Underclocking

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See also: Dynamic frequency and voltage scaling

Underclocking also known as downclocking is the practice of modifying a synchronous circuit's speed settings to run at a lower clock speed than the manufacturer's specification. Underclocking is the opposite of overclocking.

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[edit] Microprocessor underclocking

For microprocessors, the purpose is generally to decrease the need for heat dissipation devices or decrease the electrical power consumption. This can provide increased system stability in high-heat environments, or can allow a system to run with a lower airflow (and therefore quieter) cooling fan or without one at all. For example, a Pentium 4 processor clocked at 2.4 GHz can be underclocked to 1.8 GHz and can then be safely run with reduced fan speeds. However, this invariably comes at the expense of some system performance. Underclocking is also usually used to closely monitor a process for its behavior at lower speed where a fast speed running of the process does not allow that, it gives programmer or technicians to run an application that is running abnormally fast. Also it is being used where extra CPU power is not needed to avoid high power consumption or even less heat generating from the processor and better stability.

[edit] Memory underclocking

Newer and faster RAM may be underclocked to match older systems as an inexpensive way to replace rare or discontinued memory. This might also be necessary if stability problems are encountered at higher settings.

[edit] When used

Dynamic frequency scaling (automatic underclocking) is very common on laptop computers and is beginning to emerge on desktop computers as well. In laptops, the processor is usually underclocked automatically whenever the computer is operating on batteries. Most newer notebook and some desktop processors (see Cool'n'Quiet) will also underclock themselves automatically when under a light processing load. Intel has also used this method on their Core 2 Duo processors, through a utility called SpeedStep.

Some processors underclock automatically as a defensive measure, to prevent overheating which could cause permanent damage. When such a processor reaches a temperature level deemed too high for safe operation, the thermal control circuit activates, automatically decreasing the clock and CPU core voltage until the temperature has returned to a safe level. In a properly cooled environment, this mechanism should trigger rarely (if ever).

There are several different underclocking competitions similar in format to overclocking competitions, except the goal is to have the lowest clocked computer, as opposed to the highest.

[edit] Advantages

  • Reduced heat generation (and hence dissipation).
  • Reduced electrical power consumption.
  • Considerably longer hardware lifespan.
  • Increased stability.
  • Reduced noise from mechanical cooling parts (i.e. removing fan)


[edit] In practice

[edit] Linux

The Linux kernel, as well as other open-source kernels, include a feature known as CPU frequency modulation. This feature, often known as cpufreq, gives the system administrator a variable level of control over the CPU's clock speed. The kernel includes five governors by default: conservative, ondemand, performance, powersave, and userspace. The conservative and ondemand governors adjust the clock speed depending on the CPU load, but each with different algorithms. The ondemand governor jumps to maximum frequency on CPU load and decreases the frequency step by step on CPU idle, whereas the conservative governor increases the frequency step by step on CPU load and jumps to lowest frequency on CPU idle. The performance, powersave and userspace governors set the clock speed statically: performance to the highest available, powersave to the lowest available, and userspace to a frequency determined and controlled by the user.

[edit] Windows

Underclocking can be done manually in the BIOS or with Windows applications, or dynamically using features such as Intel's SpeedStep or AMD's Cool'n'Quiet.

[edit] Apple products

A 1.0 GHz Intel Pentium-M based chip (codenamed "Crofton") with 2 MB of L2 cache, underclocked to 350 MHz bus, runs the Apple TV.

The 612 MHz ARM processor in the iPhone and iPod touch is underclocked to 412 MHz.

[edit] Asus Eee PC

The Eee PC uses a 900 MHz Intel Celeron M processor underclocked to 630 MHz.

[edit] Performance

The performance of an underclocked machine will often be better than might be expected. Under normal desktop use, the full power of the CPU is rarely needed. Even when the system is busy a large amount of time is usually spent waiting for data from memory, disk, or other devices. Such devices communicate with the CPU through a bus which operates at a much lower speed. Generally speaking, the lower the speed of a CPU, the closer its speed will be to that of the bus, and the less power it spends waiting.

[edit] See also

v • d • e
CPU power saving technologies

[edit] External links