Clarkdale Efficiency: The Perfect Clock Rate For Intel's Core i5-661


We don’t want to spend too much time talking about the Clarkdale-based Core i5/i3 dual-core processors themselves. We’ve done that in great detail, and it’s obvious that these CPUs deliver great performance and impressive efficiency, albeit at a significant price premium compared to AMD’s offerings. Instead, this article looks at the performance gains and the impact on power efficiency when overclocking a Core i5-661.

Our testing revealed that the processor shows fairly steady efficiency up to about 3.9 GHz. This means that every voltage and clock speed increase causes an increase in power consumption that is equivalent to the performance impact. However, if you exceed 4 GHz nominal clock speed, you will have to turn up the processor voltage (Vcore), and this clearly decreases performance per watt. Our recommendation is to stay in the range of 3.7 to 3.9 GHz nominal speed. Intel’s Turbo Boost technology will still be there to add another one or two clock speed increments.

Two conclusions can be drawn: first of all, the difference in power consumption between the stock settings and our highest overclock—4.0 GHz nominal and 4.32 GHz with Turbo Boost max'ed out—is relatively small. Our measurements say that the delta is around 5W at idle and roughly 30W at peak load. Even though efficiency decreases above 4 GHz, it might still make sense to go higher. The chips have decent clock speed margins, and the additional power draw isn’t a huge deal. All you’ll need is a better cooling solution. Last but not least, there's room for improvement on the power supply side, as the 750W PC Power and Cooling device we’ve been using for various reviews is by no means an ideal partner for this test system. Go for a more conservative PSU to match power requirements so you can operate within power supply’s highest efficiency range, and thus save some power.

Our second conclusion refers to Turbo Boost: we want more of it! Obviously, there's headroom. Intel could take better advantage of the existing architecture, so why not work on more sophisticated power schemes? Turbo Boost and SpeedStep are conceptually similar, but they originate from different requirements. The PCU, Intel’s on-die power control unit, is already there to take care of dynamic clock speed adjustments. We would love to see processors differentiated mainly by their thermal specifications, meaning that a 65W desktop CPU would feature large power saving potential, but limited (reasonable) “overclocks” to stay within the power envelope. At the same time, a 130W high-end CPU could offer smaller power saving options but much higher clock speed boosts at peak loads, given the available thermal headroom, of course. Finally, the Extreme Edition flagship could allow users to modify thermal thresholds at their own risk. This is only a vision, but it would make more sense than keeping the processor world revolving around clock speeds.