Copyright by William Lloyd Bircher 2010 - The Laboratory for ...

amplitude distributions is made in Figure 4.7. Note that the CPU distribution is truncated
at 60 Watts to prevent obscuring results from the other subsystems. A small number of
phases (6.5%) exist above 60 Watts and extending to 163 Watts.
These distributions suggest that there are significant opportunities for phase-based power
savings for CPU, I/O, and disk. These subsystems have more wider and/or multimodal
distributions. The larger variations in power consumption provide greater opportunity to
use runtime detection techniques such as [In06] [IsMa06]. In contrast, chipset and
memory have homogeneous behavior suggesting nearly constant power consumption and
less opportunity for phase detection.
The presence of power variation is not sufficient to motivate the application of power
adaptation. Due to the overhead of detection and transition, adapting for short duration
phases may not be worthwhile. Table 4.3 presents the percentage of samples that are
classifiable as phases with durations of 1 ms, 10ms, 100ms and 1000ms. A group of
power samples is considered a phase if the power level within the group remains
constant. To quantify the similarity, the coefficient of variation (CoV) is calculated for
the group. The group is considered a phase if the CoV does not exceed a specified
threshold. The boundaries of a phase are determined by samples which cause the CoV to
exceed the threshold. Results for CoV of 0.25, 0.1 and 0.05 are presented. At thresholds
of 0.25 and 0.1 excessive error exists especially in I/O subsystem phase classifications.
A probable cause of the error is the greater sample-to-sample variability of the I/O power
trace. The disk subsystem, which has higher than average error, also has a wider than
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