Dynamic frequency scaling

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Dynamic frequency scaling (also known as CPU throttling) is a technique in computer architecture where a processor is run at a less-than-maximum frequency in order to conserve power. 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, further decreasing power consumption.

The switching power dissipated by a chip using static CMOS gates is C·V²·f, where C is the capacitance being switched per clock cycle, V is voltage, and f is the switching frequency,^[1] so this part of the power consumption decreases linearly with frequency. The formula is not exact however, as many modern chips are not implemented using 100% CMOS, but also uses pseudo nMOS gates, domino logic etc. Moreover, there is also a static leakage current, which has become more and more accentuated as feature sizes has become smaller and threshold levels lower.

Dynamic voltage scaling is another power conservation technique that works on the same principles as dynamic frequency scaling. Both dynamic frequency scaling and dynamic voltage scaling can be used to prevent computer system overheating, which can result in program or operating system crashes, and possibly hardware damage.

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 faster than the CMOS formula indicates, and vice-versa. ^[2][3]

[edit] 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 low. 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.

[edit] 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 their desktop and server processor lines. The aim of Cool'n'Quiet is not to save battery life, as it is not used in their 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.

[edit] References

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