Dynamic frequency scaling
Dynamic frequency scaling (also known as CPU throttling) is a technique in computer architecture whereby the frequency of a microprocessor can be automatically adjusted "on the fly," either to conserve power or to reduce the amount of heat generated by the chip. Dynamic frequency scaling is commonly used in laptops and other mobile devices, where energy comes from a battery and thus is limited. It is also used in quiet computing settings and to decrease energy and cooling costs for lightly loaded machines. Less heat output, in turn, allows the system cooling fans to be throttled down or turned off, reducing noise levels and further decreasing power consumption. It is also used for reducing heat in insufficiently cooled systems when the temperature reaches a certain threshold, such as in poorly cooled overclocked systems.
The dynamic power (switching power) dissipated per unit of time by a chip is C·V2·A·f, where C is the capacitance being switched per clock cycle, V is voltage, A is the Activity Factor indicating the average number of switching events undergone by the transistors in the chip and f is the switching frequency (as a unitless quantity). The voltage required for stable operation is determined by the frequency at which the circuit is clocked, and can be reduced if the frequency is also reduced. Dynamic power does not account for the total power of the chip, however, as there is also static power, which is primarily because of various leakage currents. Due to static power consumption and asymptotic execution time it has been shown that the energy consumption of a piece of software shows convex energy behavior, i.e., there exists an optimal CPU frequency at which energy consumption is minimal. Leakage current has become more and more important as transistor sizes have become smaller and threshold voltage levels lower. A decade ago, dynamic power accounted for approximately two-thirds of the total chip power. The power loss due to leakage currents in contemporary CPUs and SoCs tend to dominate the total power consumption. In the attempt to control the leakage power high-k metal-gates and power gating have been common methods.
Dynamic voltage scaling is another power conservation technique that is often used in conjunction with frequency scaling, as the frequency that a chip may run at is related to the operating voltage.
The efficiency of some electrical components, such as voltage regulators, decreases with increasing temperature, so the power used may increase with temperature. Since increasing power use may increase the temperature, increases in voltage or frequency may increase system power demands even further than the CMOS formula indicates, and vice-versa.
Dynamic frequency scaling reduces the number of instructions a processor can issue in a given amount of time, thus reducing performance. Hence, it is generally used when the workload is not CPU-bound.
Dynamic frequency scaling by itself is rarely worthwhile as a way to conserve switching power. Saving the most power requires dynamic voltage scaling too, because of the V2 component and the fact that modern CPUs are strongly optimized for low power idle states. In most constant-voltage cases it is more efficient to run briefly at peak speed and stay in a deep idle state for longer (called "race to idle"), than it is to run at a reduced clock rate for a long time and only stay briefly in a light idle state. However, reducing voltage along with clock rate can change those tradeoffs.
A related-but-opposite technique is overclocking, whereby processor performance is increased by ramping the processor's (dynamic) frequency beyond the manufacturer's design specifications.
One major difference between the two is that in modern PC systems overclocking is mostly done over the Front Side Bus (mainly because the multiplier is normally locked), but dynamic frequency scaling is done with the multiplier. Moreover, overclocking is often static, while dynamic frequency scaling is always dynamic. Software can often incorporate overclocked frequencies into the frequency scaling algorithm, if the chip degradation risks are allowable.
AMD employs two different CPU throttling technologies. AMD's Cool'n'Quiet technology is used on its desktop and server processor lines. The aim of Cool'n'Quiet is not to save battery life, as it is not used in AMD's mobile processor line, but instead with the purpose of producing less heat, which in turn allows the system fan to spin down to slower speeds, resulting in cooler and quieter operation, hence the name of the technology. AMD's PowerNow! CPU throttling technology is used in its mobile processor line, though some supporting CPUs like the AMD K6-2+ can be found in desktops as well.
The 36-processor AsAP 1 chip is among the first multi-core processor chips to support completely unconstrained clock operation (requiring only that frequencies are below the maximum allowed) including arbitrary changes in frequency, starts, and stops. The 167-processor AsAP 2 chip is the first multi-core processor chip which enables individual processors to make fully unconstrained changes to their own clock frequencies.
According to the ACPI Specs, the C0 working state of a modern-day CPU can be divided into the so-called "P"-states (performance states) which allow clock rate reduction and "T"-states (throttling states) which will further throttle down a CPU (but not the actual clock rate) by inserting STPCLK (stop clock) signals and thus omitting duty cycles.
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