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Performance Vs. Power

Energy-Efficient Computing Options

With the exception of special low-power applications that do not require much performance, the best low-power PC has to be a solution that has reasonably low idle-power requirements, yet provides sufficient performance to take on demanding workloads. Finally, the system must not be considerably more expensive than a mainstream PC. Four things have to be considered:

  • Maximum power requirement of key components
  • Idle power requirement of key components
  • Performance of key components
  • Cost of these components (mainstream component prices compared to low-power options.

Imagine two systems, A and B, which both show an idle power requirement of 60 W and a maximum requirement of 100 W when the processors are at a 100% load. If we assume that both systems are based on dual-core processors, but processor B delivers more performance for a certain workload, it will be able to complete the task faster than system A. As a result, the overall power requirement of system B over time would be smaller, because it completes the workloads faster and spends more time in idle.

This example applies to the current situation between AMD and Intel. AMD systems tend to be more efficient in idle, as the processors include the memory controller, providing better energy efficiency compared to when the memory controller is part of the chipset. Recent Core 2 Duo L2 and G0 steppings have reached idle power requirements similar to those the Athlon 64 X2 offers, which puts Intel in the position of having smaller or similar maximum and comparable idle power requirements, while Core 2 Duo provides somewhat more performance than an Athlon 64 X2 at comparable price points. The issue of processing tasks quicker in order to return to an idle state earlier typically isn't measured by the term "performance per Watt." It also means that most of the power requirement measurements available on the Web today - including many of ours - are very much useless unless systems are compared under a specific, realistic workload over a certain period of time. The result of power measurements has to be expressed in kilo Watt hours (kWh).

The same principle basically applies to hard drives, as a quicker hard drive can finish its tasks earlier to go into an idle state earlier. However, the difference in power requirement between a mainstream 7,200 RPM hard drive and a 10,000 WD Raptor drive is only a few Watts, which only becomes important if you already tackled potential power savings in the processor, the platform, the power supply unit and the graphics subsystem.

Graphics is an excellent topic to talk about regarding performance vs. power requirements, because something like an energy-efficient graphics solution doesn't exist in the desktop space today. For low-power PCs, you can only go for a graphics solution that is integrated into the chipset, which requires only a few Watts for 2D and no more than 20 W in 3D operation. However, and despite all the improvements, 3D performance of integrated solutions is pathetic and largely unsuitable for serious gaming or other 3D applications. As you will see in the graphics section of this article, even a low-end discrete graphics card will increase the system idle power requirement by at least 20 W. Numbers of 30-40 W are much more realistic for mainstream cards, and high-end 3D graphics monsters consume 60 to 90 W in 2D idle, when they effectively do nothing! One possible solution would be moving mobile graphics product into the desktop space, as these are power optimized, yet provide good performance.

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