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

memory. Since the number of main memory accesses is directly proportional to the
number of cache misses, it is possible to approximate the number of accesses using only
cache misses. Since these memory accesses must go off-chip, power is consumed
proportionally in the memory controller and DRAM. In reality the relation is not so
simple, but there is still a strong causal relationship between cache misses and main
memory accesses.
5.2 Complete-System Server Power Model
Though the initial selection of performance events for modeling is dictated by an
understanding of subsystem interactions (as in the previous example), the final selection
of which event type(s) to use is determined by the average error rate and a qualitative
comparison of the measured and modeled power traces. The dominant, power-related
performance events are described below.
Cycles – Execution time in terms of CPU clock cycles. The cycles metric is combined
with most other metrics to create per cycle metrics. This corrects for slight differences in
sampling rate. Though sampling is periodic, the actual sampling rate varies slightly due
to cache effects and interrupt latency.
Halted Cycles – Cycles in which clock gating is active. When the Pentium IV processor
is idle, it saves power by gating the clock signal to portions of itself. Idle phases of
execution are “detected” by the processor through the use of the HLT (halt) instruction.
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