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

exhibits little variation in power consumption. Therefore, a constant power model is an
obvious choice. Further, it is difficult to identify the effect performance events have on
power consumption compared to induced electrical noise in the sensors. The second, and
more critical reason, is a limitation in the power sampling environment. Since the chipset
subsystem uses power from more than one power domain, the total power cannot be
measured directly. Instead, it is derived by finding the average measured difference in
power between multiple domains. The average chipset power is 19.9W.
5.2.6 Model Validation
Tables 5.1 and 5.2 present summaries of average errors for the five models applied to
twelve workloads. Errors are determined by comparing modeled and measured error at
each sample. A sample corresponds to one second of program execution or
approximately 1.5 billion instructions per processor. For performance counter sampling,
the total number of events during the previous one second is used. For power
consumption, the average of all samples in the previous second (ten thousand) is used.
One second sample intervals provide a compromise between trace size and accuracy.
Reducing the sample interval to as low as 100 microseconds does increase the magnitude
of error in worst-case samples. However, the cumulative average error as shown in
Equation 5.6 is nearly identical to that obtained with one second sample intervals. The
benefit is that trace size is reduced to a practical level that allows tracing complete,
realistic workloads.
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