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

the standard deviation of subsystem power. Two major differences are apparent
comparing desktop to server power consumption: desktop power in each subsystem is
much less while relative variability is much greater. In both cases, power management
plays a large role. Effective power management through DVFS, clock gating and link
management reduce average power during idle and low utilization phases. This leads to a
greater difference in sample-to-sample power. Additionally, semiconductor process
improvements have a major effect.
The CPU subsystem is considered first. Not surprisingly, the desktop processor has
average power that is an order of magnitude less than the server processor. This is
largely influenced by process (130nM vs. 45nM), DVFS (desktop-only) and idle power
management. While the server idle power represents at least 24% of average power,
desktop idle power is no more than 4%. These large power savings require the CPU
model to include additional metrics such as frequency, voltage and temperature. It is not
sufficient to consider metrics associated only with the instruction stream (IPC, cache
accesses).
Like CPU, the chipset also exhibits much greater power variation. Unlike the server
chipset which has nearly uniform power consumption, the desktop chipset has much
greater variation with standard deviation representing as much as 10% of average power.
The difference illustrates the impact of link (Hypertransport) power management.
Average power values are also much less due to the lack of an L3 cache in the desktop
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