Underclocking

See also: Dynamic frequency and voltage scaling

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. It may be said to be the computer equivalent of driving a car below the speed limit. Usually, underclocking is used to reduce a computer's power consumption and heat emission, sometimes also to increase the system's stability and compatibility. Underclocking may be implemented by the factory, but many computers and components are end user underclockable.

Types of underclocking

CPU 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. This invariably comes at the expense of some system performance. However, the performance usually is reduced less than the reduction of clock speed because the performance often is limited by other bottlenecks: the hard disk, disk controller, Internet, network, etc.

Graphics cards

"Underclocking" can also be performed on graphics card processors (graphics processing units; 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 and word processing), 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, especially in a PC with several memory modules of different clock speed. If you underclock a PC processor, and do not change the clock factor or multipilier (the ratio between the processor and the memory clock speed), the memory will also be underclocked.

When used

Dynamic frequency scaling (automatic underclocking) is very common on laptop computers and has become common on desktop computers as well. In laptops, the processor is usually underclocked automatically whenever the computer is operating on batteries. Most modern notebook and desktop processors (utilizing power-saving schemes like AMD's Cool'n'Quiet and PowerNow!^[1]) will underclock themselves automatically under a light processing load, when the machine BIOS and the operating system support it. Intel has also used this method on their Core 2 Duo and later 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 electrical power consumption, especially when combined with undervolting (i.e., reducing the component’s voltage below the nominal). For instance, by underclocking an Athlon XP 1700+ processor from 1466 to 1000 MHz and reducing the core voltage from 1.75 to 1.15V, a computer user reduced the power consumption from 64.0 to 21.6W, i.e., 66% power reduction, with only 26% less performance.^[2] The same is true for newer processors: When a single-core Intel CPU was 20% underclocked, the PC’s performance was down only 13% with a 49% power reduction.^[3]

In general, the power consumed by a CPU with a capacitance C, running at frequency f and voltage V is approximately^[4]

P = CV²f.

• Reduced heat generation, which is exactly proportional to the power consumption.
• Less noise because the cooling fans may be slowed down, or even eliminated. A cooling fan’s efficiency is proportional to its rotation speed, but it noise grows much more.
• Longer hardware lifespan.
• Increased stability.
• Reduced noise from cooling parts due to reduced heat dissipation requirements.
• Increased battery life.
• Better compatibility with old applications.

In practice

Linux

Linux kernel supports CPU frequency modulation. In supported processors, using cpufreq to gain access to this feature 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 Asus Eee PC uses a 900 MHz Intel Celeron M processor underclocked to 630 MHz.

Smartphones and PDAs

Most smartphones and PDAs, such as the Motorola Droid, Palm Pre, and Apple iPhone, iPhone 3G, iPhone 3GS, iPhone 4 and iPhone 4s, use the "underclocking" of a more powerful processor, rather than the full clocking of a less powerful processor, to maximize battery life. The designers for such mobile devices often discover that a slower processor gives worse battery life than a more powerful processor at a lower clock rate. They select a processor on the basis of the performance per watt of the processor.^[5]

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, the lower the CPU multiplier (and thus clockrate of a CPU), the closer its performance will be to that of the bus, and the less time it will spend waiting.

See also

• Overclocking
• SpeedStep
• Undervolting

External links

1. ^ http://www.amd.com/uk/products/technologies/amd-powernow-technology/Pages/amd-powernow-technology.aspx
2. ^ Ultimate Underclock & Undervolt Project; http://www.silentpcreview.com/article33-page1.html
3. ^ Fra overklokking til underklokking; http://www.datarespons.com/templates/interrupt.aspx?id=30477 (in Norwegian)
4. ^ Enhanced Intel SpeedStep Technology for the Intel Pentium M Processor - White Paper, Intel Corporation, March 2004, ftp://download.intel.com/design/network/papers/30117401.pdf 
5. ^ "Microprocessor Design: Performance Metrics"

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