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Energy-Efficient Processors vs. Downclocking

As a matter of fact, you don’t really need to have an energy-efficient S or T series processor to build your own compact and high-performance computer. Some enthusiasts take a different approach. They prefer downclocking, which means that they deliberately lower the clock rate of an ordinary CPU. It works because a processor’s heat dissipation and power consumption are directly linked to its clock rate and voltage. Lowering the clock rate below the default level helps lower the voltage, which leads to power savings.

Moreover, Intel’s energy-efficient processors are in fact a product of factory downclocking of standard models, so we can be really sure that manual downclocking is going to work. The question is if it will produce better results than using S and T series processors. The answer isn’t obvious. On one hand, the specialized processors come with optimized settings and have an aggressively set-up turbo mode which improves their performance. And on the other hand, regular processors, when downclocked, have no additional and virtually unnecessary limitations in terms of operating temperature but provide much set-up flexibility because their specified frequency multiplier is higher and can be reduced.

Talking about downclocking, it is important to note that Haswell-based processors let you do it in a much easier way since they’ve got a smart integrated voltage regulator. As you may know, the voltage of the execution cores of any Haswell-based processor is not a fixed value. It is a function of its frequency. So the voltage of each processor sample varies dynamically, among other factors, on the clock rate it currently uses. The regulator automatically steps the voltage up when the clock rate goes up and steps it down when the clock rate is dropped. By the way, this is why the Haswell automatically increases its voltage when overclocked and why the voltage grows up in the turbo modes. Obviously, it also means that downclocking a regular Haswell-based CPU ensures a reduction in its power consumption without your having to change any voltage-related settings in the mainboard’s BIOS. The voltage will go down automatically.

To illustrate this, the next diagram shows the correlation between a CPU’s voltage and frequency. We’ve taken our Core i5-4670 for this test:

The voltage varies in a broad range, as you can see. As the clock rate changes from 2.8 to 3.6 GHz, the Haswell’s integrated regulator increases it by 0.15 volts. Interestingly, if we put down dots corresponding to the Core i5-4670T and Core i5-4670S (for the first processor: clock rate of 2.9 GHz and voltage of 1.04 volts; for the second processor: 3.4 GHz and 1.06 volts) into the Core i5-4670 graph above, we will find that the energy-efficient processors have no special properties. When the clock rate of the regular Core i5-4670 is reduced, its voltage goes down to the same levels as with the specialized Core i5-4670T and Core i5-4670S. In other words, these three processor models (Core i5-4670T, Core i5-4670S and Core i5-4670) only differ in default clock rates and Turbo Boost 2.0 settings. The integrated voltage regulator is set up identically in them. So if we downclock a Core i5-4670 to the frequencies the energy-efficient processors work at, we automatically get the same level of heat dissipation and power consumption as with a Core i5-4670T or Core i5-4670S.

To prove our point, we will show you a graph of the correlation between the peak power consumption of a Core i5-4670 (measured in LinX 0.6.5) and its clock rate. The voltage was left at its default level as set by the integrated regulator. That is, it changed according to the graph above. The power consumption of the energy-efficient Core i5-4670T and Core i5-4670S at the same load is indicated in the diagram, too.

As you can see, the regular Core i5-4670, if downclocked, can act as an energy-efficient processor without any additional setting-up. By lowering its frequency multiplier you can easily get the same power consumption and heat dissipation as with the specialized Core i5-4670T and Core i5-4670S models, which means that these three processors are indistinguishable in terms of their semiconductor die design.

That said, an ordinary LGA1150 processor cannot be a full replacement for an energy-efficient one, even if handled properly. Reducing the frequency multiplier manually, of course, helps drop the power consumption to any level you want but you disable the Turbo Boost technology by doing so. There’s no automatic overclocking for a CPU with a reduced frequency multiplier. And it means that downclocking cannot deliver the same performance at single-threaded loads as you can get with S and T series models which boost their clock rate aggressively in the turbo mode.

Considering that Intel’s S and T series processors are not more expensive than their regular counterparts, there’s no reason to prefer the latter when building compact, economical and quiet computers. The standard-TDP models may only be interesting for their higher flexibility in terms of frequency and heat dissipation adjustment for specific operating conditions. This is hardly a hefty advantage, though. The problem of performance should also be taken into account, however, so now we’ll proceed to checking it out.

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