Dynamic frequency scaling

For the CPU design principle, see 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 by a chip is C·V²·f, where C is the capacitance being switched per clock cycle, V is voltage, and f is the switching frequency (as a unitless quantity).^[1] As frequency changes, the dynamic power will change linearly with it. Dynamic power does not account the total power of the chip however, as there is also static power, which is primarily due to various leakage currents. Leakage current has become more and more important as feature sizes has become smaller and threshold levels lower. In state-of-the-art deep submicrometre technologies in 2008, dynamic power accounts for approximately two-thirds of the total chip power, which limits the effectiveness of frequency scaling.^{[citation needed]}

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.^[2][3]

Performance impact

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.

Implementations

Intel's CPU throttling technology, SpeedStep, is used in its mobile CPU line. 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.

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.

References

1. ^ J. M. Rabaey. Digital Integrated Circuits. Prentice Hall, 1996.
2. ^ Mike Chin. "Asus EN9600GT Silent Edition Graphics Card". Silent PC Review. p. 5. http://www.silentpcreview.com/article821-page5.html. Retrieved 2008-04-21. 
3. ^ MIke Chin. "80 Plus expands podium for Bronze, Silver & Gold". Silent PC Review. http://www.silentpcreview.com/article814-page1.html. Retrieved 2008-04-21.