Underclocking

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"Underclocking" also known as "Downclocking" is the practice of modifying a synchronous circuit's timing settings to run at a lower clock rate than it was specified to operate at.

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" can also be performed on graphics card processors (GPUs), usually with the aim of reducing heat output. For instance, it is possible to set a GPU to run at lower clock rates when performing everyday tasks (e.g. internet browsing), thus allowing the card to operate at lower temperature and thus lower, quieter fan speeds. The GPU can then be overclocked for more graphically intense applications, such as games. "Underclocking" a GPU will reduce performance, but this decrease will probably not be noticeable except in graphically intensive applications.

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.

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 feature 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.

Advantages

• Reduced heat generation.
• Reduced electrical power consumption.
• Longer hardware lifespan.
• Increased stability.
• Increased battery life.
• Reduced noise from cooling parts due to reduced heat dissipation requirements.

In practice

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 rate. The kernel includes five governors by default: conservative, Ondemand, Performance, Powersave, and Userspace. The conservative and Ondemand governors adjust the clock rate 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 rate statically: performance to the highest available, Powersave to the lowest available, and Userspace to a frequency determined and controlled by the user.

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.

Asus Eee PC

Some versions of the Eee PC uses a 900 MHz Intel Celeron M processor underclocked to 630 MHz.

Smartphones/PDA's

The Motorolla Droid, Palm Pre, and Apple's iPhone and iPhone 3G use the "underclocking" of a more powerful processor, rather than the full clocking of a less powerful processor, to maximize battery life. The power consumed by using a slower processor is more than using the more powerful processor at a lower clock rate.

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 bandwidth. Generally speaking, the lower the clock rate of a CPU, the closer its performance will be to that of the bus, and the less power it spends waiting.

See also

• SpeedStep
• Cool'n'Quiet
• Undervolting
• Crystal CPUID - a tool that uses Cool'n'Quiet to set custom clock/voltage step settings
• Overclocking

External links

• CPU "Undervolting" & "Underclocking" A Primer From SilentPCReview.com
• "Underclocking" a Game Boy classic tutorial