn reduces the
throughput, and power dissipation, by a proportional amount. The energy consumption remains unchanged,
as shown in Fig. 17.2(b), because energy/operation is independent of fCLK.
This approach actually
increases the ETR with respect to the previous approach, and is therefore more energy inefficient, because
the processor delivers the same amount of computation per battery life, but at a lower level of peak
throughput.